/Community.CsharpSqlite/src/pager_c.cs

using System;
using System.Diagnostics;
using System.IO;

using i16 = System.Int16;
using i64 = System.Int64;

using u8 = System.Byte;
using u16 = System.UInt16;
using u32 = System.UInt32;

using Pgno = System.UInt32;
using sqlite3_int64 = System.Int64;

namespace Community.CsharpSqlite
{
  using System.Text;
  using DbPage = Sqlite3.PgHdr;
  public partial class Sqlite3
  {
    /*
    ** 2001 September 15
    **
    ** The author disclaims copyright to this source code.  In place of
    ** a legal notice, here is a blessing:
    **
    **    May you do good and not evil.
    **    May you find forgiveness for yourself and forgive others.
    **    May you share freely, never taking more than you give.
    **
    *************************************************************************
    ** This is the implementation of the page cache subsystem or "pager".
    **
    ** The pager is used to access a database disk file.  It implements
    ** atomic commit and rollback through the use of a journal file that
    ** is separate from the database file.  The pager also implements file
    ** locking to prevent two processes from writing the same database
    ** file simultaneously, or one process from reading the database while
    ** another is writing.
    *************************************************************************
    **  Included in SQLite3 port to C#-SQLite;  2008 Noah B Hart
    **  C#-SQLite is an independent reimplementation of the SQLite software library
    **
    **  SQLITE_SOURCE_ID: 2011-06-23 19:49:22 4374b7e83ea0a3fbc3691f9c0c936272862f32f2
    **
    *************************************************************************
    */
#if !SQLITE_OMIT_DISKIO
    //#include "sqliteInt.h"
    //#include "wal.h"


    /******************* NOTES ON THE DESIGN OF THE PAGER ************************
    **
    ** This comment block describes invariants that hold when using a rollback
    ** journal.  These invariants do not apply for journal_mode=WAL,
    ** journal_mode=MEMORY, or journal_mode=OFF.
    **
    ** Within this comment block, a page is deemed to have been synced
    ** automatically as soon as it is written when PRAGMA synchronous=OFF.
    ** Otherwise, the page is not synced until the xSync method of the VFS
    ** is called successfully on the file containing the page.
    **
    ** Definition:  A page of the database file is said to be "overwriteable" if
    ** one or more of the following are true about the page:
    ** 
    **     (a)  The original content of the page as it was at the beginning of
    **          the transaction has been written into the rollback journal and
    **          synced.
    ** 
    **     (b)  The page was a freelist leaf page at the start of the transaction.
    ** 
    **     (c)  The page number is greater than the largest page that existed in
    **          the database file at the start of the transaction.
    ** 
    ** (1) A page of the database file is never overwritten unless one of the
    **     following are true:
    ** 
    **     (a) The page and all other pages on the same sector are overwriteable.
    ** 
    **     (b) The atomic page write optimization is enabled, and the entire
    **         transaction other than the update of the transaction sequence
    **         number consists of a single page change.
    ** 
    ** (2) The content of a page written into the rollback journal exactly matches
    **     both the content in the database when the rollback journal was written
    **     and the content in the database at the beginning of the current
    **     transaction.
    ** 
    ** (3) Writes to the database file are an integer multiple of the page size
    **     in length and are aligned on a page boundary.
    ** 
    ** (4) Reads from the database file are either aligned on a page boundary and
    **     an integer multiple of the page size in length or are taken from the
    **     first 100 bytes of the database file.
    ** 
    ** (5) All writes to the database file are synced prior to the rollback journal
    **     being deleted, truncated, or zeroed.
    ** 
    ** (6) If a master journal file is used, then all writes to the database file
    **     are synced prior to the master journal being deleted.
    ** 
    ** Definition: Two databases (or the same database at two points it time)
    ** are said to be "logically equivalent" if they give the same answer to
    ** all queries.  Note in particular the the content of freelist leaf
    ** pages can be changed arbitarily without effecting the logical equivalence
    ** of the database.
    ** 
    ** (7) At any time, if any subset, including the empty set and the total set,
    **     of the unsynced changes to a rollback journal are removed and the 
    **     journal is rolled back, the resulting database file will be logical
    **     equivalent to the database file at the beginning of the transaction.
    ** 
    ** (8) When a transaction is rolled back, the xTruncate method of the VFS
    **     is called to restore the database file to the same size it was at
    **     the beginning of the transaction.  (In some VFSes, the xTruncate
    **     method is a no-op, but that does not change the fact the SQLite will
    **     invoke it.)
    ** 
    ** (9) Whenever the database file is modified, at least one bit in the range
    **     of bytes from 24 through 39 inclusive will be changed prior to releasing
    **     the EXCLUSIVE lock, thus signaling other connections on the same
    **     database to flush their caches.
    **
    ** (10) The pattern of bits in bytes 24 through 39 shall not repeat in less
    **      than one billion transactions.
    **
    ** (11) A database file is well-formed at the beginning and at the conclusion
    **      of every transaction.
    **
    ** (12) An EXCLUSIVE lock is held on the database file when writing to
    **      the database file.
    **
    ** (13) A SHARED lock is held on the database file while reading any
    **      content out of the database file.
    **
    ******************************************************************************/

    /*
    ** Macros for troubleshooting.  Normally turned off
    */
#if TRACE

static bool sqlite3PagerTrace = false;  /* True to enable tracing */
//#define sqlite3DebugPrintf printf
//#define PAGERTRACE(X)     if( sqlite3PagerTrace ){ sqlite3DebugPrintf X; }
static void PAGERTRACE( string T, params object[] ap ) { if ( sqlite3PagerTrace )sqlite3DebugPrintf( T, ap ); }
#else
    //#define PAGERTRACE(X)
    static void PAGERTRACE( string T, params object[] ap )
    {
    }
#endif

    /*
** The following two macros are used within the PAGERTRACE() macros above
** to print out file-descriptors.
**
** PAGERID() takes a pointer to a Pager struct as its argument. The
** associated file-descriptor is returned. FILEHANDLEID() takes an sqlite3_file
** struct as its argument.
*/
    //#define PAGERID(p) ((int)(p.fd))
    static int PAGERID( Pager p )
    {
      return p.GetHashCode();
    }

    //#define FILEHANDLEID(fd) ((int)fd)
    static int FILEHANDLEID( sqlite3_file fd )
    {
      return fd.GetHashCode();
    }

    /*
    ** The Pager.eState variable stores the current 'state' of a pager. A
    ** pager may be in any one of the seven states shown in the following
    ** state diagram.
    **
    **                            OPEN <------+------+
    **                              |         |      |
    **                              V         |      |
    **               +---------> READER-------+      |
    **               |              |                |
    **               |              V                |
    **               |<-------WRITER_LOCKED------> ERROR
    **               |              |                ^  
    **               |              V                |
    **               |<------WRITER_CACHEMOD-------->|
    **               |              |                |
    **               |              V                |
    **               |<-------WRITER_DBMOD---------->|
    **               |              |                |
    **               |              V                |
    **               +<------WRITER_FINISHED-------->+
    **
    **
    ** List of state transitions and the C [function] that performs each:
    ** 
    **   OPEN              -> READER              [sqlite3PagerSharedLock]
    **   READER            -> OPEN                [pager_unlock]
    **
    **   READER            -> WRITER_LOCKED       [sqlite3PagerBegin]
    **   WRITER_LOCKED     -> WRITER_CACHEMOD     [pager_open_journal]
    **   WRITER_CACHEMOD   -> WRITER_DBMOD        [syncJournal]
    **   WRITER_DBMOD      -> WRITER_FINISHED     [sqlite3PagerCommitPhaseOne]
    **   WRITER_***        -> READER              [pager_end_transaction]
    **
    **   WRITER_***        -> ERROR               [pager_error]
    **   ERROR             -> OPEN                [pager_unlock]
    ** 
    **
    **  OPEN:
    **
    **    The pager starts up in this state. Nothing is guaranteed in this
    **    state - the file may or may not be locked and the database size is
    **    unknown. The database may not be read or written.
    **
    **    * No read or write transaction is active.
    **    * Any lock, or no lock at all, may be held on the database file.
    **    * The dbSize, dbOrigSize and dbFileSize variables may not be trusted.
    **
    **  READER:
    **
    **    In this state all the requirements for reading the database in 
    **    rollback (non-WAL) mode are met. Unless the pager is (or recently
    **    was) in exclusive-locking mode, a user-level read transaction is 
    **    open. The database size is known in this state.
    **
    **    A connection running with locking_mode=normal enters this state when
    **    it opens a read-transaction on the database and returns to state
    **    OPEN after the read-transaction is completed. However a connection
    **    running in locking_mode=exclusive (including temp databases) remains in
    **    this state even after the read-transaction is closed. The only way
    **    a locking_mode=exclusive connection can transition from READER to OPEN
    **    is via the ERROR state (see below).
    ** 
    **    * A read transaction may be active (but a write-transaction cannot).
    **    * A SHARED or greater lock is held on the database file.
    **    * The dbSize variable may be trusted (even if a user-level read 
    **      transaction is not active). The dbOrigSize and dbFileSize variables
    **      may not be trusted at this point.
    **    * If the database is a WAL database, then the WAL connection is open.
    **    * Even if a read-transaction is not open, it is guaranteed that 
    **      there is no hot-journal in the file-system.
    **
    **  WRITER_LOCKED:
    **
    **    The pager moves to this state from READER when a write-transaction
    **    is first opened on the database. In WRITER_LOCKED state, all locks 
    **    required to start a write-transaction are held, but no actual 
    **    modifications to the cache or database have taken place.
    **
    **    In rollback mode, a RESERVED or (if the transaction was opened with 
    **    BEGIN EXCLUSIVE) EXCLUSIVE lock is obtained on the database file when
    **    moving to this state, but the journal file is not written to or opened 
    **    to in this state. If the transaction is committed or rolled back while 
    **    in WRITER_LOCKED state, all that is required is to unlock the database 
    **    file.
    **
    **    IN WAL mode, WalBeginWriteTransaction() is called to lock the log file.
    **    If the connection is running with locking_mode=exclusive, an attempt
    **    is made to obtain an EXCLUSIVE lock on the database file.
    **
    **    * A write transaction is active.
    **    * If the connection is open in rollback-mode, a RESERVED or greater 
    **      lock is held on the database file.
    **    * If the connection is open in WAL-mode, a WAL write transaction
    **      is open (i.e. sqlite3WalBeginWriteTransaction() has been successfully
    **      called).
    **    * The dbSize, dbOrigSize and dbFileSize variables are all valid.
    **    * The contents of the pager cache have not been modified.
    **    * The journal file may or may not be open.
    **    * Nothing (not even the first header) has been written to the journal.
    **
    **  WRITER_CACHEMOD:
    **
    **    A pager moves from WRITER_LOCKED state to this state when a page is
    **    first modified by the upper layer. In rollback mode the journal file
    **    is opened (if it is not already open) and a header written to the
    **    start of it. The database file on disk has not been modified.
    **
    **    * A write transaction is active.
    **    * A RESERVED or greater lock is held on the database file.
    **    * The journal file is open and the first header has been written 
    **      to it, but the header has not been synced to disk.
    **    * The contents of the page cache have been modified.
    **
    **  WRITER_DBMOD:
    **
    **    The pager transitions from WRITER_CACHEMOD into WRITER_DBMOD state
    **    when it modifies the contents of the database file. WAL connections
    **    never enter this state (since they do not modify the database file,
    **    just the log file).
    **
    **    * A write transaction is active.
    **    * An EXCLUSIVE or greater lock is held on the database file.
    **    * The journal file is open and the first header has been written 
    **      and synced to disk.
    **    * The contents of the page cache have been modified (and possibly
    **      written to disk).
    **
    **  WRITER_FINISHED:
    **
    **    It is not possible for a WAL connection to enter this state.
    **
    **    A rollback-mode pager changes to WRITER_FINISHED state from WRITER_DBMOD
    **    state after the entire transaction has been successfully written into the
    **    database file. In this state the transaction may be committed simply
    **    by finalizing the journal file. Once in WRITER_FINISHED state, it is 
    **    not possible to modify the database further. At this point, the upper 
    **    layer must either commit or rollback the transaction.
    **
    **    * A write transaction is active.
    **    * An EXCLUSIVE or greater lock is held on the database file.
    **    * All writing and syncing of journal and database data has finished.
    **      If no error occured, all that remains is to finalize the journal to
    **      commit the transaction. If an error did occur, the caller will need
    **      to rollback the transaction. 
    **
    **  ERROR:
    **
    **    The ERROR state is entered when an IO or disk-full error (including
    **    SQLITE_IOERR_NOMEM) occurs at a point in the code that makes it 
    **    difficult to be sure that the in-memory pager state (cache contents, 
    **    db size etc.) are consistent with the contents of the file-system.
    **
    **    Temporary pager files may enter the ERROR state, but in-memory pagers
    **    cannot.
    **
    **    For example, if an IO error occurs while performing a rollback, 
    **    the contents of the page-cache may be left in an inconsistent state.
    **    At this point it would be dangerous to change back to READER state
    **    (as usually happens after a rollback). Any subsequent readers might
    **    report database corruption (due to the inconsistent cache), and if
    **    they upgrade to writers, they may inadvertently corrupt the database
    **    file. To avoid this hazard, the pager switches into the ERROR state
    **    instead of READER following such an error.
    **
    **    Once it has entered the ERROR state, any attempt to use the pager
    **    to read or write data returns an error. Eventually, once all 
    **    outstanding transactions have been abandoned, the pager is able to
    **    transition back to OPEN state, discarding the contents of the 
    **    page-cache and any other in-memory state at the same time. Everything
    **    is reloaded from disk (and, if necessary, hot-journal rollback peformed)
    **    when a read-transaction is next opened on the pager (transitioning
    **    the pager into READER state). At that point the system has recovered 
    **    from the error.
    **
    **    Specifically, the pager jumps into the ERROR state if:
    **
    **      1. An error occurs while attempting a rollback. This happens in
    **         function sqlite3PagerRollback().
    **
    **      2. An error occurs while attempting to finalize a journal file
    **         following a commit in function sqlite3PagerCommitPhaseTwo().
    **
    **      3. An error occurs while attempting to write to the journal or
    **         database file in function pagerStress() in order to free up
    **         memory.
    **
    **    In other cases, the error is returned to the b-tree layer. The b-tree
    **    layer then attempts a rollback operation. If the error condition 
    **    persists, the pager enters the ERROR state via condition (1) above.
    **
    **    Condition (3) is necessary because it can be triggered by a read-only
    **    statement executed within a transaction. In this case, if the error
    **    code were simply returned to the user, the b-tree layer would not
    **    automatically attempt a rollback, as it assumes that an error in a
    **    read-only statement cannot leave the pager in an internally inconsistent 
    **    state.
    **
    **    * The Pager.errCode variable is set to something other than SQLITE_OK.
    **    * There are one or more outstanding references to pages (after the
    **      last reference is dropped the pager should move back to OPEN state).
    **    * The pager is not an in-memory pager.
    **    
    **
    ** Notes:
    **
    **   * A pager is never in WRITER_DBMOD or WRITER_FINISHED state if the
    **     connection is open in WAL mode. A WAL connection is always in one
    **     of the first four states.
    **
    **   * Normally, a connection open in exclusive mode is never in PAGER_OPEN
    **     state. There are two exceptions: immediately after exclusive-mode has
    **     been turned on (and before any read or write transactions are 
    **     executed), and when the pager is leaving the "error state".
    **
    **   * See also: assert_pager_state().
    */

    //#define PAGER_OPEN                  0
    //#define PAGER_READER                1
    //#define PAGER_WRITER_LOCKED         2
    //#define PAGER_WRITER_CACHEMOD       3
    //#define PAGER_WRITER_DBMOD          4
    //#define PAGER_WRITER_FINISHED       5
    //#define PAGER_ERROR                 6
    const int PAGER_OPEN = 0;
    const int PAGER_READER = 1;
    const int PAGER_WRITER_LOCKED = 2;
    const int PAGER_WRITER_CACHEMOD = 3;
    const int PAGER_WRITER_DBMOD = 4;
    const int PAGER_WRITER_FINISHED = 5;
    const int PAGER_ERROR = 6;

    /*
    ** The Pager.eLock variable is almost always set to one of the 
    ** following locking-states, according to the lock currently held on
    ** the database file: NO_LOCK, SHARED_LOCK, RESERVED_LOCK or EXCLUSIVE_LOCK.
    ** This variable is kept up to date as locks are taken and released by
    ** the pagerLockDb() and pagerUnlockDb() wrappers.
    **
    ** If the VFS xLock() or xUnlock() returns an error other than SQLITE_BUSY
    ** (i.e. one of the SQLITE_IOERR subtypes), it is not clear whether or not
    ** the operation was successful. In these circumstances pagerLockDb() and
    ** pagerUnlockDb() take a conservative approach - eLock is always updated
    ** when unlocking the file, and only updated when locking the file if the
    ** VFS call is successful. This way, the Pager.eLock variable may be set
    ** to a less exclusive (lower) value than the lock that is actually held
    ** at the system level, but it is never set to a more exclusive value.
    **
    ** This is usually safe. If an xUnlock fails or appears to fail, there may 
    ** be a few redundant xLock() calls or a lock may be held for longer than
    ** required, but nothing really goes wrong.
    **
    ** The exception is when the database file is unlocked as the pager moves
    ** from ERROR to OPEN state. At this point there may be a hot-journal file 
    ** in the file-system that needs to be rolled back (as part of a OPEN->SHARED
    ** transition, by the same pager or any other). If the call to xUnlock()
    ** fails at this point and the pager is left holding an EXCLUSIVE lock, this
    ** can confuse the call to xCheckReservedLock() call made later as part
    ** of hot-journal detection.
    **
    ** xCheckReservedLock() is defined as returning true "if there is a RESERVED 
    ** lock held by this process or any others". So xCheckReservedLock may 
    ** return true because the caller itself is holding an EXCLUSIVE lock (but
    ** doesn't know it because of a previous error in xUnlock). If this happens
    ** a hot-journal may be mistaken for a journal being created by an active
    ** transaction in another process, causing SQLite to read from the database
    ** without rolling it back.
    **
    ** To work around this, if a call to xUnlock() fails when unlocking the
    ** database in the ERROR state, Pager.eLock is set to UNKNOWN_LOCK. It
    ** is only changed back to a real locking state after a successful call
    ** to xLock(EXCLUSIVE). Also, the code to do the OPEN->SHARED state transition
    ** omits the check for a hot-journal if Pager.eLock is set to UNKNOWN_LOCK 
    ** lock. Instead, it assumes a hot-journal exists and obtains an EXCLUSIVE
    ** lock on the database file before attempting to roll it back. See function
    ** PagerSharedLock() for more detail.
    **
    ** Pager.eLock may only be set to UNKNOWN_LOCK when the pager is in 
    ** PAGER_OPEN state.
    */
    //#define UNKNOWN_LOCK                (EXCLUSIVE_LOCK+1)
    const int UNKNOWN_LOCK = ( EXCLUSIVE_LOCK + 1 );

    /*
    ** A macro used for invoking the codec if there is one
    */

    // The E parameter is what executes when there is an error, 
    // cannot implement here, since this is not really a macro
    // calling code must be modified to call E when truen

#if SQLITE_HAS_CODEC
    //# define CODEC1(P,D,N,X,E) \
    //if( P.xCodec && P.xCodec(P.pCodec,D,N,X)==0 ){ E; }
    static bool CODEC1( Pager P, byte[] D, uint N /* page number */, int X /* E (moved to caller */)
    {
      return ( ( P.xCodec != null ) && ( P.xCodec( P.pCodec, D, N, X ) == null ) );
    }

    // The E parameter is what executes when there is an error, 
    // cannot implement here, since this is not really a macro
    // calling code must be modified to call E when truen

    //# define CODEC2(P,D,N,X,E,O) \
    //if( P.xCodec==0 ){ O=(char*)D; }else \
    //if( (O=(char*)(P.xCodec(P.pCodec,D,N,X)))==0 ){ E; }
    static bool CODEC2( Pager P, byte[] D, uint N, int X, ref byte[] O )
    {
      if ( P.xCodec == null )
      {
        O = D; // do nothing
        return false;
      }
      else
      {
        return ( ( O = P.xCodec( P.pCodec, D, N, X ) ) == null );
      }
    }
#else
//# define CODEC1(P,D,N,X,E)   /* NO-OP */
static bool CODEC1 (Pager P, byte[] D, uint N /* page number */, int X /* E (moved to caller */)  { return false; }
//# define CODEC2(P,D,N,X,E,O) O=(char*)D
static bool CODEC2( Pager P, byte[] D, uint N, int X, ref byte[] O ) { O = D; return false; }
#endif

    /*
** The maximum allowed sector size. 64KiB. If the xSectorsize() method 
** returns a value larger than this, then MAX_SECTOR_SIZE is used instead.
** This could conceivably cause corruption following a power failure on
** such a system. This is currently an undocumented limit.
*/
    //#define MAX_SECTOR_SIZE 0x10000
    const int MAX_SECTOR_SIZE = 0x10000;

    /*
    ** An instance of the following structure is allocated for each active
    ** savepoint and statement transaction in the system. All such structures
    ** are stored in the Pager.aSavepoint[] array, which is allocated and
    ** resized using sqlite3Realloc().
    **
    ** When a savepoint is created, the PagerSavepoint.iHdrOffset field is
    ** set to 0. If a journal-header is written into the main journal while
    ** the savepoint is active, then iHdrOffset is set to the byte offset
    ** immediately following the last journal record written into the main
    ** journal before the journal-header. This is required during savepoint
    ** rollback (see pagerPlaybackSavepoint()).
    */
    //typedef struct PagerSavepoint PagerSavepoint;
    public class PagerSavepoint
    {
      public i64 iOffset;                 /* Starting offset in main journal */
      public i64 iHdrOffset;              /* See above */
      public Bitvec pInSavepoint;         /* Set of pages in this savepoint */
      public Pgno nOrig;                  /* Original number of pages in file */
      public Pgno iSubRec;                /* Index of first record in sub-journal */
#if !SQLITE_OMIT_WAL
public u32 aWalData[WAL_SAVEPOINT_NDATA];        /* WAL savepoint context */
#else
      public object aWalData = null;      /* Used for C# convenience */
#endif
      public static implicit operator bool( PagerSavepoint b )
      {
        return ( b != null );
      }
    };


    /*
    ** A open page cache is an instance of struct Pager. A description of
    ** some of the more important member variables follows:
    **
    ** eState
    **
    **   The current 'state' of the pager object. See the comment and state
    **   diagram above for a description of the pager state.
    **
    ** eLock
    **
    **   For a real on-disk database, the current lock held on the database file -
    **   NO_LOCK, SHARED_LOCK, RESERVED_LOCK or EXCLUSIVE_LOCK.
    **
    **   For a temporary or in-memory database (neither of which require any
    **   locks), this variable is always set to EXCLUSIVE_LOCK. Since such
    **   databases always have Pager.exclusiveMode==1, this tricks the pager
    **   logic into thinking that it already has all the locks it will ever
    **   need (and no reason to release them).
    **
    **   In some (obscure) circumstances, this variable may also be set to
    **   UNKNOWN_LOCK. See the comment above the #define of UNKNOWN_LOCK for
    **   details.
    **
    ** changeCountDone
    **
    **   This boolean variable is used to make sure that the change-counter 
    **   (the 4-byte header field at byte offset 24 of the database file) is 
    **   not updated more often than necessary. 
    **
    **   It is set to true when the change-counter field is updated, which 
    **   can only happen if an exclusive lock is held on the database file.
    **   It is cleared (set to false) whenever an exclusive lock is 
    **   relinquished on the database file. Each time a transaction is committed,
    **   The changeCountDone flag is inspected. If it is true, the work of
    **   updating the change-counter is omitted for the current transaction.
    **
    **   This mechanism means that when running in exclusive mode, a connection 
    **   need only update the change-counter once, for the first transaction
    **   committed.
    **
    ** setMaster
    **
    **   When PagerCommitPhaseOne() is called to commit a transaction, it may
    **   (or may not) specify a master-journal name to be written into the 
    **   journal file before it is synced to disk.
    **
    **   Whether or not a journal file contains a master-journal pointer affects 
    **   the way in which the journal file is finalized after the transaction is 
    **   committed or rolled back when running in "journal_mode=PERSIST" mode.
    **   If a journal file does not contain a master-journal pointer, it is
    **   finalized by overwriting the first journal header with zeroes. If
    **   it does contain a master-journal pointer the journal file is finalized 
    **   by truncating it to zero bytes, just as if the connection were 
    **   running in "journal_mode=truncate" mode.
    **
    **   Journal files that contain master journal pointers cannot be finalized
    **   simply by overwriting the first journal-header with zeroes, as the
    **   master journal pointer could interfere with hot-journal rollback of any
    **   subsequently interrupted transaction that reuses the journal file.
    **
    **   The flag is cleared as soon as the journal file is finalized (either
    **   by PagerCommitPhaseTwo or PagerRollback). If an IO error prevents the
    **   journal file from being successfully finalized, the setMaster flag
    **   is cleared anyway (and the pager will move to ERROR state).
    **
    ** doNotSpill, doNotSyncSpill
    **
    **   These two boolean variables control the behaviour of cache-spills
    **   (calls made by the pcache module to the pagerStress() routine to
    **   write cached data to the file-system in order to free up memory).
    **
    **   When doNotSpill is non-zero, writing to the database from pagerStress()
    **   is disabled altogether. This is done in a very obscure case that
    **   comes up during savepoint rollback that requires the pcache module
    **   to allocate a new page to prevent the journal file from being written
    **   while it is being traversed by code in pager_playback().
    ** 
    **   If doNotSyncSpill is non-zero, writing to the database from pagerStress()
    **   is permitted, but syncing the journal file is not. This flag is set
    **   by sqlite3PagerWrite() when the file-system sector-size is larger than
    **   the database page-size in order to prevent a journal sync from happening 
    **   in between the journalling of two pages on the same sector. 
    **
    ** subjInMemory
    **
    **   This is a boolean variable. If true, then any required sub-journal
    **   is opened as an in-memory journal file. If false, then in-memory
    **   sub-journals are only used for in-memory pager files.
    **
    **   This variable is updated by the upper layer each time a new 
    **   write-transaction is opened.
    **
    ** dbSize, dbOrigSize, dbFileSize
    **
    **   Variable dbSize is set to the number of pages in the database file.
    **   It is valid in PAGER_READER and higher states (all states except for
    **   OPEN and ERROR). 
    **
    **   dbSize is set based on the size of the database file, which may be 
    **   larger than the size of the database (the value stored at offset
    **   28 of the database header by the btree). If the size of the file
    **   is not an integer multiple of the page-size, the value stored in
    **   dbSize is rounded down (i.e. a 5KB file with 2K page-size has dbSize==2).
    **   Except, any file that is greater than 0 bytes in size is considered
    **   to have at least one page. (i.e. a 1KB file with 2K page-size leads
    **   to dbSize==1).
    **
    **   During a write-transaction, if pages with page-numbers greater than
    **   dbSize are modified in the cache, dbSize is updated accordingly.
    **   Similarly, if the database is truncated using PagerTruncateImage(), 
    **   dbSize is updated.
    **
    **   Variables dbOrigSize and dbFileSize are valid in states 
    **   PAGER_WRITER_LOCKED and higher. dbOrigSize is a copy of the dbSize
    **   variable at the start of the transaction. It is used during rollback,
    **   and to determine whether or not pages need to be journalled before
    **   being modified.
    **
    **   Throughout a write-transaction, dbFileSize contains the size of
    **   the file on disk in pages. It is set to a copy of dbSize when the
    **   write-transaction is first opened, and updated when VFS calls are made
    **   to write or truncate the database file on disk. 
    **
    **   The only reason the dbFileSize variable is required is to suppress 
    **   unnecessary calls to xTruncate() after committing a transaction. If, 
    **   when a transaction is committed, the dbFileSize variable indicates 
    **   that the database file is larger than the database image (Pager.dbSize), 
    **   pager_truncate() is called. The pager_truncate() call uses xFilesize()
    **   to measure the database file on disk, and then truncates it if required.
    **   dbFileSize is not used when rolling back a transaction. In this case
    **   pager_truncate() is called unconditionally (which means there may be
    **   a call to xFilesize() that is not strictly required). In either case,
    **   pager_truncate() may cause the file to become smaller or larger.
    **
    ** dbHintSize
    **
    **   The dbHintSize variable is used to limit the number of calls made to
    **   the VFS xFileControl(FCNTL_SIZE_HINT) method. 
    **
    **   dbHintSize is set to a copy of the dbSize variable when a
    **   write-transaction is opened (at the same time as dbFileSize and
    **   dbOrigSize). If the xFileControl(FCNTL_SIZE_HINT) method is called,
    **   dbHintSize is increased to the number of pages that correspond to the
    **   size-hint passed to the method call. See pager_write_pagelist() for 
    **   details.
    **
    ** errCode
    **
    **   The Pager.errCode variable is only ever used in PAGER_ERROR state. It
    **   is set to zero in all other states. In PAGER_ERROR state, Pager.errCode 
    **   is always set to SQLITE_FULL, SQLITE_IOERR or one of the SQLITE_IOERR_XXX 
    **   sub-codes.
    */
    public class Pager
    {
      public sqlite3_vfs pVfs;           /* OS functions to use for IO */
      public bool exclusiveMode;         /* Boolean. True if locking_mode==EXCLUSIVE */
      public u8 journalMode;             /* One of the PAGER_JOURNALMODE_* values */
      public u8 useJournal;              /* Use a rollback journal on this file */
      public u8 noReadlock;              /* Do not bother to obtain readlocks */
      public bool noSync;                /* Do not sync the journal if true */
      public bool fullSync;              /* Do extra syncs of the journal for robustness */
      public u8 ckptSyncFlags;           /* SYNC_NORMAL or SYNC_FULL for checkpoint */
      public u8 syncFlags;               /* SYNC_NORMAL or SYNC_FULL otherwise */
      public bool tempFile;              /* zFilename is a temporary file */
      public bool readOnly;              /* True for a read-only database */
      public bool alwaysRollback;        /* Disable DontRollback() for all pages */
      public u8 memDb;                   /* True to inhibit all file I/O */
      /**************************************************************************
      ** The following block contains those class members that change during
      ** routine opertion.  Class members not in this block are either fixed
      ** when the pager is first created or else only change when there is a
      ** significant mode change (such as changing the page_size, locking_mode,
      ** or the journal_mode).  From another view, these class members describe
      ** the "state" of the pager, while other class members describe the
      ** "configuration" of the pager.
      */
      public u8 eState;                  /* Pager state (OPEN, READER, WRITER_LOCKED..) */
      public u8 eLock;                   /* Current lock held on database file */
      public bool changeCountDone;       /* Set after incrementing the change-counter */
      public int setMaster;              /* True if a m-j name has been written to jrnl */
      public u8 doNotSpill;              /* Do not spill the cache when non-zero */
      public u8 doNotSyncSpill;          /* Do not do a spill that requires jrnl sync */
      public u8 subjInMemory;            /* True to use in-memory sub-journals */
      public Pgno dbSize;                /* Number of pages in the database */
      public Pgno dbOrigSize;            /* dbSize before the current transaction */
      public Pgno dbFileSize;            /* Number of pages in the database file */
      public Pgno dbHintSize;            /* Value passed to FCNTL_SIZE_HINT call */
      public int errCode;                /* One of several kinds of errors */
      public int nRec;                   /* Pages journalled since last j-header written */
      public u32 cksumInit;              /* Quasi-random value added to every checksum */
      public u32 nSubRec;                /* Number of records written to sub-journal */
      public Bitvec pInJournal;          /* One bit for each page in the database file */
      public sqlite3_file fd;            /* File descriptor for database */
      public sqlite3_file jfd;           /* File descriptor for main journal */
      public sqlite3_file sjfd;          /* File descriptor for sub-journal */
      public i64 journalOff;             /* Current write offset in the journal file */
      public i64 journalHdr;             /* Byte offset to previous journal header */
      public sqlite3_backup pBackup;     /* Pointer to list of ongoing backup processes */
      public PagerSavepoint[] aSavepoint;/* Array of active savepoints */
      public int nSavepoint;             /* Number of elements in aSavepoint[] */
      public u8[] dbFileVers = new u8[16];/* Changes whenever database file changes */
      /*
      ** End of the routinely-changing class members
      ***************************************************************************/

      public u16 nExtra;                 /* Add this many bytes to each in-memory page */
      public i16 nReserve;               /* Number of unused bytes at end of each page */
      public u32 vfsFlags;               /* Flags for sqlite3_vfs.xOpen() */
      public u32 sectorSize;             /* Assumed sector size during rollback */
      public int pageSize;               /* Number of bytes in a page */
      public Pgno mxPgno;                /* Maximum allowed size of the database */
      public i64 journalSizeLimit;       /* Size limit for persistent journal files */
      public string zFilename;           /* Name of the database file */
      public string zJournal;            /* Name of the journal file */
      public dxBusyHandler xBusyHandler; /* Function to call when busy */
      public object pBusyHandlerArg;     /* Context argument for xBusyHandler */
#if SQLITE_TEST || DEBUG
      public int nHit, nMiss;              /* Cache hits and missing */
      public int nRead, nWrite;            /* Database pages read/written */
#else
public int nHit;
#endif
      public dxReiniter xReiniter; //(DbPage*,int);/* Call this routine when reloading pages */
#if SQLITE_HAS_CODEC
      //void *(*xCodec)(void*,void*,Pgno,int); 
      public dxCodec xCodec;                 /* Routine for en/decoding data */
      //void (*xCodecSizeChng)(void*,int,int); 
      public dxCodecSizeChng xCodecSizeChng; /* Notify of page size changes */
      //void (*xCodecFree)(void*);             
      public dxCodecFree xCodecFree;         /* Destructor for the codec */
      public codec_ctx pCodec;               /* First argument to xCodec... methods */
#endif
      public byte[] pTmpSpace;               /* Pager.pageSize bytes of space for tmp use */
      public PCache pPCache;                 /* Pointer to page cache object */
#if !SQLITE_OMIT_WAL
public Wal pWal;                       /* Write-ahead log used by "journal_mode=wal" */
public string zWal;                    /* File name for write-ahead log */
#else
      public sqlite3_vfs pWal = null;             /* Having this dummy here makes C# easier */
#endif
    };

    /*
    ** The following global variables hold counters used for
    ** testing purposes only.  These variables do not exist in
    ** a non-testing build.  These variables are not thread-safe.
    */
#if SQLITE_TEST
#if !TCLSH
    static int sqlite3_pager_readdb_count = 0;    /* Number of full pages read from DB */
    static int sqlite3_pager_writedb_count = 0;   /* Number of full pages written to DB */
    static int sqlite3_pager_writej_count = 0;    /* Number of pages written to journal */
#else
    static tcl.lang.Var.SQLITE3_GETSET sqlite3_pager_readdb_count = new tcl.lang.Var.SQLITE3_GETSET( "sqlite3_pager_readdb_count" );
    static tcl.lang.Var.SQLITE3_GETSET sqlite3_pager_writedb_count = new tcl.lang.Var.SQLITE3_GETSET( "sqlite3_pager_writedb_count" );
    static tcl.lang.Var.SQLITE3_GETSET sqlite3_pager_writej_count = new tcl.lang.Var.SQLITE3_GETSET( "sqlite3_pager_writej_count" );
#endif
    static void PAGER_INCR( ref int v )
    {
      v++;
    }
#else
//# define PAGER_INCR(v)
static void PAGER_INCR(ref int v) {}
#endif

    /*
** Journal files begin with the following magic string.  The data
** was obtained from /dev/random.  It is used only as a sanity check.
**
** Since version 2.8.0, the journal format contains additional sanity
** checking information.  If the power fails while the journal is being
** written, semi-random garbage data might appear in the journal
** file after power is restored.  If an attempt is then made
** to roll the journal back, the database could be corrupted.  The additional
** sanity checking data is an attempt to discover the garbage in the
** journal and ignore it.
**
** The sanity checking information for the new journal format consists
** of a 32-bit checksum on each page of data.  The checksum covers both
** the page number and the pPager.pageSize bytes of data for the page.
** This cksum is initialized to a 32-bit random value that appears in the
** journal file right after the header.  The random initializer is important,
** because garbage data that appears at the end of a journal is likely
** data that was once in other files that have now been deleted.  If the
** garbage data came from an obsolete journal file, the checksums might
** be correct.  But by initializing the checksum to random value which
** is different for every journal, we minimize that risk.
*/
    static byte[] aJournalMagic = new byte[] {
0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, 0xd7,
};
    /*
    ** The size of the of each page record in the journal is given by
    ** the following macro.
    */
    //#define JOURNAL_PG_SZ(pPager)  ((pPager.pageSize) + 8)
    static int JOURNAL_PG_SZ( Pager pPager )
    {
      return ( pPager.pageSize + 8 );
    }

    /*
    ** The journal header size for this pager. This is usually the same
    ** size as a single disk sector. See also setSectorSize().
    */
    //#define JOURNAL_HDR_SZ(pPager) (pPager.sectorSize)
    static u32 JOURNAL_HDR_SZ( Pager pPager )
    {
      return ( pPager.sectorSize );
    }

    /*
    ** The macro MEMDB is true if we are dealing with an in-memory database.
    ** We do this as a macro so that if the SQLITE_OMIT_MEMORYDB macro is set,
    ** the value of MEMDB will be a constant and the compiler will optimize
    ** out code that would never execute.
    */
#if SQLITE_OMIT_MEMORYDB
//# define MEMDB 0
const int MEMDB = 0;
#else
    //# define MEMDB pPager.memDb
#endif

    /*
** The maximum legal page number is (2^31 - 1).
*/
    //#define PAGER_MAX_PGNO 2147483647
    const int PAGER_MAX_PGNO = 2147483647;

    /*
    ** The argument to this macro is a file descriptor (type sqlite3_file*).
    ** Return 0 if it is not open, or non-zero (but not 1) if it is.
    **
    ** This is so that expressions can be written as:
    **
    **   if( isOpen(pPager.jfd) ){ ...
    **
    ** instead of
    **
    **   if( pPager.jfd->pMethods ){ ...
    */
    //#define isOpen(pFd) ((pFd)->pMethods)
    static bool isOpen( sqlite3_file pFd )
    {
      return pFd.pMethods != null;
    }

    /*
    ** Return true if this pager uses a write-ahead log instead of the usual
    ** rollback journal. Otherwise false.
    */
#if !SQLITE_OMIT_WAL
static int pagerUseWal(Pager *pPager){
return (pPager->pWal!=0);
}
#else
    //# define pagerUseWal(x) 0
    static bool pagerUseWal( Pager x )
    {
      return false;
    }
    //# define pagerRollbackWal(x) 0
    static int pagerRollbackWal( Pager x )
    {
      return 0;
    }
    //# define pagerWalFrames(v,w,x,y,z) 0
    static int pagerWalFrames( Pager v, PgHdr w, Pgno x, int y, int z )
    {
      return 0;
    }
    //# define pagerOpenWalIfPresent(z) SQLITE_OK
    static int pagerOpenWalIfPresent( Pager z )
    {
      return SQLITE_OK;
    }
    //# define pagerBeginReadTransaction(z) SQLITE_OK
    static int pagerBeginReadTransaction( Pager z )
    {
      return SQLITE_OK;
    }
#endif

#if NDEBUG
    /*
** Usage:
**
**   Debug.Assert( assert_pager_state(pPager) );
**
** This function runs many Debug.Asserts to try to find inconsistencies in
** the internal state of the Pager object.
*/
    static bool assert_pager_state( Pager p )
    {
      Pager pPager = p;

      /* State must be valid. */
      Debug.Assert( p.eState == PAGER_OPEN
      || p.eState == PAGER_READER
      || p.eState == PAGER_WRITER_LOCKED
      || p.eState == PAGER_WRITER_CACHEMOD
      || p.eState == PAGER_WRITER_DBMOD
      || p.eState == PAGER_WRITER_FINISHED
      || p.eState == PAGER_ERROR
      );

      /* Regardless of the current state, a temp-file connection always behaves
      ** as if it has an exclusive lock on the database file. It never updates
      ** the change-counter field, so the changeCountDone flag is always set.
      */
      Debug.Assert( p.tempFile == false || p.eLock == EXCLUSIVE_LOCK );
      Debug.Assert( p.tempFile == false || pPager.changeCountDone );

      /* If the useJournal flag is clear, the journal-mode must be "OFF". 
      ** And if the journal-mode is "OFF", the journal file must not be open.
      */
      Debug.Assert( p.journalMode == PAGER_JOURNALMODE_OFF || p.useJournal != 0 );
      Debug.Assert( p.journalMode != PAGER_JOURNALMODE_OFF || !isOpen( p.jfd ) );

      /* Check that MEMDB implies noSync. And an in-memory journal. Since 
      ** this means an in-memory pager performs no IO at all, it cannot encounter 
      ** either SQLITE_IOERR or SQLITE_FULL during rollback or while finalizing 
      ** a journal file. (although the in-memory journal implementation may 
      ** return SQLITE_IOERR_NOMEM while the journal file is being written). It 
      ** is therefore not possible for an in-memory pager to enter the ERROR 
      ** state.
      */
      if (
#if SQLITE_OMIT_MEMORYDB
0!=MEMDB
#else
 0 != pPager.memDb
#endif
 )
      {
        Debug.Assert( p.noSync );
        Debug.Assert( p.journalMode == PAGER_JOURNALMODE_OFF
        || p.journalMode == PAGER_JOURNALMODE_MEMORY
        );
        Debug.Assert( p.eState != PAGER_ERROR && p.eState != PAGER_OPEN );
        Debug.Assert( pagerUseWal( p ) == false );
      }

      /* If changeCountDone is set, a RESERVED lock or greater must be held
      ** on the file.
      */
      Debug.Assert( pPager.changeCountDone == false || pPager.eLock >= RESERVED_LOCK );
      Debug.Assert( p.eLock != PENDING_LOCK );

      switch ( p.eState )
      {
        case PAGER_OPEN:
          Debug.Assert(
#if SQLITE_OMIT_MEMORYDB
0==MEMDB
#else
 0 == pPager.memDb
#endif
 );
          Debug.Assert( pPager.errCode == SQLITE_OK );
          Debug.Assert( sqlite3PcacheRefCount( pPager.pPCache ) == 0 || pPager.tempFile );
          break;

        case PAGER_READER:
          Debug.Assert( pPager.errCode == SQLITE_OK );
          Debug.Assert( p.eLock != UNKNOWN_LOCK );
          Debug.Assert( p.eLock >= SHARED_LOCK || p.noReadlock != 0 );
          break;

        case PAGER_WRITER_LOCKED:
          Debug.Assert( p.eLock != UNKNOWN_LOCK );
          Debug.Assert( pPager.errCode == SQLITE_OK );
          if ( !pagerUseWal( pPager ) )
          {
            Debug.Assert( p.eLock >= RESERVED_LOCK );
          }
          Debug.Assert( pPager.dbSize == pPager.dbOrigSize );
          Debug.Assert( pPager.dbOrigSize == pPager.dbFileSize );
          Debug.Assert( pPager.dbOrigSize == pPager.dbHintSize );
          Debug.Assert( pPager.setMaster == 0 );
          break;

        case PAGER_WRITER_CACHEMOD:
          Debug.Assert( p.eLock != UNKNOWN_LOCK );
          Debug.Assert( pPager.errCode == SQLITE_OK );
          if ( !pagerUseWal( pPager ) )
          {
            /* It is possible that if journal_mode=wal here that neither the
            ** journal file nor the WAL file are open. This happens during
            ** a rollback transaction that switches from journal_mode=off
            ** to journal_mode=wal.
            */
            Debug.Assert( p.eLock >= RESERVED_LOCK );
            Debug.Assert( isOpen( p.jfd )
            || p.journalMode == PAGER_JOURNALMODE_OFF
            || p.journalMode == PAGER_JOURNALMODE_WAL
            );
          }
          Debug.Assert( pPager.dbOrigSize == pPager.dbFileSize );
          Debug.Assert( pPager.dbOrigSize == pPager.dbHintSize );
          break;

        case PAGER_WRITER_DBMOD:
          Debug.Assert( p.eLock == EXCLUSIVE_LOCK );
          Debug.Assert( pPager.errCode == SQLITE_OK );
          Debug.Assert( !pagerUseWal( pPager ) );
          Debug.Assert( p.eLock >= EXCLUSIVE_LOCK );
          Debug.Assert( isOpen( p.jfd )
          || p.journalMode == PAGER_JOURNALMODE_OFF
          || p.journalMode == PAGER_JOURNALMODE_WAL
          );
          Debug.Assert( pPager.dbOrigSize <= pPager.dbHintSize );
          break;

        case PAGER_WRITER_FINISHED:
          Debug.Assert( p.eLock == EXCLUSIVE_LOCK );
          Debug.Assert( pPager.errCode == SQLITE_OK );
          Debug.Assert( !pagerUseWal( pPager ) );
          Debug.Assert( isOpen( p.jfd )
          || p.journalMode == PAGER_JOURNALMODE_OFF
          || p.journalMode == PAGER_JOURNALMODE_WAL
          );
          break;

        case PAGER_ERROR:
          /* There must be at least one outstanding reference to the pager if
          ** in ERROR state. Otherwise the pager should have already dropped
          ** back to OPEN state.
          */
          Debug.Assert( pPager.errCode != SQLITE_OK );
          Debug.Assert( sqlite3PcacheRefCount( pPager.pPCache ) > 0 );
          break;
      }

      return true;
    }
#else
    static bool assert_pager_state( Pager pPager )
    {
      return true;
    }
#endif //* ifndef NDEBUG */

#if SQLITE_DEBUG
    /*
** Return a pointer to a human readable string in a static buffer
** containing the state of the Pager object passed as an argument. This
** is intended to be used within debuggers. For example, as an alternative
** to "print *pPager" in gdb:
**
** (gdb) printf "%s", print_pager_state(pPager)
*/
    static string print_pager_state( Pager p )
    {
      StringBuilder zRet = new StringBuilder( 1024 );

      sqlite3_snprintf( 1024, zRet,
      "Filename:      %s\n" +
      "State:         %s errCode=%d\n" +
      "Lock:          %s\n" +
      "Locking mode:  locking_mode=%s\n" +
      "Journal mode:  journal_mode=%s\n" +
      "Backing store: tempFile=%d memDb=%d useJournal=%d\n" +
      "Journal:       journalOff=%lld journalHdr=%lld\n" +
      "Size:          dbsize=%d dbOrigSize=%d dbFileSize=%d\n"
      , p.zFilename
      , p.eState == PAGER_OPEN ? "OPEN" :
      p.eState == PAGER_READER ? "READER" :
      p.eState == PAGER_WRITER_LOCKED ? "WRITER_LOCKED" :
      p.eState == PAGER_WRITER_CACHEMOD ? "WRITER_CACHEMOD" :
      p.eState == PAGER_WRITER_DBMOD ? "WRITER_DBMOD" :
      p.eState == PAGER_WRITER_FINISHED ? "WRITER_FINISHED" :
      p.eState == PAGER_ERROR ? "ERROR" : "?error?"
      , (int)p.errCode
      , p.eLock == NO_LOCK ? "NO_LOCK" :
      p.eLock == RESERVED_LOCK ? "RESERVED" :
      p.eLock == EXCLUSIVE_LOCK ? "EXCLUSIVE" :
      p.eLock == SHARED_LOCK ? "SHARED" :
      p.eLock == UNKNOWN_LOCK ? "UNKNOWN" : "?error?"
      , p.exclusiveMode ? "exclusive" : "normal"
      , p.journalMode == PAGER_JOURNALMODE_MEMORY ? "memory" :
      p.journalMode == PAGER_JOURNALMODE_OFF ? "off" :
      p.journalMode == PAGER_JOURNALMODE_DELETE ? "delete" :
      p.journalMode == PAGER_JOURNALMODE_PERSIST ? "persist" :
      p.journalMode == PAGER_JOURNALMODE_TRUNCATE ? "truncate" :
      p.journalMode == PAGER_JOURNALMODE_WAL ? "wal" : "?error?"
      , p.tempFile ? 1 : 0, (int)p.memDb, (int)p.useJournal
      , p.journalOff, p.journalHdr
      , (int)p.dbSize, (int)p.dbOrigSize, (int)p.dbFileSize
      );

      return zRet.ToString();
    }
#endif

    /*
** Return true if it is necessary to write page *pPg into the sub-journal.
** A page needs to be written into the sub-journal if there exists one
** or more open savepoints for which:
**
**   * The page-number is less than or equal to PagerSavepoint.nOrig, and
**   * The bit corresponding to the page-number is not set in
**     PagerSavepoint.pInSavepoint.
*/
    static bool subjRequiresPage( PgHdr pPg )
    {
      u32 pgno = pPg.pgno;
      Pager pPager = pPg.pPager;
      int i;
      for ( i = 0; i < pPager.nSavepoint; i++ )
      {
        PagerSavepoint p = pPager.aSavepoint[i];
        if ( p.nOrig >= pgno && 0 == sqlite3BitvecTest( p.pInSavepoint, pgno ) )
        {
          return true;
        }
      }
      return false;
    }

    /*
    ** Return true if the page is already in the journal file.
    */
    static bool pageInJournal( PgHdr pPg )
    {
      return sqlite3BitvecTest( pPg.pPager.pInJournal, pPg.pgno ) != 0;
    }

    /*
    ** Read a 32-bit integer from the given file descriptor.  Store the integer
    ** that is read in pRes.  Return SQLITE_OK if everything worked, or an
    ** error code is something goes wrong.
    **
    ** All values are stored on disk as big-endian.
    */
    static int read32bits( sqlite3_file fd, int offset, ref int pRes )
    {
      u32 u32_pRes = 0;
      int rc = read32bits( fd, offset, ref u32_pRes );
      pRes = (int)u32_pRes;
      return rc;
    }
    static int read32bits( sqlite3_file fd, i64 offset, ref u32 pRes )
    {
      int rc = read32bits( fd, (int)offset, ref pRes );
      return rc;
    }
    static int read32bits( sqlite3_file fd, int offset, ref u32 pRes )
    {
      byte[] ac = new byte[4];
      int rc = sqlite3OsRead( fd, ac, ac.Length, offset );
      if ( rc == SQLITE_OK )
      {
        pRes = sqlite3Get4byte( ac );
      }
      else
        pRes = 0;
      return rc;
    }

    /*
    ** Write a 32-bit integer into a string buffer in big-endian byte order.
    */
    //#define put32bits(A,B)  sqlite3sqlite3Put4byte((u8*)A,B)
    static void put32bits( string ac, int offset, int val )
    {
      byte[] A = new byte[4];
      A[0] = (byte)ac[offset + 0];
      A[1] = (byte)ac[offset + 1];
      A[2] = (byte)ac[offset + 2];
      A[3] = (byte)ac[offset + 3];
      sqlite3Put4byte( A, 0, val );
    }
    static void put32bits( byte[] ac, int offset, int val )
    {
      sqlite3Put4byte( ac, offset, (u32)val );
    }
    static void put32bits( byte[] ac, u32 val )
    {
      sqlite3Put4byte( ac, 0U, val );
    }
    static void put32bits( byte[] ac, int offset, u32 val )
    {
      sqlite3Put4byte( ac, offset, val );
    }

    /*
    ** Write a 32-bit integer into the given file descriptor.  Return SQLITE_OK
    ** on success or an error code is something goes wrong.
    */
    static int write32bits( sqlite3_file fd, i64 offset, u32 val )
    {
      byte[] ac = new byte[4];
      put32bits( ac, val );
      return sqlite3OsWrite( fd, ac, 4, offset );
    }

    /*
    ** Unlock the database file to level eLock, which must be either NO_LOCK
    ** or SHARED_LOCK. Regardless of whether or not the call to xUnlock()
    ** succeeds, set the Pager.eLock variable to match the (attempted) new lock.
    **
    ** Except, if Pager.eLock is set to UNKNOWN_LOCK when this function is
    ** called, do not modify it. See the comment above the #define of 
    ** UNKNOWN_LOCK for an explanation of this.
    */
    static int pagerUnlockDb( Pager pPager, int eLock )
    {
      int rc = SQLITE_OK;

      Debug.Assert( !pPager.exclusiveMode || pPager.eLock == eLock );
      Debug.Assert( eLock == NO_LOCK || eLock == SHARED_LOCK );
      Debug.Assert( eLock != NO_LOCK || pagerUseWal( pPager ) == false );
      if ( isOpen( pPager.fd ) )
      {
        Debug.Assert( pPager.eLock >= eLock );
        rc = sqlite3OsUnlock( pPager.fd, eLock );
        if ( pPager.eLock != UNKNOWN_LOCK )
        {
          pPager.eLock = (u8)eLock;
        }
        IOTRACE( "UNLOCK %p %d\n", pPager, eLock );
      }
      return rc;
    }

    /*
    ** Lock the database file to level eLock, which must be either SHARED_LOCK,
    ** RESERVED_LOCK or EXCLUSIVE_LOCK. If the caller is successful, set the
    ** Pager.eLock variable to the new locking state. 
    **
    ** Except, if Pager.eLock is set to UNKNOWN_LOCK when this function is 
    ** called, do not modify it unless the new locking state is EXCLUSIVE_LOCK. 
    ** See the comment above the #define of UNKNOWN_LOCK for an explanation 
    ** of this.
    */
    static int pagerLockDb( Pager pPager, int eLock )
    {
      int rc = SQLITE_OK;

      Debug.Assert( eLock == SHARED_LOCK || eLock == RESERVED_LOCK || eLock == EXCLUSIVE_LOCK );
      if ( pPager.eLock < eLock || pPager.eLock == UNKNOWN_LOCK )
      {
        rc = sqlite3OsLock( pPager.fd, eLock );
        if ( rc == SQLITE_OK && ( pPager.eLock != UNKNOWN_LOCK || eLock == EXCLUSIVE_LOCK ) )
        {
          pPager.eLock = (u8)eLock;
          IOTRACE( "LOCK %p %d\n", pPager, eLock );
        }
      }
      return rc;
    }

    /*
    ** This function determines whether or not the atomic-write optimization
    ** can be used with this pager. The optimization can be used if:
    **
    **  (a) the value returned by OsDeviceCharacteristics() indicates that
    **      a database page may be written atomically, and
    **  (b) the value returned by OsSectorSize() is less than or equal
    **      to the page size.
    **
    ** The optimization is also always enabled for temporary files. It is
    ** an error to call this function if pPager is opened on an in-memory
    ** database.
    **
    ** If the optimization cannot be used, 0 is returned. If it can be used,
    ** then the value returned is the size of the journal file when it
    ** contains rollback data for exactly one page.
    */
#if SQLITE_ENABLE_ATOMIC_WRITE
static int jrnlBufferSize(Pager *pPager){
Debug.Assert( 0==MEMDB );
if( !pPager.tempFile ){
int dc;                           /* Device characteristics */
int nSector;                      /* Sector size */
int szPage;                       /* Page size */

Debug.Assert( isOpen(pPager.fd) );
dc = sqlite3OsDeviceCharacteristics(pPager.fd);
nSector = pPager.sectorSize;
szPage = pPager.pageSize;

Debug.Assert(SQLITE_IOCAP_ATOMIC512==(512>>8));
Debug.Assert(SQLITE_IOCAP_ATOMIC64K==(65536>>8));
if( 0==(dc&(SQLITE_IOCAP_ATOMIC|(szPage>>8)) || nSector>szPage) ){
return 0;
}
}

return JOURNAL_HDR_SZ(pPager) + JOURNAL_PG_SZ(pPager);
}
#endif

    /*
** If SQLITE_CHECK_PAGES is defined then we do some sanity checking
** on the cache using a hash function.  This is used for testing
** and debugging only.
*/
#if SQLITE_CHECK_PAGES
/*
** Return a 32-bit hash of the page data for pPage.
*/
static u32 pager_datahash(int nByte, unsigned char pData){
u32 hash = 0;
int i;
for(i=0; i<nByte; i++){
hash = (hash*1039) + pData[i];
}
return hash;
}
static void pager_pagehash(PgHdr pPage){
return pager_datahash(pPage.pPager.pageSize, (unsigned char *)pPage.pData);
}
static u32 pager_set_pagehash(PgHdr pPage){
pPage.pageHash = pager_pagehash(pPage);
}

/*
** The CHECK_PAGE macro takes a PgHdr* as an argument. If SQLITE_CHECK_PAGES
** is defined, and NDEBUG is not defined, an Debug.Assert() statement checks
** that the page is either dirty or still matches the calculated page-hash.
*/
//#define CHECK_PAGE(x) checkPage(x)
static void checkPage(PgHdr pPg){
Pager pPager = pPg.pPager;
assert( pPager->eState!=PAGER_ERROR );
assert( (pPg->flags&PGHDR_DIRTY) || pPg->pageHash==pager_pagehash(pPg) );
}

#else
    //#define pager_datahash(X,Y)  0
    static int pager_datahash( int X, byte[] Y )
    {
      return 0;
    }

    //#define pager_pagehash(X)  0
    static int pager_pagehash( PgHdr X )
    {
      return 0;
    }

    //#define pager_set_pagehash(X)
    static void pager_set_pagehash( PgHdr X )
    {
    }

    //#define CHECK_PAGE(x)
#endif //* SQLITE_CHECK_PAGES */


    /*
** When this is called the journal file for pager pPager must be open.
** This function attempts to read a master journal file name from the
** end of the file and, if successful, copies it into memory supplied
** by the caller. See comments above writeMasterJournal() for the format
** used to store a master journal file name at the end of a journal file.
**
** zMaster must point to a buffer of at least nMaster bytes allocated by
** the caller. This should be sqlite3_vfs.mxPathname+1 (to ensure there is
** enough space to write the master journal name). If the master journal
** name in the journal is longer than nMaster bytes (including a
** nul-terminator), then this is handled as if no master journal name
** were present in the journal.
**
** If a master journal file name is present at the end of the journal
** file, then it is copied into the buffer pointed to by zMaster. A
** nul-terminator byte is appended to the buffer following the master
** journal file name.
**
** If it is determined that no master journal file name is present
** zMaster[0] is set to 0 and SQLITE_OK returned.
**
** If an error occurs while reading from the journal file, an SQLite
** error code is returned.
*/
    static int readMasterJournal( sqlite3_file pJrnl, byte[] zMaster, u32 nMaster )
    {
      int rc;                       /* Return code */
      int len = 0;                  /* Length in bytes of master journal name */
      i64 szJ = 0;                 /* Total size in bytes of journal file pJrnl */
      u32 cksum = 0;                /* MJ checksum value read from journal */
      int u;                        /* Unsigned loop counter */
      byte[] aMagic = new byte[8];  /* A buffer to hold the magic header */

      zMaster[0] = 0;

      if ( SQLITE_OK != ( rc = sqlite3OsFileSize( pJrnl, ref szJ ) )
      || szJ < 16
      || SQLITE_OK != ( rc = read32bits( pJrnl, (int)( szJ - 16 ), ref len ) )
      || len >= nMaster
      || SQLITE_OK != ( rc = read32bits( pJrnl, szJ - 12, ref cksum ) )
      || SQLITE_OK != ( rc = sqlite3OsRead( pJrnl, aMagic, 8, szJ - 8 ) )
      || memcmp( aMagic, aJournalMagic, 8 ) != 0
      || SQLITE_OK != ( rc = sqlite3OsRead( pJrnl, zMaster, len, (long)( szJ - 16 - len ) ) )
      )
      {
        return rc;
      }

      /* See if the checksum matches the master journal name */
      for ( u = 0; u < len; u++ )
      {
        cksum -= zMaster[u];
      }
      if ( cksum != 0 )
      {
        /* If the checksum doesn't add up, then one or more of the disk sectors
        ** containing the master journal filename is corrupted. This means
        ** definitely roll back, so just return SQLITE_OK and report a (nul)
        ** master-journal filename.
        */
        len = 0;
      }
      if ( len == 0 )
        zMaster[0] = 0;

      return SQLITE_OK;
    }

    /*
    ** Return the offset of the sector boundary at or immediately
    ** following the value in pPager.journalOff, assuming a sector
    ** size of pPager.sectorSize bytes.
    **
    ** i.e for a sector size of 512:
    **
    **   Pager.journalOff          Return value
    **   ---------------------------------------
    **   0                         0
    **   512                       512
    **   100                       512
    **   2000                      2048
    **
    */
    static i64 journalHdrOffset( Pager pPager )
    {
      i64 offset = 0;
      i64 c = pPager.journalOff;
      if ( c != 0 )
      {
        offset = (int)( ( ( c - 1 ) / pPager.sectorSize + 1 ) * pPager.sectorSize );//offset = ((c-1)/JOURNAL_HDR_SZ(pPager) + 1) * JOURNAL_HDR_SZ(pPager);
      }
      Debug.Assert( offset % pPager.sectorSize == 0 ); //Debug.Assert(offset % JOURNAL_HDR_SZ(pPager) == 0);
      Debug.Assert( offset >= c );
      Debug.Assert( ( offset - c ) < pPager.sectorSize );//Debug.Assert( (offset-c)<JOURNAL_HDR_SZ(pPager) );
      return offset;
    }
    static void seekJournalHdr( Pager pPager )
    {
      pPager.journalOff = journalHdrOffset( pPager );
    }

    /*
    ** The journal file must be open when this function is called.
    **
    ** This function is a no-op if the journal file has not been written to
    ** within the current transaction (i.e. if Pager.journalOff==0).
    **
    ** If doTruncate is non-zero or the Pager.journalSizeLimit variable is
    ** set to 0, then truncate the journal file to zero bytes in size. Otherwise,
    ** zero the 28-byte header at the start of the journal file. In either case,
    ** if the pager is not in no-sync mode, sync the journal file immediately
    ** after writing or truncating it.
    **
    ** If Pager.journalSizeLimit is set to a positive, non-zero value, and
    ** following the truncation or zeroing described above the size of the
    ** journal file in bytes is larger than this value, then truncate the
    ** journal file to Pager.journalSizeLimit bytes. The journal file does
    ** not need to be synced following this operation.
    **
    ** If an IO error occurs, abandon processing and return the IO error code.
    ** Otherwise, return SQLITE_OK.
    */
    static int zeroJournalHdr( Pager pPager, int doTruncate )
    {
      int rc = SQLITE_OK;                               /* Return code */
      Debug.Assert( isOpen( pPager.jfd ) );

      if ( pPager.journalOff != 0 )
      {
        i64 iLimit = pPager.journalSizeLimit;           /* Local cache of jsl */
        IOTRACE( "JZEROHDR %p\n", pPager );
        if ( doTruncate != 0 || iLimit == 0 )
        {
          rc = sqlite3OsTruncate( pPager.jfd, 0 );
        }
        else
        {
          byte[] zeroHdr = new byte[28];// = {0};
          rc = sqlite3OsWrite( pPager.jfd, zeroHdr, zeroHdr.Length, 0 );
        }
        if ( rc == SQLITE_OK && !pPager.noSync )
        {
          rc = sqlite3OsSync( pPager.jfd, SQLITE_SYNC_DATAONLY | pPager.syncFlags );
        }

        /* At this point the transaction is committed but the write lock
        ** is still held on the file. If there is a size limit configured for
        ** the persistent journal and the journal file currently consumes more
        ** space than that limit allows for, truncate it now. There is no need
        ** to sync the file following this operation.
        */
        if ( rc == SQLITE_OK && iLimit > 0 )
        {
          i64 sz = 0;
          rc = sqlite3OsFileSize( pPager.jfd, ref sz );
          if ( rc == SQLITE_OK && sz > iLimit )
          {
            rc = sqlite3OsTruncate( pPager.jfd, iLimit );
          }
        }
      }
      return rc;
    }

    /*
    ** The journal file must be open when this routine is called. A journal
    ** header (JOURNAL_HDR_SZ bytes) is written into the journal file at the
    ** current location.
    **
    ** The format for the journal header is as follows:
    ** - 8 bytes: Magic identifying journal format.
    ** - 4 bytes: Number of records in journal, or -1 no-sync mode is on.
    ** - 4 bytes: Random number used for page hash.
    ** - 4 bytes: Initial database page count.
    ** - 4 bytes: Sector size used by the process that wrote this journal.
    ** - 4 bytes: Database page size.
    **
    ** Followed by (JOURNAL_HDR_SZ - 28) bytes of unused space.
    */
    static int writeJournalHdr( Pager pPager )
    {

      int rc = SQLITE_OK;                 /* Return code */
      byte[] zHeader = pPager.pTmpSpace;  /* Temporary space used to build header */
      u32 nHeader = (u32)pPager.pageSize; /* Size of buffer pointed to by zHeader */
      u32 nWrite;                         /* Bytes of header sector written */
      int ii;                             /* Loop counter */

      Debug.Assert( isOpen( pPager.jfd ) );      /* Journal file must be open. */

      if ( nHeader > JOURNAL_HDR_SZ( pPager ) )
      {
        nHeader = JOURNAL_HDR_SZ( pPager );
      }
      /* If there are active savepoints and any of them were created
      ** since the most recent journal header was written, update the
      ** PagerSavepoint.iHdrOffset fields now.
      */
      for ( ii = 0; ii < pPager.nSavepoint; ii++ )
      {
        if ( pPager.aSavepoint[ii].iHdrOffset == 0 )
        {
          pPager.aSavepoint[ii].iHdrOffset = pPager.journalOff;
        }
      }
      pPager.journalHdr = pPager.journalOff = journalHdrOffset( pPager );

      /*
      ** Write the nRec Field - the number of page records that follow this
      ** journal header. Normally, zero is written to this value at this time.
      ** After the records are added to the journal (and the journal synced,
      ** if in full-sync mode), the zero is overwritten with the true number
      ** of records (see syncJournal()).
      **
      ** A faster alternative is to write 0xFFFFFFFF to the nRec field. When
      ** reading the journal this value tells SQLite to assume that the
      ** rest of the journal file contains valid page records. This assumption
      ** is dangerous, as if a failure occurred whilst writing to the journal
      ** file it may contain some garbage data. There are two scenarios
      ** where this risk can be ignored:
      **
      **   * When the pager is in no-sync mode. Corruption can follow a
      **     power failure in this case anyway.
      **
      **   * When the SQLITE_IOCAP_SAFE_APPEND flag is set. This guarantees
      **     that garbage data is never appended to the journal file.
      */
      Debug.Assert( isOpen( pPager.fd ) || pPager.noSync );
      if ( pPager.noSync || ( pPager.journalMode == PAGER_JOURNALMODE_MEMORY )
      || ( sqlite3OsDeviceCharacteristics( pPager.fd ) & SQLITE_IOCAP_SAFE_APPEND ) != 0
      )
      {
        aJournalMagic.CopyTo( zHeader, 0 );// memcpy(zHeader, aJournalMagic, sizeof(aJournalMagic));
        put32bits( zHeader, aJournalMagic.Length, 0xffffffff );
      }
      else
      {
        Array.Clear( zHeader, 0, aJournalMagic.Length + 4 );//memset(zHeader, 0, sizeof(aJournalMagic)+4);
      }

      /* The random check-hash initialiser */
      i64 i64Temp = 0;
      sqlite3_randomness( sizeof( i64 ), ref i64Temp );
      pPager.cksumInit = (u32)i64Temp;
      put32bits( zHeader, aJournalMagic.Length + 4, pPager.cksumInit );
      /* The initial database size */
      put32bits( zHeader, aJournalMagic.Length + 8, pPager.dbOrigSize );
      /* The assumed sector size for this process */
      put32bits( zHeader, aJournalMagic.Length + 12, pPager.sectorSize );
      /* The page size */
      put32bits( zHeader, aJournalMagic.Length + 16, (u32)pPager.pageSize );

      /* Initializing the tail of the buffer is not necessary.  Everything
      ** works find if the following memset() is omitted.  But initializing
      ** the memory prevents valgrind from complaining, so we are willing to
      ** take the performance hit.
      */
      //  memset(&zHeader[sizeof(aJournalMagic)+20], 0,
      //  nHeader-(sizeof(aJournalMagic)+20));
      Array.Clear( zHeader, aJournalMagic.Length + 20, (int)nHeader - ( aJournalMagic.Length + 20 ) );

      /* In theory, it is only necessary to write the 28 bytes that the
      ** journal header consumes to the journal file here. Then increment the
      ** Pager.journalOff variable by JOURNAL_HDR_SZ so that the next
      ** record is written to the following sector (leaving a gap in the file
      ** that will be implicitly filled in by the OS).
      **
      ** However it has been discovered that on some systems this pattern can
      ** be significantly slower than contiguously writing data to the file,
      ** even if that means explicitly writing data to the block of
      ** (JOURNAL_HDR_SZ - 28) bytes that will not be used. So that is what
      ** is done.
      **
      ** The loop is required here in case the sector-size is larger than the
      ** database page size. Since the zHeader buffer is only Pager.pageSize
      ** bytes in size, more than one call to sqlite3OsWrite() may be required
      ** to populate the entire journal header sector.
      */
      for ( nWrite = 0; rc == SQLITE_OK && nWrite < JOURNAL_HDR_SZ( pPager ); nWrite += nHeader )
      {
        IOTRACE( "JHDR %p %lld %d\n", pPager, pPager.journalHdr, nHeader );
        rc = sqlite3OsWrite( pPager.jfd, zHeader, (int)nHeader, pPager.journalOff );
        Debug.Assert( pPager.journalHdr <= pPager.journalOff );
        pPager.journalOff += (int)nHeader;
      }
      return rc;
    }

    /*
    ** The journal file must be open when this is called. A journal header file
    ** (JOURNAL_HDR_SZ bytes) is read from the current location in the journal
    ** file. The current location in the journal file is given by
    ** pPager.journalOff. See comments above function writeJournalHdr() for
    ** a description of the journal header format.
    **
    ** If the header is read successfully, *pNRec is set to the number of
    ** page records following this header and *pDbSize is set to the size of the
    ** database before the transaction began, in pages. Also, pPager.cksumInit
    ** is set to the value read from the journal header. SQLITE_OK is returned
    ** in this case.
    **
    ** If the journal header file appears to be corrupted, SQLITE_DONE is
    ** returned and *pNRec and *PDbSize are undefined.  If JOURNAL_HDR_SZ bytes
    ** cannot be read from the journal file an error code is returned.
    */
    static int readJournalHdr(
    Pager pPager,               /* Pager object */
    int isHot,
    i64 journalSize,            /* Size of the open journal file in bytes */
    out u32 pNRec,              /* OUT: Value read from the nRec field */
    out u32 pDbSize             /* OUT: Value of original database size field */
    )
    {
      int rc;                      /* Return code */
      byte[] aMagic = new byte[8]; /* A buffer to hold the magic header */
      i64 iHdrOff;                 /* Offset of journal header being read */

      Debug.Assert( isOpen( pPager.jfd ) );      /* Journal file must be open. */

      pNRec = 0;
      pDbSize = 0;

      /* Advance Pager.journalOff to the start of the next sector. If the
      ** journal file is too small for there to be a header stored at this
      ** point, return SQLITE_DONE.
      */
      pPager.journalOff = journalHdrOffset( pPager );
      if ( pPager.journalOff + JOURNAL_HDR_SZ( pPager ) > journalSize )
      {
        return SQLITE_DONE;
      }
      iHdrOff = pPager.journalOff;

      /* Read in the first 8 bytes of the journal header. If they do not match
      ** the  magic string found at the start of each journal header, return
      ** SQLITE_DONE. If an IO error occurs, return an error code. Otherwise,
      ** proceed.
      */
      if ( isHot != 0 || iHdrOff != pPager.journalHdr )
      {
        rc = sqlite3OsRead( pPager.jfd, aMagic, aMagic.Length, iHdrOff );
        if ( rc != 0 )
        {
          return rc;
        }
        if ( memcmp( aMagic, aJournalMagic, aMagic.Length ) != 0 )
        {
          return SQLITE_DONE;
        }
      }
      /* Read the first three 32-bit fields of the journal header: The nRec
      ** field, the checksum-initializer and the database size at the start
      ** of the transaction. Return an error code if anything goes wrong.
      */
      if ( SQLITE_OK != ( rc = read32bits( pPager.jfd, iHdrOff + 8, ref pNRec ) )
      || SQLITE_OK != ( rc = read32bits( pPager.jfd, iHdrOff + 12, ref pPager.cksumInit ) )
      || SQLITE_OK != ( rc = read32bits( pPager.jfd, iHdrOff + 16, ref pDbSize ) )
      )
      {
        return rc;
      }

      if ( pPager.journalOff == 0 )
      {
        u32 iPageSize = 0;           /* Page-size field of journal header */
        u32 iSectorSize = 0;         /* Sector-size field of journal header */

        /* Read the page-size and sector-size journal header fields. */
        if ( SQLITE_OK != ( rc = read32bits( pPager.jfd, iHdrOff + 20, ref iSectorSize ) )
        || SQLITE_OK != ( rc = read32bits( pPager.jfd, iHdrOff + 24, ref iPageSize ) )
        )
        {
          return rc;
        }

        /* Versions of SQLite prior to 3.5.8 set the page-size field of the
        ** journal header to zero. In this case, assume that the Pager.pageSize
        ** variable is already set to the correct page size.
        */
        if ( iPageSize == 0 )
        {
          iPageSize = (u32)pPager.pageSize;
        }

        /* Check that the values read from the page-size and sector-size fields
        ** are within range. To be 'in range', both values need to be a power
        ** of two greater than or equal to 512 or 32, and not greater than their 
        ** respective compile time maximum limits.
        */
        if ( iPageSize < 512 || iSectorSize < 32
        || iPageSize > SQLITE_MAX_PAGE_SIZE || iSectorSize > MAX_SECTOR_SIZE
        || ( ( iPageSize - 1 ) & iPageSize ) != 0 || ( ( iSectorSize - 1 ) & iSectorSize ) != 0
        )
        {
          /* If the either the page-size or sector-size in the journal-header is
          ** invalid, then the process that wrote the journal-header must have
          ** crashed before the header was synced. In this case stop reading
          ** the journal file here.
          */
          return SQLITE_DONE;
        }

        /* Update the page-size to match the value read from the journal.
        ** Use a testcase() macro to make sure that malloc failure within
        ** PagerSetPagesize() is tested.
        */
        rc = sqlite3PagerSetPagesize( pPager, ref iPageSize, -1 );
        testcase( rc != SQLITE_OK );

        /* Update the assumed sector-size to match the value used by
        ** the process that created this journal. If this journal was
        ** created by a process other than this one, then this routine
        ** is being called from within pager_playback(). The local value
        ** of Pager.sectorSize is restored at the end of that routine.
        */
        pPager.sectorSize = iSectorSize;
      }

      pPager.journalOff += (int)JOURNAL_HDR_SZ( pPager );
      return rc;
    }

    /*
    ** Write the supplied master journal name into the journal file for pager
    ** pPager at the current location. The master journal name must be the last
    ** thing written to a journal file. If the pager is in full-sync mode, the
    ** journal file descriptor is advanced to the next sector boundary before
    ** anything is written. The format is:
    **
    **   + 4 bytes: PAGER_MJ_PGNO.
    **   + N bytes: Master journal filename in utf-8.
    **   + 4 bytes: N (length of master journal name in bytes, no nul-terminator).
    **   + 4 bytes: Master journal name checksum.
    **   + 8 bytes: aJournalMagic[].
    **
    ** The master journal page checksum is the sum of the bytes in the master
    ** journal name, where each byte is interpreted as a signed 8-bit integer.
    **
    ** If zMaster is a NULL pointer (occurs for a single database transaction),
    ** this call is a no-op.
    */
    static int writeMasterJournal( Pager pPager, string zMaster )
    {
      int rc;                          /* Return code */
      int nMaster;                     /* Length of string zMaster */
      i64 iHdrOff;                     /* Offset of header in journal file */
      i64 jrnlSize = 0;                  /* Size of journal file on disk */
      u32 cksum = 0;                   /* Checksum of string zMaster */

      Debug.Assert( pPager.setMaster == 0 );
      Debug.Assert( !pagerUseWal( pPager ) );
      if ( null == zMaster
      || pPager.journalMode == PAGER_JOURNALMODE_MEMORY
      || pPager.journalMode == PAGER_JOURNALMODE_OFF
      )
      {
        return SQLITE_OK;
      }

      pPager.setMaster = 1;
      Debug.Assert( isOpen( pPager.jfd ) );
      Debug.Assert( pPager.journalHdr <= pPager.journalOff );

      /* Calculate the length in bytes and the checksum of zMaster */
      for ( nMaster = 0; nMaster < zMaster.Length && zMaster[nMaster] != 0; nMaster++ )
      {
        cksum += zMaster[nMaster];
      }

      /* If in full-sync mode, advance to the next disk sector before writing
      ** the master journal name. This is in case the previous page written to
      ** the journal has already been synced.
      */
      if ( pPager.fullSync )
      {
        pPager.journalOff = journalHdrOffset( pPager );
      }
      iHdrOff = pPager.journalOff;
      /* Write the master journal data to the end of the journal file. If
      ** an error occurs, return the error code to the caller.
      */
      if ( ( 0 != ( rc = write32bits( pPager.jfd, iHdrOff, (u32)PAGER_MJ_PGNO( pPager ) ) ) )
      || ( 0 != ( rc = sqlite3OsWrite( pPager.jfd, Encoding.UTF8.GetBytes( zMaster ), nMaster, iHdrOff + 4 ) ) )
      || ( 0 != ( rc = write32bits( pPager.jfd, iHdrOff + 4 + nMaster, (u32)nMaster ) ) )
      || ( 0 != ( rc = write32bits( pPager.jfd, iHdrOff + 4 + nMaster + 4, cksum ) ) )
      || ( 0 != ( rc = sqlite3OsWrite( pPager.jfd, aJournalMagic, 8, iHdrOff + 4 + nMaster + 8 ) ) )
      )
      {
        return rc;
      }
      pPager.journalOff += ( nMaster + 20 );

      /* If the pager is in peristent-journal mode, then the physical
      ** journal-file may extend past the end of the master-journal name
      ** and 8 bytes of magic data just written to the file. This is
      ** dangerous because the code to rollback a hot-journal file
      ** will not be able to find the master-journal name to determine
      ** whether or not the journal is hot.
      **
      ** Easiest thing to do in this scenario is to truncate the journal
      ** file to the required size.
      */
      if ( SQLITE_OK == ( rc = sqlite3OsFileSize( pPager.jfd, ref jrnlSize ) )
      && jrnlSize > pPager.journalOff
      )
      {
        rc = sqlite3OsTruncate( pPager.jfd, pPager.journalOff );
      }

      return rc;
    }

    /*
    ** Find a page in the hash table given its page number. Return
    ** a pointer to the page or NULL if the requested page is not
    ** already in memory.
    */
    static PgHdr pager_lookup( Pager pPager, u32 pgno )
    {
      PgHdr p = null;                         /* Return value */
      /* It is not possible for a call to PcacheFetch() with createFlag==0 to
      ** fail, since no attempt to allocate dynamic memory will be made.
      */
      sqlite3PcacheFetch( pPager.pPCache, pgno, 0, ref p );
      return p;
    }

    /*
    ** Discard the entire contents of the in-memory page-cache.
    */
    static void pager_reset( Pager pPager )
    {
      sqlite3BackupRestart( pPager.pBackup );
      sqlite3PcacheClear( pPager.pPCache );
    }

    /*
    ** Free all structures in the Pager.aSavepoint[] array and set both
    ** Pager.aSavepoint and Pager.nSavepoint to zero. Close the sub-journal
    ** if it is open and the pager is not in exclusive mode.
    */
    static void releaseAllSavepoints( Pager pPager )
    {
      int ii;               /* Iterator for looping through Pager.aSavepoint */
      for ( ii = 0; ii < pPager.nSavepoint; ii++ )
      {
        sqlite3BitvecDestroy( ref pPager.aSavepoint[ii].pInSavepoint );
      }
      if ( !pPager.exclusiveMode || sqlite3IsMemJournal( pPager.sjfd ) )
      {
        sqlite3OsClose( pPager.sjfd );
      }
      //sqlite3_free( ref pPager.aSavepoint );
      pPager.aSavepoint = null;
      pPager.nSavepoint = 0;
      pPager.nSubRec = 0;
    }

    /*
    ** Set the bit number pgno in the PagerSavepoint.pInSavepoint
    ** bitvecs of all open savepoints. Return SQLITE_OK if successful
    ** or SQLITE_NOMEM if a malloc failure occurs.
    */
    static int addToSavepointBitvecs( Pager pPager, u32 pgno )
    {
      int ii;                   /* Loop counter */
      int rc = SQLITE_OK;       /* Result code */

      for ( ii = 0; ii < pPager.nSavepoint; ii++ )
      {
        PagerSavepoint p = pPager.aSavepoint[ii];
        if ( pgno <= p.nOrig )
        {
          rc |= sqlite3BitvecSet( p.pInSavepoint, pgno );
          testcase( rc == SQLITE_NOMEM );
          Debug.Assert( rc == SQLITE_OK || rc == SQLITE_NOMEM );
        }
      }
      return rc;
    }

    /*
    ** This function is a no-op if the pager is in exclusive mode and not
    ** in the ERROR state. Otherwise, it switches the pager to PAGER_OPEN
    ** state.
    **
    ** If the pager is not in exclusive-access mode, the database file is
    ** completely unlocked. If the file is unlocked and the file-system does
    ** not exhibit the UNDELETABLE_WHEN_OPEN property, the journal file is
    ** closed (if it is open).
    **
    ** If the pager is in ERROR state when this function is called, the 
    ** contents of the pager cache are discarded before switching back to 
    ** the OPEN state. Regardless of whether the pager is in exclusive-mode
    ** or not, any journal file left in the file-system will be treated
    ** as a hot-journal and rolled back the next time a read-transaction
    ** is opened (by this or by any other connection).
    */
    static void pager_unlock( Pager pPager )
    {

      Debug.Assert( pPager.eState == PAGER_READER
      || pPager.eState == PAGER_OPEN
      || pPager.eState == PAGER_ERROR
      );

      sqlite3BitvecDestroy( ref pPager.pInJournal );
      pPager.pInJournal = null;
      releaseAllSavepoints( pPager );

      if ( pagerUseWal( pPager ) )
      {
        Debug.Assert( !isOpen( pPager.jfd ) );
        sqlite3WalEndReadTransaction( pPager.pWal );
        pPager.eState = PAGER_OPEN;
      }
      else if ( !pPager.exclusiveMode )
      {
        int rc;                       /* Error code returned by pagerUnlockDb() */
        int iDc = isOpen( pPager.fd ) ? sqlite3OsDeviceCharacteristics( pPager.fd ) : 0;

        /* If the operating system support deletion of open files, then
        ** close the journal file when dropping the database lock.  Otherwise
        ** another connection with journal_mode=delete might delete the file
        ** out from under us.
        */
        Debug.Assert( ( PAGER_JOURNALMODE_MEMORY & 5 ) != 1 );
        Debug.Assert( ( PAGER_JOURNALMODE_OFF & 5 ) != 1 );
        Debug.Assert( ( PAGER_JOURNALMODE_WAL & 5 ) != 1 );
        Debug.Assert( ( PAGER_JOURNALMODE_DELETE & 5 ) != 1 );
        Debug.Assert( ( PAGER_JOURNALMODE_TRUNCATE & 5 ) == 1 );
        Debug.Assert( ( PAGER_JOURNALMODE_PERSIST & 5 ) == 1 );
        if ( 0 == ( iDc & SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN )
        || 1 != ( pPager.journalMode & 5 )
        )
        {
          sqlite3OsClose( pPager.jfd );
        }


        /* If the pager is in the ERROR state and the call to unlock the database
        ** file fails, set the current lock to UNKNOWN_LOCK. See the comment
        ** above the #define for UNKNOWN_LOCK for an explanation of why this
        ** is necessary.
        */
        rc = pagerUnlockDb( pPager, NO_LOCK );
        if ( rc != SQLITE_OK && pPager.eState == PAGER_ERROR )
        {
          pPager.eLock = UNKNOWN_LOCK;
        }

        /* The pager state may be changed from PAGER_ERROR to PAGER_OPEN here
        ** without clearing the error code. This is intentional - the error
        ** code is cleared and the cache reset in the block below.
        */
        Debug.Assert( pPager.errCode != 0 || pPager.eState != PAGER_ERROR );
        pPager.changeCountDone = false;
        pPager.eState = PAGER_OPEN;
      }

      /* If Pager.errCode is set, the contents of the pager cache cannot be
      ** trusted. Now that there are no outstanding references to the pager,
      ** it can safely move back to PAGER_OPEN state. This happens in both
      ** normal and exclusive-locking mode.
      */
      if ( pPager.errCode != 0 )
      {
        Debug.Assert(
#if SQLITE_OMIT_MEMORYDB
0==MEMDB
#else
 0 == pPager.memDb
#endif
 );
        pager_reset( pPager );
        pPager.changeCountDone = pPager.tempFile;
        pPager.eState = PAGER_OPEN;
        pPager.errCode = SQLITE_OK;
      }

      pPager.journalOff = 0;
      pPager.journalHdr = 0;
      pPager.setMaster = 0;
    }

    /*
    ** This function is called whenever an IOERR or FULL error that requires
    ** the pager to transition into the ERROR state may ahve occurred.
    ** The first argument is a pointer to the pager structure, the second 
    ** the error-code about to be returned by a pager API function. The 
    ** value returned is a copy of the second argument to this function. 
    **
    ** If the second argument is SQLITE_FULL, SQLITE_IOERR or one of the
    ** IOERR sub-codes, the pager enters the ERROR state and the error code
    ** is stored in Pager.errCode. While the pager remains in the ERROR state,
    ** all major API calls on the Pager will immediately return Pager.errCode.
    **
    ** The ERROR state indicates that the contents of the pager-cache 
    ** cannot be trusted. This state can be cleared by completely discarding 
    ** the contents of the pager-cache. If a transaction was active when
    ** the persistent error occurred, then the rollback journal may need
    ** to be replayed to restore the contents of the database file (as if
    ** it were a hot-journal).
    */
    static int pager_error( Pager pPager, int rc )
    {
      int rc2 = rc & 0xff;
      Debug.Assert( rc == SQLITE_OK ||
#if SQLITE_OMIT_MEMORYDB
0==MEMDB
#else
 0 == pPager.memDb
#endif
 );
      Debug.Assert(
      pPager.errCode == SQLITE_FULL ||
      pPager.errCode == SQLITE_OK ||
      ( pPager.errCode & 0xff ) == SQLITE_IOERR
      );
      if (
      rc2 == SQLITE_FULL || rc2 == SQLITE_IOERR )
      {
        pPager.errCode = rc;
        pPager.eState = PAGER_ERROR;
      }
      return rc;
    }

    /*
    ** This routine ends a transaction. A transaction is usually ended by 
    ** either a COMMIT or a ROLLBACK operation. This routine may be called 
    ** after rollback of a hot-journal, or if an error occurs while opening
    ** the journal file or writing the very first journal-header of a
    ** database transaction.
    ** 
    ** This routine is never called in PAGER_ERROR state. If it is called
    ** in PAGER_NONE or PAGER_SHARED state and the lock held is less
    ** exclusive than a RESERVED lock, it is a no-op.
    **
    ** Otherwise, any active savepoints are released.
    **
    ** If the journal file is open, then it is "finalized". Once a journal 
    ** file has been finalized it is not possible to use it to roll back a 
    ** transaction. Nor will it be considered to be a hot-journal by this
    ** or any other database connection. Exactly how a journal is finalized
    ** depends on whether or not the pager is running in exclusive mode and
    ** the current journal-mode (Pager.journalMode value), as follows:
    **
    **   journalMode==MEMORY
    **     Journal file descriptor is simply closed. This destroys an 
    **     in-memory journal.
    **
    **   journalMode==TRUNCATE
    **     Journal file is truncated to zero bytes in size.
    **
    **   journalMode==PERSIST
    **     The first 28 bytes of the journal file are zeroed. This invalidates
    **     the first journal header in the file, and hence the entire journal
    **     file. An invalid journal file cannot be rolled back.
    **
    **   journalMode==DELETE
    **     The journal file is closed and deleted using sqlite3OsDelete().
    **
    **     If the pager is running in exclusive mode, this method of finalizing
    **     the journal file is never used. Instead, if the journalMode is
    **     DELETE and the pager is in exclusive mode, the method described under
    **     journalMode==PERSIST is used instead.
    **
    ** After the journal is finalized, the pager moves to PAGER_READER state.
    ** If running in non-exclusive rollback mode, the lock on the file is 
    ** downgraded to a SHARED_LOCK.
    **
    ** SQLITE_OK is returned if no error occurs. If an error occurs during
    ** any of the IO operations to finalize the journal file or unlock the
    ** database then the IO error code is returned to the user. If the 
    ** operation to finalize the journal file fails, then the code still
    ** tries to unlock the database file if not in exclusive mode. If the
    ** unlock operation fails as well, then the first error code related
    ** to the first error encountered (the journal finalization one) is
    ** returned.
    */
    static int pager_end_transaction( Pager pPager, int hasMaster )
    {
      int rc = SQLITE_OK;     /* Error code from journal finalization operation */
      int rc2 = SQLITE_OK;    /* Error code from db file unlock operation */
      /* Do nothing if the pager does not have an open write transaction
      ** or at least a RESERVED lock. This function may be called when there
      ** is no write-transaction active but a RESERVED or greater lock is
      ** held under two circumstances:
      **
      **   1. After a successful hot-journal rollback, it is called with
      **      eState==PAGER_NONE and eLock==EXCLUSIVE_LOCK.
      **
      **   2. If a connection with locking_mode=exclusive holding an EXCLUSIVE 
      **      lock switches back to locking_mode=normal and then executes a
      **      read-transaction, this function is called with eState==PAGER_READER 
      **      and eLock==EXCLUSIVE_LOCK when the read-transaction is closed.
      */
      Debug.Assert( assert_pager_state( pPager ) );
      Debug.Assert( pPager.eState != PAGER_ERROR );
      if ( pPager.eState < PAGER_WRITER_LOCKED && pPager.eLock < RESERVED_LOCK )
      {
        return SQLITE_OK;
      }

      releaseAllSavepoints( pPager );
      Debug.Assert( isOpen( pPager.jfd ) || pPager.pInJournal == null );
      if ( isOpen( pPager.jfd ) )
      {
        Debug.Assert( !pagerUseWal( pPager ) );

        /* Finalize the journal file. */
        if ( sqlite3IsMemJournal( pPager.jfd ) )
        {
          Debug.Assert( pPager.journalMode == PAGER_JOURNALMODE_MEMORY );
          sqlite3OsClose( pPager.jfd );
        }
        else if ( pPager.journalMode == PAGER_JOURNALMODE_TRUNCATE )
        {
          if ( pPager.journalOff == 0 )
          {
            rc = SQLITE_OK;
          }
          else
          {
            rc = sqlite3OsTruncate( pPager.jfd, 0 );
          }
          pPager.journalOff = 0;
        }
        else if ( pPager.journalMode == PAGER_JOURNALMODE_PERSIST
        || ( pPager.exclusiveMode && pPager.journalMode != PAGER_JOURNALMODE_WAL )
        )
        {
          rc = zeroJournalHdr( pPager, hasMaster );
          pPager.journalOff = 0;
        }
        else
        {
          /* This branch may be executed with Pager.journalMode==MEMORY if
          ** a hot-journal was just rolled back. In this case the journal
          ** file should be closed and deleted. If this connection writes to
          ** the database file, it will do so using an in-memory journal. 
          */
          Debug.Assert( pPager.journalMode == PAGER_JOURNALMODE_DELETE
          || pPager.journalMode == PAGER_JOURNALMODE_MEMORY
          || pPager.journalMode == PAGER_JOURNALMODE_WAL
          );
          sqlite3OsClose( pPager.jfd );
          if ( !pPager.tempFile )
          {
            rc = sqlite3OsDelete( pPager.pVfs, pPager.zJournal, 0 );
          }
        }
      }
#if SQLITE_CHECK_PAGES
sqlite3PcacheIterateDirty(pPager.pPCache, pager_set_pagehash);
if( pPager.dbSize==0 && sqlite3PcacheRefCount(pPager.pPCache)>0 ){
PgHdr p = pager_lookup(pPager, 1);
if( p != null ){
p.pageHash = null;
sqlite3PagerUnref(p);
}
}
#endif
      sqlite3BitvecDestroy( ref pPager.pInJournal );
      pPager.pInJournal = null;
      pPager.nRec = 0;
      sqlite3PcacheCleanAll( pPager.pPCache );
      sqlite3PcacheTruncate( pPager.pPCache, pPager.dbSize );

      if ( pagerUseWal( pPager ) )
      {
        /* Drop the WAL write-lock, if any. Also, if the connection was in 
        ** locking_mode=exclusive mode but is no longer, drop the EXCLUSIVE 
        ** lock held on the database file.
        */
        rc2 = sqlite3WalEndWriteTransaction( pPager.pWal );
        Debug.Assert( rc2 == SQLITE_OK );
      }
      if ( !pPager.exclusiveMode
      && ( !pagerUseWal( pPager ) || sqlite3WalExclusiveMode( pPager.pWal, 0 ) )
      )
      {
        rc2 = pagerUnlockDb( pPager, SHARED_LOCK );
        pPager.changeCountDone = false;
      }
      pPager.eState = PAGER_READER;
      pPager.setMaster = 0;

      return ( rc == SQLITE_OK ? rc2 : rc );
    }

    /*
    ** Execute a rollback if a transaction is active and unlock the 
    ** database file. 
    **
    ** If the pager has already entered the ERROR state, do not attempt 
    ** the rollback at this time. Instead, pager_unlock() is called. The
    ** call to pager_unlock() will discard all in-memory pages, unlock
    ** the database file and move the pager back to OPEN state. If this 
    ** means that there is a hot-journal left in the file-system, the next 
    ** connection to obtain a shared lock on the pager (which may be this one) 
    ** will roll it back.
    **
    ** If the pager has not already entered the ERROR state, but an IO or
    ** malloc error occurs during a rollback, then this will itself cause 
    ** the pager to enter the ERROR state. Which will be cleared by the
    ** call to pager_unlock(), as described above.
    */
    static void pagerUnlockAndRollback( Pager pPager )
    {
      if ( pPager.eState != PAGER_ERROR && pPager.eState != PAGER_OPEN )
      {
        Debug.Assert( assert_pager_state( pPager ) );
        if ( pPager.eState >= PAGER_WRITER_LOCKED )
        {
          sqlite3BeginBenignMalloc();
          sqlite3PagerRollback( pPager );
          sqlite3EndBenignMalloc();
        }
        else if ( !pPager.exclusiveMode )
        {
          Debug.Assert( pPager.eState == PAGER_READER );
          pager_end_transaction( pPager, 0 );
        }
      }
      pager_unlock( pPager );
    }

    /*
    ** Parameter aData must point to a buffer of pPager.pageSize bytes
    ** of data. Compute and return a checksum based ont the contents of the
    ** page of data and the current value of pPager.cksumInit.
    **
    ** This is not a real checksum. It is really just the sum of the
    ** random initial value (pPager.cksumInit) and every 200th byte
    ** of the page data, starting with byte offset (pPager.pageSize%200).
    ** Each byte is interpreted as an 8-bit unsigned integer.
    **
    ** Changing the formula used to compute this checksum results in an
    ** incompatible journal file format.
    **
    ** If journal corruption occurs due to a power failure, the most likely
    ** scenario is that one end or the other of the record will be changed.
    ** It is much less likely that the two ends of the journal record will be
    ** correct and the middle be corrupt.  Thus, this "checksum" scheme,
    ** though fast and simple, catches the mostly likely kind of corruption.
    */
    static u32 pager_cksum( Pager pPager, byte[] aData )
    {
      u32 cksum = pPager.cksumInit;         /* Checksum value to return */
      int i = pPager.pageSize - 200;        /* Loop counter */
      while ( i > 0 )
      {
        cksum += aData[i];
        i -= 200;
      }
      return cksum;
    }

    /*
    ** Report the current page size and number of reserved bytes back
    ** to the codec.
    */
#if SQLITE_HAS_CODEC
    static void pagerReportSize( Pager pPager )
    {
      if ( pPager.xCodecSizeChng != null )
      {
        pPager.xCodecSizeChng( pPager.pCodec, pPager.pageSize,
        pPager.nReserve );
      }
    }
#else
//# define pagerReportSize(X)     /* No-op if we do not support a codec */
static void pagerReportSize(Pager X){}
#endif

    /*
** Read a single page from either the journal file (if isMainJrnl==1) or
** from the sub-journal (if isMainJrnl==0) and playback that page.
** The page begins at offset *pOffset into the file. The *pOffset
** value is increased to the start of the next page in the journal.
**
** The main rollback journal uses checksums - the statement journal does 
** not.
**
** If the page number of the page record read from the (sub-)journal file
** is greater than the current value of Pager.dbSize, then playback is
** skipped and SQLITE_OK is returned.
**
** If pDone is not NULL, then it is a record of pages that have already
** been played back.  If the page at *pOffset has already been played back
** (if the corresponding pDone bit is set) then skip the playback.
** Make sure the pDone bit corresponding to the *pOffset page is set
** prior to returning.
**
** If the page record is successfully read from the (sub-)journal file
** and played back, then SQLITE_OK is returned. If an IO error occurs
** while reading the record from the (sub-)journal file or while writing
** to the database file, then the IO error code is returned. If data
** is successfully read from the (sub-)journal file but appears to be
** corrupted, SQLITE_DONE is returned. Data is considered corrupted in
** two circumstances:
** 
**   * If the record page-number is illegal (0 or PAGER_MJ_PGNO), or
**   * If the record is being rolled back from the main journal file
**     and the checksum field does not match the record content.
**
** Neither of these two scenarios are possible during a savepoint rollback.
**
** If this is a savepoint rollback, then memory may have to be dynamically
** allocated by this function. If this is the case and an allocation fails,
** SQLITE_NOMEM is returned.
*/
    static int pager_playback_one_page(
    Pager pPager,                /* The pager being played back */
    ref i64 pOffset,             /* Offset of record to playback */
    Bitvec pDone,                /* Bitvec of pages already played back */
    int isMainJrnl,              /* True for main rollback journal. False for Stmt jrnl */
    int isSavepnt                /* True for a savepoint rollback */
    )
    {
      int rc;
      PgHdr pPg;                    /* An existing page in the cache */
      Pgno pgno = 0;                /* The page number of a page in journal */
      u32 cksum = 0;                /* Checksum used for sanity checking */
      byte[] aData;                 /* Temporary storage for the page */
      sqlite3_file jfd;             /* The file descriptor for the journal file */
      bool isSynced;                /* True if journal page is synced */

      Debug.Assert( ( isMainJrnl & ~1 ) == 0 );   /* isMainJrnl is 0 or 1 */
      Debug.Assert( ( isSavepnt & ~1 ) == 0 );    /* isSavepnt is 0 or 1 */
      Debug.Assert( isMainJrnl != 0 || pDone != null );        /* pDone always used on sub-journals */
      Debug.Assert( isSavepnt != 0 || pDone == null );    /* pDone never used on non-savepoint */

      aData = pPager.pTmpSpace;
      Debug.Assert( aData != null );         /* Temp storage must have already been allocated */
      Debug.Assert( pagerUseWal( pPager ) == false || ( 0 == isMainJrnl && isSavepnt != 0 ) );

      /* Either the state is greater than PAGER_WRITER_CACHEMOD (a transaction 
      ** or savepoint rollback done at the request of the caller) or this is
      ** a hot-journal rollback. If it is a hot-journal rollback, the pager
      ** is in state OPEN and holds an EXCLUSIVE lock. Hot-journal rollback
      ** only reads from the main journal, not the sub-journal.
      */
      Debug.Assert( pPager.eState >= PAGER_WRITER_CACHEMOD
      || ( pPager.eState == PAGER_OPEN && pPager.eLock == EXCLUSIVE_LOCK )
      );
      Debug.Assert( pPager.eState >= PAGER_WRITER_CACHEMOD || isMainJrnl != 0 );

      /* Read the page number and page data from the journal or sub-journal
      ** file. Return an error code to the caller if an IO error occurs.
      */
      jfd = isMainJrnl != 0 ? pPager.jfd : pPager.sjfd;

      rc = read32bits( jfd, pOffset, ref pgno );
      if ( rc != SQLITE_OK )
        return rc;
      rc = sqlite3OsRead( jfd, aData, pPager.pageSize, ( pOffset ) + 4 );
      if ( rc != SQLITE_OK )
        return rc;
      pOffset += pPager.pageSize + 4 + isMainJrnl * 4;

      /* Sanity checking on the page.  This is more important that I originally
      ** thought.  If a power failure occurs while the journal is being written,
      ** it could cause invalid data to be written into the journal.  We need to
      ** detect this invalid data (with high probability) and ignore it.
      */
      if ( pgno == 0 || pgno == PAGER_MJ_PGNO( pPager ) )
      {
        Debug.Assert( 0 == isSavepnt );
        return SQLITE_DONE;
      }
      if ( pgno > pPager.dbSize || sqlite3BitvecTest( pDone, pgno ) != 0 )
      {
        return SQLITE_OK;
      }
      if ( isMainJrnl != 0 )
      {
        rc = read32bits( jfd, ( pOffset ) - 4, ref cksum );
        if ( rc != 0 )
          return rc;
        if ( 0 == isSavepnt && pager_cksum( pPager, aData ) != cksum )
        {
          return SQLITE_DONE;
        }
      }

      /* If this page has already been played by before during the current
      ** rollback, then don't bother to play it back again.
      */
      if ( pDone != null && ( rc = sqlite3BitvecSet( pDone, pgno ) ) != SQLITE_OK )
      {
        return rc;
      }

      /* When playing back page 1, restore the nReserve setting
      */
      if ( pgno == 1 && pPager.nReserve != ( aData )[20] )
      {
        pPager.nReserve = ( aData )[20];
        pagerReportSize( pPager );
      }

      /* If the pager is in CACHEMOD state, then there must be a copy of this
      ** page in the pager cache. In this case just update the pager cache,
      ** not the database file. The page is left marked dirty in this case.
      **
      ** An exception to the above rule: If the database is in no-sync mode
      ** and a page is moved during an incremental vacuum then the page may
      ** not be in the pager cache. Later: if a malloc() or IO error occurs
      ** during a Movepage() call, then the page may not be in the cache
      ** either. So the condition described in the above paragraph is not
      ** assert()able.
      **
      ** If in WRITER_DBMOD, WRITER_FINISHED or OPEN state, then we update the
      ** pager cache if it exists and the main file. The page is then marked 
      ** not dirty. Since this code is only executed in PAGER_OPEN state for
      ** a hot-journal rollback, it is guaranteed that the page-cache is empty
      ** if the pager is in OPEN state.
      **
      ** Ticket #1171:  The statement journal might contain page content that is
      ** different from the page content at the start of the transaction.
      ** This occurs when a page is changed prior to the start of a statement
      ** then changed again within the statement.  When rolling back such a
      ** statement we must not write to the original database unless we know
      ** for certain that original page contents are synced into the main rollback
      ** journal.  Otherwise, a power loss might leave modified data in the
      ** database file without an entry in the rollback journal that can
      ** restore the database to its original form.  Two conditions must be
      ** met before writing to the database files. (1) the database must be
      ** locked.  (2) we know that the original page content is fully synced
      ** in the main journal either because the page is not in cache or else
      ** the page is marked as needSync==0.
      **
      ** 2008-04-14:  When attempting to vacuum a corrupt database file, it
      ** is possible to fail a statement on a database that does not yet exist.
      ** Do not attempt to write if database file has never been opened.
      */
      if ( pagerUseWal( pPager ) )
      {
        pPg = null;
      }
      else
      {
        pPg = pager_lookup( pPager, pgno );
      }
      Debug.Assert( pPg != null ||
#if SQLITE_OMIT_MEMORYDB
0==MEMDB
#else
 pPager.memDb == 0
#endif
 );
      Debug.Assert( pPager.eState != PAGER_OPEN || pPg == null );

      PAGERTRACE( "PLAYBACK %d page %d hash(%08x) %s\n",
      PAGERID( pPager ), pgno, pager_datahash( pPager.pageSize, aData ),
      ( isMainJrnl != 0 ? "main-journal" : "sub-journal" )
      );
      if ( isMainJrnl != 0 )
      {
        isSynced = pPager.noSync || ( pOffset <= pPager.journalHdr );
      }
      else
      {
        isSynced = ( pPg == null || 0 == ( pPg.flags & PGHDR_NEED_SYNC ) );
      }
      if ( isOpen( pPager.fd )
      && ( pPager.eState >= PAGER_WRITER_DBMOD || pPager.eState == PAGER_OPEN )
      && isSynced
      )
      {
        i64 ofst = ( pgno - 1 ) * pPager.pageSize;
        testcase( 0 == isSavepnt && pPg != null && ( pPg.flags & PGHDR_NEED_SYNC ) != 0 );
        Debug.Assert( !pagerUseWal( pPager ) );
        rc = sqlite3OsWrite( pPager.fd, aData, pPager.pageSize, ofst );
        if ( pgno > pPager.dbFileSize )
        {
          pPager.dbFileSize = pgno;
        }
        if ( pPager.pBackup != null )
        {
          if ( CODEC1( pPager, aData, pgno, SQLITE_DECRYPT ) )
            rc = SQLITE_NOMEM; // CODEC1( pPager, aData, pgno, 3, rc = SQLITE_NOMEM );
          sqlite3BackupUpdate( pPager.pBackup, pgno, (u8[])aData );
          if ( CODEC2( pPager, aData, pgno, SQLITE_ENCRYPT_READ_CTX, ref aData ) )
            rc = SQLITE_NOMEM;//CODEC2( pPager, aData, pgno, 7, rc = SQLITE_NOMEM, aData);
        }
      }
      else if ( 0 == isMainJrnl && pPg == null )
      {
        /* If this is a rollback of a savepoint and data was not written to
        ** the database and the page is not in-memory, there is a potential
        ** problem. When the page is next fetched by the b-tree layer, it
        ** will be read from the database file, which may or may not be
        ** current.
        **
        ** There are a couple of different ways this can happen. All are quite
        ** obscure. When running in synchronous mode, this can only happen
        ** if the page is on the free-list at the start of the transaction, then
        ** populated, then moved using sqlite3PagerMovepage().
        **
        ** The solution is to add an in-memory page to the cache containing
        ** the data just read from the sub-journal. Mark the page as dirty
        ** and if the pager requires a journal-sync, then mark the page as
        ** requiring a journal-sync before it is written.
        */
        Debug.Assert( isSavepnt != 0 );
        Debug.Assert( pPager.doNotSpill == 0 );
        pPager.doNotSpill++;
        rc = sqlite3PagerAcquire( pPager, pgno, ref pPg, 1 );
        Debug.Assert( pPager.doNotSpill == 1 );
        pPager.doNotSpill--;
        if ( rc != SQLITE_OK )
          return rc;
        pPg.flags &= ~PGHDR_NEED_READ;
        sqlite3PcacheMakeDirty( pPg );
      }
      if ( pPg != null )
      {
        /* No page should ever be explicitly rolled back that is in use, except
        ** for page 1 which is held in use in order to keep the lock on the
        ** database active. However such a page may be rolled back as a result
        ** of an internal error resulting in an automatic call to
        ** sqlite3PagerRollback().
        */
        byte[] pData = pPg.pData;
        Buffer.BlockCopy( aData, 0, pData, 0, pPager.pageSize );// memcpy(pData, (u8[])aData, pPager.pageSize);
        pPager.xReiniter( pPg );
        if ( isMainJrnl != 0 && ( 0 == isSavepnt || pOffset <= pPager.journalHdr ) )
        {
          /* If the contents of this page were just restored from the main
          ** journal file, then its content must be as they were when the
          ** transaction was first opened. In this case we can mark the page
          ** as clean, since there will be no need to write it out to the
          ** database.
          **
          ** There is one exception to this rule. If the page is being rolled
          ** back as part of a savepoint (or statement) rollback from an
          ** unsynced portion of the main journal file, then it is not safe
          ** to mark the page as clean. This is because marking the page as
          ** clean will clear the PGHDR_NEED_SYNC flag. Since the page is
          ** already in the journal file (recorded in Pager.pInJournal) and
          ** the PGHDR_NEED_SYNC flag is cleared, if the page is written to
          ** again within this transaction, it will be marked as dirty but
          ** the PGHDR_NEED_SYNC flag will not be set. It could then potentially
          ** be written out into the database file before its journal file
          ** segment is synced. If a crash occurs during or following this,
          ** database corruption may ensue.
          */
          Debug.Assert( !pagerUseWal( pPager ) );
          sqlite3PcacheMakeClean( pPg );
        }
        pager_set_pagehash( pPg );
        /* If this was page 1, then restore the value of Pager.dbFileVers.
        ** Do this before any decoding. */
        if ( pgno == 1 )
        {
          Buffer.BlockCopy( pData, 24, pPager.dbFileVers, 0, pPager.dbFileVers.Length ); //memcpy(pPager.dbFileVers, ((u8*)pData)[24], sizeof(pPager.dbFileVers));
        }

        /* Decode the page just read from disk */
        if ( CODEC1( pPager, pData, pPg.pgno, SQLITE_DECRYPT ) )
          rc = SQLITE_NOMEM; //CODEC1(pPager, pData, pPg.pgno, 3, rc=SQLITE_NOMEM);
        sqlite3PcacheRelease( pPg );
      }
      return rc;
    }

    /*
    ** Parameter zMaster is the name of a master journal file. A single journal
    ** file that referred to the master journal file has just been rolled back.
    ** This routine checks if it is possible to delete the master journal file,
    ** and does so if it is.
    **
    ** Argument zMaster may point to Pager.pTmpSpace. So that buffer is not
    ** available for use within this function.
    **
    ** When a master journal file is created, it is populated with the names
    ** of all of its child journals, one after another, formatted as utf-8
    ** encoded text. The end of each child journal file is marked with a
    ** nul-terminator byte (0x00). i.e. the entire contents of a master journal
    ** file for a transaction involving two databases might be:
    **
    **   "/home/bill/a.db-journal\x00/home/bill/b.db-journal\x00"
    **
    ** A master journal file may only be deleted once all of its child
    ** journals have been rolled back.
    **
    ** This function reads the contents of the master-journal file into
    ** memory and loops through each of the child journal names. For
    ** each child journal, it checks if:
    **
    **   * if the child journal exists, and if so
    **   * if the child journal contains a reference to master journal
    **     file zMaster
    **
    ** If a child journal can be found that matches both of the criteria
    ** above, this function returns without doing anything. Otherwise, if
    ** no such child journal can be found, file zMaster is deleted from
    ** the file-system using sqlite3OsDelete().
    **
    ** If an IO error within this function, an error code is returned. This
    ** function allocates memory by calling sqlite3Malloc(). If an allocation
    ** fails, SQLITE_NOMEM is returned. Otherwise, if no IO or malloc errors
    ** occur, SQLITE_OK is returned.
    **
    ** TODO: This function allocates a single block of memory to load
    ** the entire contents of the master journal file. This could be
    ** a couple of kilobytes or so - potentially larger than the page
    ** size.
    */
    static int pager_delmaster( Pager pPager, string zMaster )
    {
      sqlite3_vfs pVfs = pPager.pVfs;
      int rc;                       /* Return code */
      sqlite3_file pMaster;         /* Malloc'd master-journal file descriptor */
      sqlite3_file pJournal;        /* Malloc'd child-journal file descriptor */
      //string zMasterJournal = null; /* Contents of master journal file */
      i64 nMasterJournal;           /* Size of master journal file */
      string zJournal;              /* Pointer to one journal within MJ file */
      string zMasterPtr;            /* Space to hold MJ filename from a journal file */
      int nMasterPtr;               /* Amount of space allocated to zMasterPtr[] */

      /* Allocate space for both the pJournal and pMaster file descriptors.
      ** If successful, open the master journal file for reading.
      */
      pMaster = new sqlite3_file();// (sqlite3_file*)sqlite3MallocZero( pVfs.szOsFile * 2 );
      pJournal = new sqlite3_file();// (sqlite3_file*)( ( (u8*)pMaster ) + pVfs.szOsFile );
      //if ( null == pMaster )
      //{
      //  rc = SQLITE_NOMEM;
      //}
      //else
      {
        const int flags = ( SQLITE_OPEN_READONLY | SQLITE_OPEN_MASTER_JOURNAL );
        int iDummy = 0;
        rc = sqlite3OsOpen( pVfs, zMaster, pMaster, flags, ref iDummy );
      }
      if ( rc != SQLITE_OK )
        goto delmaster_out;

      Debugger.Break();    //TODO --

      /* Load the entire master journal file into space obtained from
      ** sqlite3_malloc() and pointed to by zMasterJournal.   Also obtain
      ** sufficient space (in zMasterPtr) to hold the names of master
      ** journal files extracted from regular rollback-journals.
      */
      //rc = sqlite3OsFileSize(pMaster, &nMasterJournal);
      //if (rc != SQLITE_OK) goto delmaster_out;
      //nMasterPtr = pVfs.mxPathname + 1;
      //  zMasterJournal = sqlite3Malloc((int)nMasterJournal + nMasterPtr + 1);
      //  if ( !zMasterJournal )
      //  {
      //    rc = SQLITE_NOMEM;
      //    goto delmaster_out;
      //  }
      //  zMasterPtr = &zMasterJournal[nMasterJournal+1];
      //  rc = sqlite3OsRead( pMaster, zMasterJournal, (int)nMasterJournal, 0 );
      //  if ( rc != SQLITE_OK ) goto delmaster_out;
      //  zMasterJournal[nMasterJournal] = 0;


      //  zJournal = zMasterJournal;
      //  while ( ( zJournal - zMasterJournal ) < nMasterJournal )
      //  {
      //    int exists;
      //    rc = sqlite3OsAccess( pVfs, zJournal, SQLITE_ACCESS_EXISTS, &exists );
      //    if ( rc != SQLITE_OK )
      //    {
      //      goto delmaster_out;
      //    }
      //    if ( exists )
      //    {
      //      /* One of the journals pointed to by the master journal exists.
      //      ** Open it and check if it points at the master journal. If
      //      ** so, return without deleting the master journal file.
      //      */
      //      int c;
      //      int flags = ( SQLITE_OPEN_READONLY | SQLITE_OPEN_MAIN_JOURNAL );
      //      rc = sqlite3OsOpen( pVfs, zJournal, pJournal, flags, 0 );
      //      if ( rc != SQLITE_OK )
      //      {
      //        goto delmaster_out;
      //      }

      //      rc = readMasterJournal( pJournal, zMasterPtr, nMasterPtr );
      //      sqlite3OsClose( pJournal );
      //      if ( rc != SQLITE_OK )
      //      {
      //        goto delmaster_out;
      //      }

      //      c = zMasterPtr[0] != 0 && strcmp( zMasterPtr, zMaster ) == 0;
      //      if ( c )
      //      {
      //        /* We have a match. Do not delete the master journal file. */
      //        goto delmaster_out;
      //      }
      //    }
      //    zJournal += ( sqlite3Strlen30( zJournal ) + 1 );
      //   }
      //
      //sqlite3OsClose(pMaster);
      //rc = sqlite3OsDelete( pVfs, zMaster, 0 );


      goto delmaster_out;
delmaster_out:
      //sqlite3_free( ref zMasterJournal );
      if ( pMaster != null )
      {
        sqlite3OsClose( pMaster );
        Debug.Assert( !isOpen( pJournal ) );
        //sqlite3_free( ref pMaster );
      }
      return rc;
    }



    /*
    ** This function is used to change the actual size of the database 
    ** file in the file-system. This only happens when committing a transaction,
    ** or rolling back a transaction (including rolling back a hot-journal).
    **
    ** If the main database file is not open, or the pager is not in either
    ** DBMOD or OPEN state, this function is a no-op. Otherwise, the size 
    ** of the file is changed to nPage pages (nPage*pPager.pageSize bytes). 
    ** If the file on disk is currently larger than nPage pages, then use the VFS
    ** xTruncate() method to truncate it.
    **
    ** Or, it might might be the case that the file on disk is smaller than 
    ** nPage pages. Some operating system implementations can get confused if 
    ** you try to truncate a file to some size that is larger than it 
    ** currently is, so detect this case and write a single zero byte to 
    ** the end of the new file instead.
    **
    ** If successful, return SQLITE_OK. If an IO error occurs while modifying
    ** the database file, return the error code to the caller.
    */
    static int pager_truncate( Pager pPager, u32 nPage )
    {
      int rc = SQLITE_OK;
      Debug.Assert( pPager.eState != PAGER_ERROR );
      Debug.Assert( pPager.eState != PAGER_READER );

      if ( isOpen( pPager.fd )
      && ( pPager.eState >= PAGER_WRITER_DBMOD || pPager.eState == PAGER_OPEN )
      )
      {
        i64 currentSize = 0, newSize;
        int szPage = pPager.pageSize;
        Debug.Assert( pPager.eLock == EXCLUSIVE_LOCK );
        /* TODO: Is it safe to use Pager.dbFileSize here? */
        rc = sqlite3OsFileSize( pPager.fd, ref currentSize );
        newSize = szPage * nPage;
        if ( rc == SQLITE_OK && currentSize != newSize )
        {
          if ( currentSize > newSize )
          {
            rc = sqlite3OsTruncate( pPager.fd, newSize );
          }
          else
          {
            byte[] pTmp = pPager.pTmpSpace;
            Array.Clear( pTmp, 0, szPage );//memset( pTmp, 0, szPage );
            testcase( ( newSize - szPage ) < currentSize );
            testcase( ( newSize - szPage ) == currentSize );
            testcase( ( newSize - szPage ) > currentSize );
            rc = sqlite3OsWrite( pPager.fd, pTmp, szPage, newSize - szPage );
          }
          if ( rc == SQLITE_OK )
          {
            pPager.dbSize = nPage;
          }
        }
      }
      return rc;
    }

    /*
    ** Set the value of the Pager.sectorSize variable for the given
    ** pager based on the value returned by the xSectorSize method
    ** of the open database file. The sector size will be used used
    ** to determine the size and alignment of journal header and
    ** master journal pointers within created journal files.
    **
    ** For temporary files the effective sector size is always 512 bytes.
    **
    ** Otherwise, for non-temporary files, the effective sector size is
    ** the value returned by the xSectorSize() method rounded up to 512 if
    ** it is less than 32, or rounded down to MAX_SECTOR_SIZE if it
    ** is greater than MAX_SECTOR_SIZE.
    */
    static void setSectorSize( Pager pPager )
    {
      Debug.Assert( isOpen( pPager.fd ) || pPager.tempFile );
      if ( !pPager.tempFile )
      {
        /* Sector size doesn't matter for temporary files. Also, the file
        ** may not have been opened yet, in which case the OsSectorSize()
        ** call will segfault.
        */
        pPager.sectorSize = (u32)sqlite3OsSectorSize( pPager.fd );
      }
      if ( pPager.sectorSize < 32 )
      {
        Debug.Assert( MAX_SECTOR_SIZE >= 512 );
        pPager.sectorSize = 512;
      }
      if ( pPager.sectorSize > MAX_SECTOR_SIZE )
      {
        pPager.sectorSize = MAX_SECTOR_SIZE;
      }
    }


    /*
    ** Playback the journal and thus restore the database file to
    ** the state it was in before we started making changes.
    **
    ** The journal file format is as follows:
    **
    **  (1)  8 byte prefix.  A copy of aJournalMagic[].
    **  (2)  4 byte big-endian integer which is the number of valid page records
    **       in the journal.  If this value is 0xffffffff, then compute the
    **       number of page records from the journal size.
    **  (3)  4 byte big-endian integer which is the initial value for the
    **       sanity checksum.
    **  (4)  4 byte integer which is the number of pages to truncate the
    **       database to during a rollback.
    **  (5)  4 byte big-endian integer which is the sector size.  The header
    **       is this many bytes in size.
    **  (6)  4 byte big-endian integer which is the page size.
    **  (7)  zero padding out to the next sector size.
    **  (8)  Zero or more pages instances, each as follows:
    **
    ** When we speak of the journal header, we mean the first 7 items above.
    ** Each entry in the journal is an instance of the 8th item.
    **
    ** Call the value from the second bullet "nRec".  nRec is the number of
    ** valid page entries in the journal.  In most cases, you can compute the
    ** value of nRec from the size of the journal file.  But if a power
    ** failure occurred while the journal was being written, it could be the
    ** case that the size of the journal file had already been increased but
    ** the extra entries had not yet made it safely to disk.  In such a case,
    ** the value of nRec computed from the file size would be too large.  For
    ** that reason, we always use the nRec value in the header.
    **
    ** If the nRec value is 0xffffffff it means that nRec should be computed
    ** from the file size.  This value is used when the user selects the
    ** no-sync option for the journal.  A power failure could lead to corruption
    ** in this case.  But for things like temporary table (which will be
    ** deleted when the power is restored) we don't care.
    **
    ** If the file opened as the journal file is not a well-formed
    ** journal file then all pages up to the first corrupted page are rolled
    ** back (or no pages if the journal header is corrupted). The journal file
    ** is then deleted and SQLITE_OK returned, just as if no corruption had
    ** been encountered.
    **
    ** If an I/O or malloc() error occurs, the journal-file is not deleted
    ** and an error code is returned.
    **
    ** The isHot parameter indicates that we are trying to rollback a journal
    ** that might be a hot journal.  Or, it could be that the journal is
    ** preserved because of JOURNALMODE_PERSIST or JOURNALMODE_TRUNCATE.
    ** If the journal really is hot, reset the pager cache prior rolling
    ** back any content.  If the journal is merely persistent, no reset is
    ** needed.
    */
    static int pager_playback( Pager pPager, int isHot )
    {
      sqlite3_vfs pVfs = pPager.pVfs;
      i64 szJ = 0;            /* Size of the journal file in bytes */
      u32 nRec = 0;            /* Number of Records in the journal */
      u32 u;                   /* Unsigned loop counter */
      u32 mxPg = 0;            /* Size of the original file in pages */
      int rc;                  /* Result code of a subroutine */
      int res = 1;             /* Value returned by sqlite3OsAccess() */
      byte[] zMaster = null;   /* Name of master journal file if any */
      int needPagerReset;      /* True to reset page prior to first page rollback */

      /* Figure out how many records are in the journal.  Abort early if
      ** the journal is empty.
      */
      Debug.Assert( isOpen( pPager.jfd ) );
      rc = sqlite3OsFileSize( pPager.jfd, ref szJ );
      if ( rc != SQLITE_OK )
      {
        goto end_playback;
      }

      /* Read the master journal name from the journal, if it is present.
      ** If a master journal file name is specified, but the file is not
      ** present on disk, then the journal is not hot and does not need to be
      ** played back.
      **
      ** TODO: Technically the following is an error because it assumes that
      ** buffer Pager.pTmpSpace is (mxPathname+1) bytes or larger. i.e. that
      ** (pPager.pageSize >= pPager.pVfs.mxPathname+1). Using os_unix.c,
      **  mxPathname is 512, which is the same as the minimum allowable value
      ** for pageSize.
      */
      zMaster = new byte[pPager.pVfs.mxPathname + 1];// pPager.pTmpSpace );
      rc = readMasterJournal( pPager.jfd, zMaster, (u32)pPager.pVfs.mxPathname + 1 );
      if ( rc == SQLITE_OK && zMaster[0] != 0 )
      {
        rc = sqlite3OsAccess( pVfs, Encoding.UTF8.GetString( zMaster, 0, zMaster.Length ), SQLITE_ACCESS_EXISTS, ref res );
      }
      zMaster = null;
      if ( rc != SQLITE_OK || res == 0 )
      {
        goto end_playback;
      }
      pPager.journalOff = 0;
      needPagerReset = isHot;

      /* This loop terminates either when a readJournalHdr() or
      ** pager_playback_one_page() call returns SQLITE_DONE or an IO error
      ** occurs.
      */
      while ( true )
      {

        /* Read the next journal header from the journal file.  If there are
        ** not enough bytes left in the journal file for a complete header, or
        ** it is corrupted, then a process must have failed while writing it.
        ** This indicates nothing more needs to be rolled back.
        */
        rc = readJournalHdr( pPager, isHot, szJ, out nRec, out mxPg );
        if ( rc != SQLITE_OK )
        {
          if ( rc == SQLITE_DONE )
          {
            rc = SQLITE_OK;
          }
          goto end_playback;
        }

        /* If nRec is 0xffffffff, then this journal was created by a process
        ** working in no-sync mode. This means that the rest of the journal
        ** file consists of pages, there are no more journal headers. Compute
        ** the value of nRec based on this assumption.
        */
        if ( nRec == 0xffffffff )
        {
          Debug.Assert( pPager.journalOff == JOURNAL_HDR_SZ( pPager ) );
          nRec = (u32)( ( szJ - JOURNAL_HDR_SZ( pPager ) ) / JOURNAL_PG_SZ( pPager ) );
        }

        /* If nRec is 0 and this rollback is of a transaction created by this
        ** process and if this is the final header in the journal, then it means
        ** that this part of the journal was being filled but has not yet been
        ** synced to disk.  Compute the number of pages based on the remaining
        ** size of the file.
        **
        ** The third term of the test was added to fix ticket #2565.
        ** When rolling back a hot journal, nRec==0 always means that the next
        ** chunk of the journal contains zero pages to be rolled back.  But
        ** when doing a ROLLBACK and the nRec==0 chunk is the last chunk in
        ** the journal, it means that the journal might contain additional
        ** pages that need to be rolled back and that the number of pages
        ** should be computed based on the journal file size.
        */
        if ( nRec == 0 && 0 == isHot &&
        pPager.journalHdr + JOURNAL_HDR_SZ( pPager ) == pPager.journalOff )
        {
          nRec = (u32)( ( szJ - pPager.journalOff ) / JOURNAL_PG_SZ( pPager ) );
        }

        /* If this is the first header read from the journal, truncate the
        ** database file back to its original size.
        */
        if ( pPager.journalOff == JOURNAL_HDR_SZ( pPager ) )
        {
          rc = pager_truncate( pPager, mxPg );
          if ( rc != SQLITE_OK )
          {
            goto end_playback;
          }
          pPager.dbSize = mxPg;
        }

        /* Copy original pages out of the journal and back into the
        ** database file and/or page cache.
        */
        for ( u = 0; u < nRec; u++ )
        {
          if ( needPagerReset != 0 )
          {
            pager_reset( pPager );
            needPagerReset = 0;
          }
          rc = pager_playback_one_page( pPager, ref pPager.journalOff, null, 1, 0 );
          if ( rc != SQLITE_OK )
          {
            if ( rc == SQLITE_DONE )
            {
              rc = SQLITE_OK;
              pPager.journalOff = szJ;
              break;
            }
            else if ( rc == SQLITE_IOERR_SHORT_READ )
            {
              /* If the journal has been truncated, simply stop reading and
              ** processing the journal. This might happen if the journal was
              ** not completely written and synced prior to a crash.  In that
              ** case, the database should have never been written in the
              ** first place so it is OK to simply abandon the rollback. */
              rc = SQLITE_OK;
              goto end_playback;
            }
            else
            {
              /* If we are unable to rollback, quit and return the error
              ** code.  This will cause the pager to enter the error state
              ** so that no further harm will be done.  Perhaps the next
              ** process to come along will be able to rollback the database.
              */
              goto end_playback;
            }
          }
        }
      }
/*NOTREACHED*/

    end_playback:
      /* Following a rollback, the database file should be back in its original
      ** state prior to the start of the transaction, so invoke the
      ** SQLITE_FCNTL_DB_UNCHANGED file-control method to disable the
      ** assertion that the transaction counter was modified.
      */
      sqlite3_int64 iDummy = 0;
      Debug.Assert(
      pPager.fd.pMethods == null ||
      sqlite3OsFileControl( pPager.fd, SQLITE_FCNTL_DB_UNCHANGED, ref iDummy ) >= SQLITE_OK
      );

      /* If this playback is happening automatically as a result of an IO or
      ** malloc error that occurred after the change-counter was updated but
      ** before the transaction was committed, then the change-counter
      ** modification may just have been reverted. If this happens in exclusive
      ** mode, then subsequent transactions performed by the connection will not
      ** update the change-counter at all. This may lead to cache inconsistency
      ** problems for other processes at some point in the future. So, just
      ** in case this has happened, clear the changeCountDone flag now.
      */
      pPager.changeCountDone = pPager.tempFile;

      if ( rc == SQLITE_OK )
      {
        zMaster = new byte[pPager.pVfs.mxPathname + 1];//pPager.pTmpSpace );
        rc = readMasterJournal( pPager.jfd, zMaster, (u32)pPager.pVfs.mxPathname + 1 );
        testcase( rc != SQLITE_OK );
      }
      if ( rc == SQLITE_OK
      && ( pPager.eState >= PAGER_WRITER_DBMOD || pPager.eState == PAGER_OPEN )
      )
      {
        rc = sqlite3PagerSync( pPager );
      }
      if ( rc == SQLITE_OK )
      {
        rc = pager_end_transaction( pPager, zMaster[0] != '\0' ? 1 : 0 );
        testcase( rc != SQLITE_OK );
      }
      if ( rc == SQLITE_OK && zMaster[0] != '\0' && res != 0 )
      {
        /* If there was a master journal and this routine will return success,
        ** see if it is possible to delete the master journal.
        */
        rc = pager_delmaster( pPager, Encoding.UTF8.GetString( zMaster, 0, zMaster.Length ) );
        testcase( rc != SQLITE_OK );
      }

      /* The Pager.sectorSize variable may have been updated while rolling
      ** back a journal created by a process with a different sector size
      ** value. Reset it to the correct value for this process.
      */
      setSectorSize( pPager );
      return rc;
    }


    /*
    ** Read the content for page pPg out of the database file and into 
    ** pPg.pData. A shared lock or greater must be held on the database
    ** file before this function is called.
    **
    ** If page 1 is read, then the value of Pager.dbFileVers[] is set to
    ** the value read from the database file.
    **
    ** If an IO error occurs, then the IO error is returned to the caller.
    ** Otherwise, SQLITE_OK is returned.
    */
    static int readDbPage( PgHdr pPg )
    {
      Pager pPager = pPg.pPager;  /* Pager object associated with page pPg */
      Pgno pgno = pPg.pgno;       /* Page number to read */
      int rc = SQLITE_OK;         /* Return code */
      int isInWal = 0;            /* True if page is in log file */
      int pgsz = pPager.pageSize; /* Number of bytes to read */

      Debug.Assert( pPager.eState >= PAGER_READER &&
#if SQLITE_OMIT_MEMORYDB
0 == MEMDB 
#else
 0 == pPager.memDb
#endif
 );
      Debug.Assert( isOpen( pPager.fd ) );

      if ( NEVER( !isOpen( pPager.fd ) ) )
      {
        Debug.Assert( pPager.tempFile );
        Array.Clear( pPg.pData, 0, pPager.pageSize );// memset(pPg.pData, 0, pPager.pageSize);
        return SQLITE_OK;
      }

      if ( pagerUseWal( pPager ) )
      {
        /* Try to pull the page from the write-ahead log. */
        rc = sqlite3WalRead( pPager.pWal, pgno, ref isInWal, pgsz, pPg.pData );
      }
      if ( rc == SQLITE_OK && 0 == isInWal )
      {
        i64 iOffset = ( pgno - 1 ) * (i64)pPager.pageSize;
        rc = sqlite3OsRead( pPager.fd, pPg.pData, pgsz, iOffset );
        if ( rc == SQLITE_IOERR_SHORT_READ )
        {
          rc = SQLITE_OK;
        }
      }

      if ( pgno == 1 )
      {
        if ( rc != 0 )
        {
          /* If the read is unsuccessful, set the dbFileVers[] to something
          ** that will never be a valid file version.  dbFileVers[] is a copy
          ** of bytes 24..39 of the database.  Bytes 28..31 should always be
          ** zero or the size of the database in page. Bytes 32..35 and 35..39
          ** should be page numbers which are never 0xffffffff.  So filling
          ** pPager.dbFileVers[] with all 0xff bytes should suffice.
          **
          ** For an encrypted database, the situation is more complex:  bytes
          ** 24..39 of the database are white noise.  But the probability of
          ** white noising equaling 16 bytes of 0xff is vanishingly small so
          ** we should still be ok.
          */
          for ( int i = 0; i < pPager.dbFileVers.Length; pPager.dbFileVers[i++] = 0xff )
            ; // memset(pPager.dbFileVers, 0xff, sizeof(pPager.dbFileVers));
        }
        else
        {
          //u8[] dbFileVers = pPg.pData[24];
          Buffer.BlockCopy( pPg.pData, 24, pPager.dbFileVers, 0, pPager.dbFileVers.Length ); //memcpy(&pPager.dbFileVers, dbFileVers, sizeof(pPager.dbFileVers));
        }
      }
      if ( CODEC1( pPager, pPg.pData, pgno, SQLITE_DECRYPT ) )
        rc = SQLITE_NOMEM;//CODEC1(pPager, pPg.pData, pgno, 3, rc = SQLITE_NOMEM);

#if SQLITE_TEST
      //  PAGER_INCR(ref sqlite3_pager_readdb_count);
#if !TCLSH
      PAGER_INCR( ref sqlite3_pager_readdb_count );
#else
      int iValue;
      iValue = sqlite3_pager_readdb_count.iValue;
      PAGER_INCR( ref iValue );
      sqlite3_pager_readdb_count.iValue = iValue;
#endif

      PAGER_INCR( ref pPager.nRead );
#endif
      IOTRACE( "PGIN %p %d\n", pPager, pgno );
      PAGERTRACE( "FETCH %d page %d hash(%08x)\n",
      PAGERID( pPager ), pgno, pager_pagehash( pPg ) );

      return rc;
    }

    /*
    ** Update the value of the change-counter at offsets 24 and 92 in
    ** the header and the sqlite version number at offset 96.
    **
    ** This is an unconditional update.  See also the pager_incr_changecounter()
    ** routine which only updates the change-counter if the update is actually
    ** needed, as determined by the pPager.changeCountDone state variable.
    */
    static void pager_write_changecounter( PgHdr pPg )
    {
      u32 change_counter;

      /* Increment the value just read and write it back to byte 24. */
      change_counter = sqlite3Get4byte( pPg.pPager.dbFileVers, 0 ) + 1;
      put32bits(  pPg.pData, 24, change_counter );

      /* Also store the SQLite version number in bytes 96..99 and in
      ** bytes 92..95 store the change counter for which the version number
      ** is valid. */
      put32bits( pPg.pData, 92, change_counter );
      put32bits( pPg.pData, 96, SQLITE_VERSION_NUMBER );
    }

#if !SQLITE_OMIT_WAL
/*
** This function is invoked once for each page that has already been 
** written into the log file when a WAL transaction is rolled back.
** Parameter iPg is the page number of said page. The pCtx argument 
** is actually a pointer to the Pager structure.
**
** If page iPg is present in the cache, and has no outstanding references,
** it is discarded. Otherwise, if there are one or more outstanding
** references, the page content is reloaded from the database. If the
** attempt to reload content from the database is required and fails, 
** return an SQLite error code. Otherwise, SQLITE_OK.
*/
static int pagerUndoCallback(void *pCtx, Pgno iPg){
int rc = SQLITE_OK;
Pager *pPager = (Pager *)pCtx;
PgHdr *pPg;

pPg = sqlite3PagerLookup(pPager, iPg);
if( pPg ){
if( sqlite3PcachePageRefcount(pPg)==1 ){
sqlite3PcacheDrop(pPg);
}else{
rc = readDbPage(pPg);
if( rc==SQLITE_OK ){
pPager.xReiniter(pPg);
}
sqlite3PagerUnref(pPg);
}
}

/* Normally, if a transaction is rolled back, any backup processes are
** updated as data is copied out of the rollback journal and into the
** database. This is not generally possible with a WAL database, as
** rollback involves simply truncating the log file. Therefore, if one
** or more frames have already been written to the log (and therefore 
** also copied into the backup databases) as part of this transaction,
** the backups must be restarted.
*/
sqlite3BackupRestart(pPager.pBackup);

return rc;
}

/*
** This function is called to rollback a transaction on a WAL database.
*/
static int pagerRollbackWal(Pager *pPager){
int rc;                         /* Return Code */
PgHdr *pList;                   /* List of dirty pages to revert */

/* For all pages in the cache that are currently dirty or have already
** been written (but not committed) to the log file, do one of the 
** following:
**
**   + Discard the cached page (if refcount==0), or
**   + Reload page content from the database (if refcount>0).
*/
pPager.dbSize = pPager.dbOrigSize;
rc = sqlite3WalUndo(pPager.pWal, pagerUndoCallback, (void *)pPager);
pList = sqlite3PcacheDirtyList(pPager.pPCache);
while( pList && rc==SQLITE_OK ){
PgHdr *pNext = pList->pDirty;
rc = pagerUndoCallback((void *)pPager, pList->pgno);
pList = pNext;
}

return rc;
}


/*
** This function is a wrapper around sqlite3WalFrames(). As well as logging
** the contents of the list of pages headed by pList (connected by pDirty),
** this function notifies any active backup processes that the pages have
** changed.
**
** The list of pages passed into this routine is always sorted by page number.
** Hence, if page 1 appears anywhere on the list, it will be the first page.
*/ 
static int pagerWalFrames(
Pager *pPager,                  /* Pager object */
PgHdr *pList,                   /* List of frames to log */
Pgno nTruncate,                 /* Database size after this commit */
int isCommit,                   /* True if this is a commit */
int syncFlags                   /* Flags to pass to OsSync() (or 0) */
){
int rc;                         /* Return code */
#if (SQLITE_DEBUG) || (SQLITE_CHECK_PAGES)
PgHdr *p;                       /* For looping over pages */
#endif

assert( pPager.pWal );
#if SQLITE_DEBUG
/* Verify that the page list is in accending order */
for(p=pList; p && p->pDirty; p=p->pDirty){
assert( p->pgno < p->pDirty->pgno );
}
#endif

  if( isCommit ){
    /* If a WAL transaction is being committed, there is no point in writing
    ** any pages with page numbers greater than nTruncate into the WAL file.
    ** They will never be read by any client. So remove them from the pDirty
    ** list here. */
    PgHdr *p;
    PgHdr **ppNext = &pList;
    for(p=pList; (*ppNext = p); p=p->pDirty){
      if( p->pgno<=nTruncate ) ppNext = &p->pDirty;
    }
    assert( pList );
  }


if( pList->pgno==1 ) pager_write_changecounter(pList);
rc = sqlite3WalFrames(pPager.pWal, 
pPager.pageSize, pList, nTruncate, isCommit, syncFlags
);
if( rc==SQLITE_OK && pPager.pBackup ){
PgHdr *p;
for(p=pList; p; p=p->pDirty){
sqlite3BackupUpdate(pPager.pBackup, p->pgno, (u8 *)p->pData);
}
}

#if SQLITE_CHECK_PAGES
pList = sqlite3PcacheDirtyList(pPager.pPCache);
for(p=pList; p; p=p->pDirty){
pager_set_pagehash(p);
}
#endif

return rc;
}

/*
** Begin a read transaction on the WAL.
**
** This routine used to be called "pagerOpenSnapshot()" because it essentially
** makes a snapshot of the database at the current point in time and preserves
** that snapshot for use by the reader in spite of concurrently changes by
** other writers or checkpointers.
*/
static int pagerBeginReadTransaction(Pager *pPager){
int rc;                         /* Return code */
int changed = 0;                /* True if cache must be reset */

assert( pagerUseWal(pPager) );
assert( pPager.eState==PAGER_OPEN || pPager.eState==PAGER_READER );

/* sqlite3WalEndReadTransaction() was not called for the previous
** transaction in locking_mode=EXCLUSIVE.  So call it now.  If we
** are in locking_mode=NORMAL and EndRead() was previously called,
** the duplicate call is harmless.
*/
sqlite3WalEndReadTransaction(pPager.pWal);

rc = sqlite3WalBeginReadTransaction(pPager.pWal, &changed);
if( rc!=SQLITE_OK || changed ){
pager_reset(pPager);
}

return rc;
}
#endif

    /*
    ** This function is called as part of the transition from PAGER_OPEN
    ** to PAGER_READER state to determine the size of the database file
    ** in pages (assuming the page size currently stored in Pager.pageSize).
    **
    ** If no error occurs, SQLITE_OK is returned and the size of the database
    ** in pages is stored in *pnPage. Otherwise, an error code (perhaps
    ** SQLITE_IOERR_FSTAT) is returned and *pnPage is left unmodified.
    */
    static int pagerPagecount( Pager pPager, ref Pgno pnPage )
    {
      Pgno nPage;                     /* Value to return via *pnPage */

      /* Query the WAL sub-system for the database size. The WalDbsize()
      ** function returns zero if the WAL is not open (i.e. Pager.pWal==0), or
      ** if the database size is not available. The database size is not
      ** available from the WAL sub-system if the log file is empty or
      ** contains no valid committed transactions.
      */
      Debug.Assert( pPager.eState == PAGER_OPEN );
      Debug.Assert( pPager.eLock >= SHARED_LOCK || pPager.noReadlock != 0 );
      nPage = sqlite3WalDbsize( pPager.pWal );

      /* If the database size was not available from the WAL sub-system,
      ** determine it based on the size of the database file. If the size
      ** of the database file is not an integer multiple of the page-size,
      ** round down to the nearest page. Except, any file larger than 0
      ** bytes in size is considered to contain at least one page.
      */
      if ( nPage == 0 )
      {
        i64 n = 0;                    /* Size of db file in bytes */
        Debug.Assert( isOpen( pPager.fd ) || pPager.tempFile );
        if ( isOpen( pPager.fd ) )
        {
          int rc = sqlite3OsFileSize( pPager.fd, ref n );
          if ( rc != SQLITE_OK )
          {
            return rc;
          }
        }
        nPage = (Pgno)( n / pPager.pageSize );
        if ( nPage == 0 && n > 0 )
        {
          nPage = 1;
        }
      }

      /* If the current number of pages in the file is greater than the
      ** configured maximum pager number, increase the allowed limit so
      ** that the file can be read.
      */
      if ( nPage > pPager.mxPgno )
      {
        pPager.mxPgno = (Pgno)nPage;
      }

      pnPage = nPage;
      return SQLITE_OK;
    }


#if !SQLITE_OMIT_WAL
/*
** Check if the *-wal file that corresponds to the database opened by pPager
** exists if the database is not empy, or verify that the *-wal file does
** not exist (by deleting it) if the database file is empty.
**
** If the database is not empty and the *-wal file exists, open the pager
** in WAL mode.  If the database is empty or if no *-wal file exists and
** if no error occurs, make sure Pager.journalMode is not set to
** PAGER_JOURNALMODE_WAL.
**
** Return SQLITE_OK or an error code.
**
** The caller must hold a SHARED lock on the database file to call this
** function. Because an EXCLUSIVE lock on the db file is required to delete 
** a WAL on a none-empty database, this ensures there is no race condition 
** between the xAccess() below and an xDelete() being executed by some 
** other connection.
*/
static int pagerOpenWalIfPresent(Pager *pPager){
int rc = SQLITE_OK;
Debug.Assert( pPager.eState==PAGER_OPEN );
Debug.Assert( pPager.eLock>=SHARED_LOCK || pPager.noReadlock );

if( !pPager.tempFile ){
int isWal;                    /* True if WAL file exists */
Pgno nPage;                   /* Size of the database file */

rc = pagerPagecount(pPager, &nPage);
if( rc ) return rc;
if( nPage==0 ){
rc = sqlite3OsDelete(pPager.pVfs, pPager.zWal, 0);
isWal = 0;
}else{
rc = sqlite3OsAccess(
pPager.pVfs, pPager.zWal, SQLITE_ACCESS_EXISTS, &isWal
);
}
if( rc==SQLITE_OK ){
if( isWal ){
testcase( sqlite3PcachePagecount(pPager.pPCache)==0 );
rc = sqlite3PagerOpenWal(pPager, 0);
}else if( pPager.journalMode==PAGER_JOURNALMODE_WAL ){
pPager.journalMode = PAGER_JOURNALMODE_DELETE;
}
}
}
return rc;
}
#endif

    /*
** Playback savepoint pSavepoint. Or, if pSavepoint==NULL, then playback
** the entire master journal file. The case pSavepoint==NULL occurs when
** a ROLLBACK TO command is invoked on a SAVEPOINT that is a transaction
** savepoint.
**
** When pSavepoint is not NULL (meaning a non-transaction savepoint is
** being rolled back), then the rollback consists of up to three stages,
** performed in the order specified:
**
**   * Pages are played back from the main journal starting at byte
**     offset PagerSavepoint.iOffset and continuing to
**     PagerSavepoint.iHdrOffset, or to the end of the main journal
**     file if PagerSavepoint.iHdrOffset is zero.
**
**   * If PagerSavepoint.iHdrOffset is not zero, then pages are played
**     back starting from the journal header immediately following
**     PagerSavepoint.iHdrOffset to the end of the main journal file.
**
**   * Pages are then played back from the sub-journal file, starting
**     with the PagerSavepoint.iSubRec and continuing to the end of
**     the journal file.
**
** Throughout the rollback process, each time a page is rolled back, the
** corresponding bit is set in a bitvec structure (variable pDone in the
** implementation below). This is used to ensure that a page is only
** rolled back the first time it is encountered in either journal.
**
** If pSavepoint is NULL, then pages are only played back from the main
** journal file. There is no need for a bitvec in this case.
**
** In either case, before playback commences the Pager.dbSize variable
** is reset to the value that it held at the start of the savepoint
** (or transaction). No page with a page-number greater than this value
** is played back. If one is encountered it is simply skipped.
*/
    static int pagerPlaybackSavepoint( Pager pPager, PagerSavepoint pSavepoint )
    {
      i64 szJ;                 /* Effective size of the main journal */
      i64 iHdrOff;             /* End of first segment of main-journal records */
      int rc = SQLITE_OK;      /* Return code */
      Bitvec pDone = null;     /* Bitvec to ensure pages played back only once */

      Debug.Assert( pPager.eState != PAGER_ERROR );
      Debug.Assert( pPager.eState >= PAGER_WRITER_LOCKED );


      /* Allocate a bitvec to use to store the set of pages rolled back */
      if ( pSavepoint != null )
      {
        pDone = sqlite3BitvecCreate( pSavepoint.nOrig );
        //if ( null == pDone )
        //{
        //  return SQLITE_NOMEM;
        //}
      }

      /* Set the database size back to the value it was before the savepoint
      ** being reverted was opened.
      */
      pPager.dbSize = pSavepoint != null ? pSavepoint.nOrig : pPager.dbOrigSize;
      pPager.changeCountDone = pPager.tempFile;

      if ( !pSavepoint && pagerUseWal( pPager ) )
      {
        return pagerRollbackWal( pPager );
      }

      /* Use pPager.journalOff as the effective size of the main rollback
      ** journal.  The actual file might be larger than this in
      ** PAGER_JOURNALMODE_TRUNCATE or PAGER_JOURNALMODE_PERSIST.  But anything
      ** past pPager.journalOff is off-limits to us.
      */
      szJ = pPager.journalOff;
      Debug.Assert( pagerUseWal( pPager ) == false || szJ == 0 );

      /* Begin by rolling back records from the main journal starting at
      ** PagerSavepoint.iOffset and continuing to the next journal header.
      ** There might be records in the main journal that have a page number
      ** greater than the current database size (pPager.dbSize) but those
      ** will be skipped automatically.  Pages are added to pDone as they
      ** are played back.
      */
      if ( pSavepoint != null && !pagerUseWal( pPager ) )
      {
        iHdrOff = pSavepoint.iHdrOffset != 0 ? pSavepoint.iHdrOffset : szJ;
        pPager.journalOff = pSavepoint.iOffset;
        while ( rc == SQLITE_OK && pPager.journalOff < iHdrOff )
        {
          rc = pager_playback_one_page( pPager, ref pPager.journalOff, pDone, 1, 1 );
        }
        Debug.Assert( rc != SQLITE_DONE );
      }
      else
      {
        pPager.journalOff = 0;
      }

      /* Continue rolling back records out of the main journal starting at
      ** the first journal header seen and continuing until the effective end
      ** of the main journal file.  Continue to skip out-of-range pages and
      ** continue adding pages rolled back to pDone.
      */
      while ( rc == SQLITE_OK && pPager.journalOff < szJ )
      {
        u32 ii;            /* Loop counter */
        u32 nJRec;         /* Number of Journal Records */
        u32 dummy;
        rc = readJournalHdr( pPager, 0, (int)szJ, out nJRec, out dummy );
        Debug.Assert( rc != SQLITE_DONE );

        /*
        ** The "pPager.journalHdr+JOURNAL_HDR_SZ(pPager)==pPager.journalOff"
        ** test is related to ticket #2565.  See the discussion in the
        ** pager_playback() function for additional information.
        */
        if ( nJRec == 0
        && pPager.journalHdr + JOURNAL_HDR_SZ( pPager ) >= pPager.journalOff
        )
        {
          nJRec = (u32)( ( szJ - pPager.journalOff ) / JOURNAL_PG_SZ( pPager ) );
        }
        for ( ii = 0; rc == SQLITE_OK && ii < nJRec && pPager.journalOff < szJ; ii++ )
        {
          rc = pager_playback_one_page( pPager, ref pPager.journalOff, pDone, 1, 1 );
        }
        Debug.Assert( rc != SQLITE_DONE );
      }
      Debug.Assert( rc != SQLITE_OK || pPager.journalOff >= szJ );

      /* Finally,  rollback pages from the sub-journal.  Page that were
      ** previously rolled back out of the main journal (and are hence in pDone)
      ** will be skipped.  Out-of-range pages are also skipped.
      */
      if ( pSavepoint != null )
      {
        u32 ii;            /* Loop counter */
        i64 offset = pSavepoint.iSubRec * ( 4 + pPager.pageSize );
        if ( pagerUseWal( pPager ) )
        {
          rc = sqlite3WalSavepointUndo( pPager.pWal, pSavepoint.aWalData );
        }
        for ( ii = pSavepoint.iSubRec; rc == SQLITE_OK && ii < pPager.nSubRec; ii++ )
        {
          Debug.Assert( offset == ii * ( 4 + pPager.pageSize ) );
          rc = pager_playback_one_page( pPager, ref offset, pDone, 0, 1 );
        }
        Debug.Assert( rc != SQLITE_DONE );
      }

      sqlite3BitvecDestroy( ref pDone );
      if ( rc == SQLITE_OK )
      {
        pPager.journalOff = (int)szJ;
      }
      return rc;
    }

    /*
    ** Change the maximum number of in-memory pages that are allowed.
    */
    static void sqlite3PagerSetCachesize( Pager pPager, int mxPage )
    {
      sqlite3PcacheSetCachesize( pPager.pPCache, mxPage );
    }

    /*
    ** Adjust the robustness of the database to damage due to OS crashes
    ** or power failures by changing the number of syncs()s when writing
    ** the rollback journal.  There are three levels:
    **
    **    OFF       sqlite3OsSync() is never called.  This is the default
    **              for temporary and transient files.
    **
    **    NORMAL    The journal is synced once before writes begin on the
    **              database.  This is normally adequate protection, but
    **              it is theoretically possible, though very unlikely,
    **              that an inopertune power failure could leave the journal
    **              in a state which would cause damage to the database
    **              when it is rolled back.
    **
    **    FULL      The journal is synced twice before writes begin on the
    **              database (with some additional information - the nRec field
    **              of the journal header - being written in between the two
    **              syncs).  If we assume that writing a
    **              single disk sector is atomic, then this mode provides
    **              assurance that the journal will not be corrupted to the
    **              point of causing damage to the database during rollback.
    **
    ** The above is for a rollback-journal mode.  For WAL mode, OFF continues
    ** to mean that no syncs ever occur.  NORMAL means that the WAL is synced
    ** prior to the start of checkpoint and that the database file is synced
    ** at the conclusion of the checkpoint if the entire content of the WAL
    ** was written back into the database.  But no sync operations occur for
    ** an ordinary commit in NORMAL mode with WAL.  FULL means that the WAL
    ** file is synced following each commit operation, in addition to the
    ** syncs associated with NORMAL.
    **
    ** Do not confuse synchronous=FULL with SQLITE_SYNC_FULL.  The
    ** SQLITE_SYNC_FULL macro means to use the MacOSX-style full-fsync
    ** using fcntl(F_FULLFSYNC).  SQLITE_SYNC_NORMAL means to do an
    ** ordinary fsync() call.  There is no difference between SQLITE_SYNC_FULL
    ** and SQLITE_SYNC_NORMAL on platforms other than MacOSX.  But the
    ** synchronous=FULL versus synchronous=NORMAL setting determines when
    ** the xSync primitive is called and is relevant to all platforms.
    **
    ** Numeric values associated with these states are OFF==1, NORMAL=2,
    ** and FULL=3.
    */
#if !SQLITE_OMIT_PAGER_PRAGMAS
    static void sqlite3PagerSetSafetyLevel(
    Pager pPager,         /* The pager to set safety level for */
    int level,            /* PRAGMA synchronous.  1=OFF, 2=NORMAL, 3=FULL */
    int bFullFsync,       /* PRAGMA fullfsync */
    int bCkptFullFsync    /* PRAGMA checkpoint_fullfsync */
    )
    {
      Debug.Assert( level >= 1 && level <= 3 );

      pPager.noSync = ( level == 1 || pPager.tempFile );
      pPager.fullSync = ( level == 3 && !pPager.tempFile );
      if ( pPager.noSync )
      {
        pPager.syncFlags = 0;
        pPager.ckptSyncFlags = 0;
      }
      else if ( bFullFsync != 0 )
      {
        pPager.syncFlags = SQLITE_SYNC_FULL;
        pPager.ckptSyncFlags = SQLITE_SYNC_FULL;
      }
      else if ( bCkptFullFsync != 0 )
      {
        pPager.syncFlags = SQLITE_SYNC_NORMAL;
        pPager.ckptSyncFlags = SQLITE_SYNC_FULL;
      }
      else
      {
        pPager.syncFlags = SQLITE_SYNC_NORMAL;
        pPager.ckptSyncFlags = SQLITE_SYNC_NORMAL;
      }
    }
#endif

/*
** The following global variable is incremented whenever the library
** attempts to open a temporary file.  This information is used for
** testing and analysis only.
*/
#if SQLITE_TEST
#if !TCLSH
    static int sqlite3_opentemp_count = 0;
#else
    static tcl.lang.Var.SQLITE3_GETSET sqlite3_opentemp_count = new tcl.lang.Var.SQLITE3_GETSET( "sqlite3_opentemp_count" );
#endif
#endif

    /*
** Open a temporary file.
**
** Write the file descriptor into *pFile. Return SQLITE_OK on success
** or some other error code if we fail. The OS will automatically
** delete the temporary file when it is closed.
**
** The flags passed to the VFS layer xOpen() call are those specified
** by parameter vfsFlags ORed with the following:
**
**     SQLITE_OPEN_READWRITE
**     SQLITE_OPEN_CREATE
**     SQLITE_OPEN_EXCLUSIVE
**     SQLITE_OPEN_DELETEONCLOSE
*/
    static int pagerOpentemp(
    Pager pPager,           /* The pager object */
    ref sqlite3_file pFile, /* Write the file descriptor here */
    int vfsFlags            /* Flags passed through to the VFS */
    )
    {
      int rc;               /* Return code */

#if SQLITE_TEST
#if !TCLSH
      sqlite3_opentemp_count++;  /* Used for testing and analysis only */
#else
      sqlite3_opentemp_count.iValue++;  /* Used for testing and analysis only */
#endif
#endif

      vfsFlags |= SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE |
      SQLITE_OPEN_EXCLUSIVE | SQLITE_OPEN_DELETEONCLOSE;
      int dummy = 0;
      rc = sqlite3OsOpen( pPager.pVfs, null, pFile, vfsFlags, ref dummy );
      Debug.Assert( rc != SQLITE_OK || isOpen( pFile ) );
      return rc;
    }

    /*
    ** Set the busy handler function.
    **
    ** The pager invokes the busy-handler if sqlite3OsLock() returns
    ** SQLITE_BUSY when trying to upgrade from no-lock to a SHARED lock,
    ** or when trying to upgrade from a RESERVED lock to an EXCLUSIVE
    ** lock. It does *not* invoke the busy handler when upgrading from
    ** SHARED to RESERVED, or when upgrading from SHARED to EXCLUSIVE
    ** (which occurs during hot-journal rollback). Summary:
    **
    **   Transition                        | Invokes xBusyHandler
    **   --------------------------------------------------------
    **   NO_LOCK       . SHARED_LOCK      | Yes
    **   SHARED_LOCK   . RESERVED_LOCK    | No
    **   SHARED_LOCK   . EXCLUSIVE_LOCK   | No
    **   RESERVED_LOCK . EXCLUSIVE_LOCK   | Yes
    **
    ** If the busy-handler callback returns non-zero, the lock is
    ** retried. If it returns zero, then the SQLITE_BUSY error is
    ** returned to the caller of the pager API function.
    */

    static void sqlite3PagerSetBusyhandler(
    Pager pPager,                         /* Pager object */
    dxBusyHandler xBusyHandler,           /* Pointer to busy-handler function */
      //int (*xBusyHandler)(void *),
    object pBusyHandlerArg                /* Argument to pass to xBusyHandler */
    )
    {
      pPager.xBusyHandler = xBusyHandler;
      pPager.pBusyHandlerArg = pBusyHandlerArg;
    }


    /*
    ** Change the page size used by the Pager object. The new page size 
    ** is passed in *pPageSize.
    **
    ** If the pager is in the error state when this function is called, it
    ** is a no-op. The value returned is the error state error code (i.e. 
    ** one of SQLITE_IOERR, an SQLITE_IOERR_xxx sub-code or SQLITE_FULL).
    **
    ** Otherwise, if all of the following are true:
    **
    **   * the new page size (value of *pPageSize) is valid (a power 
    **     of two between 512 and SQLITE_MAX_PAGE_SIZE, inclusive), and
    **
    **   * there are no outstanding page references, and
    **
    **   * the database is either not an in-memory database or it is
    **     an in-memory database that currently consists of zero pages.
    **
    ** then the pager object page size is set to *pPageSize.
    **
    ** If the page size is changed, then this function uses sqlite3PagerMalloc() 
    ** to obtain a new Pager.pTmpSpace buffer. If this allocation attempt 
    ** fails, SQLITE_NOMEM is returned and the page size remains unchanged. 
    ** In all other cases, SQLITE_OK is returned.
    **
    ** If the page size is not changed, either because one of the enumerated
    ** conditions above is not true, the pager was in error state when this
    ** function was called, or because the memory allocation attempt failed, 
    ** then *pPageSize is set to the old, retained page size before returning.
    */
    static int sqlite3PagerSetPagesize( Pager pPager, ref u32 pPageSize, int nReserve )
    {
      int rc = SQLITE_OK;
      /* It is not possible to do a full assert_pager_state() here, as this
      ** function may be called from within PagerOpen(), before the state
      ** of the Pager object is internally consistent.
      **
      ** At one point this function returned an error if the pager was in 
      ** PAGER_ERROR state. But since PAGER_ERROR state guarantees that
      ** there is at least one outstanding page reference, this function
      ** is a no-op for that case anyhow.
      */

      u32 pageSize = pPageSize;
      Debug.Assert( pageSize == 0 || ( pageSize >= 512 && pageSize <= SQLITE_MAX_PAGE_SIZE ) );
      if ( ( pPager.memDb == 0 || pPager.dbSize == 0 )
      && sqlite3PcacheRefCount( pPager.pPCache ) == 0
      && pageSize != 0 && pageSize != (u32)pPager.pageSize
      )
      {
        //char *pNew = NULL;             /* New temp space */
        i64 nByte = 0;

        if ( pPager.eState > PAGER_OPEN && isOpen( pPager.fd ) )
        {
          rc = sqlite3OsFileSize( pPager.fd, ref nByte );
        }
        //if ( rc == SQLITE_OK )
        //{
        //pNew = (char *)sqlite3PageMalloc(pageSize);
        //if( !pNew ) rc = SQLITE_NOMEM;
        //}
        if ( rc == SQLITE_OK )
        {
          pager_reset( pPager );
          pPager.dbSize = (Pgno)( nByte / pageSize );
          pPager.pageSize = (int)pageSize;
          sqlite3PageFree( ref pPager.pTmpSpace );

          pPager.pTmpSpace = sqlite3Malloc( pageSize );// pNew;
          sqlite3PcacheSetPageSize( pPager.pPCache, (int)pageSize );
        }
      }
      pPageSize = (u32)pPager.pageSize;
      if ( rc == SQLITE_OK )
      {
        if ( nReserve < 0 )
          nReserve = pPager.nReserve;
        Debug.Assert( nReserve >= 0 && nReserve < 1000 );
        pPager.nReserve = (i16)nReserve;
        pagerReportSize( pPager );
      }
      return rc;
    }

    /*
    ** Return a pointer to the "temporary page" buffer held internally
    ** by the pager.  This is a buffer that is big enough to hold the
    ** entire content of a database page.  This buffer is used internally
    ** during rollback and will be overwritten whenever a rollback
    ** occurs.  But other modules are free to use it too, as long as
    ** no rollbacks are happening.
    */
    static byte[] sqlite3PagerTempSpace( Pager pPager )
    {
      return pPager.pTmpSpace;
    }

    /*
    ** Attempt to set the maximum database page count if mxPage is positive.
    ** Make no changes if mxPage is zero or negative.  And never reduce the
    ** maximum page count below the current size of the database.
    **
    ** Regardless of mxPage, return the current maximum page count.
    */
    static Pgno sqlite3PagerMaxPageCount( Pager pPager, int mxPage )
    {
      if ( mxPage > 0 )
      {
        pPager.mxPgno = (Pgno)mxPage;
      }
      Debug.Assert( pPager.eState != PAGER_OPEN );      /* Called only by OP_MaxPgcnt */
      Debug.Assert( pPager.mxPgno >= pPager.dbSize );  /* OP_MaxPgcnt enforces this */
      return pPager.mxPgno;
    }

    /*
    ** The following set of routines are used to disable the simulated
    ** I/O error mechanism.  These routines are used to avoid simulated
    ** errors in places where we do not care about errors.
    **
    ** Unless -DSQLITE_TEST=1 is used, these routines are all no-ops
    ** and generate no code.
    */
#if SQLITE_TEST
    //extern int sqlite3_io_error_pending;
    //extern int sqlite3_io_error_hit;
    static int saved_cnt;
    static void disable_simulated_io_errors()
    {
#if !TCLSH
      saved_cnt = sqlite3_io_error_pending;
      sqlite3_io_error_pending = -1;
#else
      saved_cnt = sqlite3_io_error_pending.iValue;
      sqlite3_io_error_pending.iValue = -1;
#endif
    }

    static void enable_simulated_io_errors()
    {
#if !TCLSH
      sqlite3_io_error_pending = saved_cnt;
#else
      sqlite3_io_error_pending.iValue = saved_cnt;
#endif
    }
#else
//# define disable_simulated_io_errors()
//# define enable_simulated_io_errors()
#endif

    /*
** Read the first N bytes from the beginning of the file into memory
** that pDest points to.
**
** If the pager was opened on a transient file (zFilename==""), or
** opened on a file less than N bytes in size, the output buffer is
** zeroed and SQLITE_OK returned. The rationale for this is that this
** function is used to read database headers, and a new transient or
** zero sized database has a header than consists entirely of zeroes.
**
** If any IO error apart from SQLITE_IOERR_SHORT_READ is encountered,
** the error code is returned to the caller and the contents of the
** output buffer undefined.
*/
    static int sqlite3PagerReadFileheader( Pager pPager, int N, byte[] pDest )
    {
      int rc = SQLITE_OK;
      Array.Clear( pDest, 0, N ); //memset(pDest, 0, N);
      Debug.Assert( isOpen( pPager.fd ) || pPager.tempFile );

      /* This routine is only called by btree immediately after creating
      ** the Pager object.  There has not been an opportunity to transition
      ** to WAL mode yet.
      */
      Debug.Assert( !pagerUseWal( pPager ) );

      if ( isOpen( pPager.fd ) )
      {
        IOTRACE( "DBHDR %p 0 %d\n", pPager, N );
        rc = sqlite3OsRead( pPager.fd, pDest, N, 0 );
        if ( rc == SQLITE_IOERR_SHORT_READ )
        {
          rc = SQLITE_OK;
        }
      }
      return rc;
    }

    /*
    ** This function may only be called when a read-transaction is open on
    ** the pager. It returns the total number of pages in the database.
    **
    ** However, if the file is between 1 and <page-size> bytes in size, then 
    ** this is considered a 1 page file.
    */
    static void sqlite3PagerPagecount( Pager pPager, out Pgno pnPage )
    {
      Debug.Assert( pPager.eState >= PAGER_READER );
      Debug.Assert( pPager.eState != PAGER_WRITER_FINISHED );
      pnPage = pPager.dbSize;
    }

    /*
    ** Try to obtain a lock of type locktype on the database file. If
    ** a similar or greater lock is already held, this function is a no-op
    ** (returning SQLITE_OK immediately).
    **
    ** Otherwise, attempt to obtain the lock using sqlite3OsLock(). Invoke
    ** the busy callback if the lock is currently not available. Repeat
    ** until the busy callback returns false or until the attempt to
    ** obtain the lock succeeds.
    **
    ** Return SQLITE_OK on success and an error code if we cannot obtain
    ** the lock. If the lock is obtained successfully, set the Pager.state
    ** variable to locktype before returning.
    */
    static int pager_wait_on_lock( Pager pPager, int locktype )
    {
      int rc;                              /* Return code */

      /* Check that this is either a no-op (because the requested lock is
      ** already held, or one of the transistions that the busy-handler
      ** may be invoked during, according to the comment above
      ** sqlite3PagerSetBusyhandler().
      */
      Debug.Assert( ( pPager.eLock >= locktype )
      || ( pPager.eLock == NO_LOCK && locktype == SHARED_LOCK )
      || ( pPager.eLock == RESERVED_LOCK && locktype == EXCLUSIVE_LOCK )
      );

      do
      {
        rc = pagerLockDb( pPager, locktype );
      } while ( rc == SQLITE_BUSY && pPager.xBusyHandler( pPager.pBusyHandlerArg ) != 0 );
      return rc;
    }

    /*
    ** Function assertTruncateConstraint(pPager) checks that one of the 
    ** following is true for all dirty pages currently in the page-cache:
    **
    **   a) The page number is less than or equal to the size of the 
    **      current database image, in pages, OR
    **
    **   b) if the page content were written at this time, it would not
    **      be necessary to write the current content out to the sub-journal
    **      (as determined by function subjRequiresPage()).
    **
    ** If the condition asserted by this function were not true, and the
    ** dirty page were to be discarded from the cache via the pagerStress()
    ** routine, pagerStress() would not write the current page content to
    ** the database file. If a savepoint transaction were rolled back after
    ** this happened, the correct behaviour would be to restore the current
    ** content of the page. However, since this content is not present in either
    ** the database file or the portion of the rollback journal and 
    ** sub-journal rolled back the content could not be restored and the
    ** database image would become corrupt. It is therefore fortunate that 
    ** this circumstance cannot arise.
    */
#if SQLITE_DEBUG
    static void assertTruncateConstraintCb( PgHdr pPg )
    {
      Debug.Assert( ( pPg.flags & PGHDR_DIRTY ) != 0 );
      Debug.Assert( !subjRequiresPage( pPg ) || pPg.pgno <= pPg.pPager.dbSize );
    }
    static void assertTruncateConstraint( Pager pPager )
    {
      sqlite3PcacheIterateDirty( pPager.pPCache, assertTruncateConstraintCb );
    }
#else
//# define assertTruncateConstraint(pPager)
static void assertTruncateConstraintCb(PgHdr pPg) { }
static void assertTruncateConstraint(Pager pPager) { }
#endif

    /*
** Truncate the in-memory database file image to nPage pages. This
** function does not actually modify the database file on disk. It
** just sets the internal state of the pager object so that the
** truncation will be done when the current transaction is committed.
*/
    static void sqlite3PagerTruncateImage( Pager pPager, u32 nPage )
    {
      Debug.Assert( pPager.dbSize >= nPage );
      Debug.Assert( pPager.eState >= PAGER_WRITER_CACHEMOD );
      pPager.dbSize = nPage;
      assertTruncateConstraint( pPager );
    }


    /*
    ** This function is called before attempting a hot-journal rollback. It
    ** syncs the journal file to disk, then sets pPager.journalHdr to the
    ** size of the journal file so that the pager_playback() routine knows
    ** that the entire journal file has been synced.
    **
    ** Syncing a hot-journal to disk before attempting to roll it back ensures 
    ** that if a power-failure occurs during the rollback, the process that
    ** attempts rollback following system recovery sees the same journal
    ** content as this process.
    **
    ** If everything goes as planned, SQLITE_OK is returned. Otherwise, 
    ** an SQLite error code.
    */
    static int pagerSyncHotJournal( Pager pPager )
    {
      int rc = SQLITE_OK;
      if ( !pPager.noSync )
      {
        rc = sqlite3OsSync( pPager.jfd, SQLITE_SYNC_NORMAL );
      }
      if ( rc == SQLITE_OK )
      {
        rc = sqlite3OsFileSize( pPager.jfd, ref pPager.journalHdr );
      }
      return rc;
    }

    /*
    ** Shutdown the page cache.  Free all memory and close all files.
    **
    ** If a transaction was in progress when this routine is called, that
    ** transaction is rolled back.  All outstanding pages are invalidated
    ** and their memory is freed.  Any attempt to use a page associated
    ** with this page cache after this function returns will likely
    ** result in a coredump.
    **
    ** This function always succeeds. If a transaction is active an attempt
    ** is made to roll it back. If an error occurs during the rollback
    ** a hot journal may be left in the filesystem but no error is returned
    ** to the caller.
    */
    static int sqlite3PagerClose( Pager pPager )
    {
#if SQLITE_TEST
      disable_simulated_io_errors();
#endif
      sqlite3BeginBenignMalloc();
      /* pPager.errCode = 0; */
      pPager.exclusiveMode = false;
#if !SQLITE_OMIT_WAL
      u8[] pTmp = pPager.pTmpSpace;
      sqlite3WalClose(pPager->pWal, pPager->ckptSyncFlags, pPager->pageSize, pTmp);
      pPager.pWal = 0;
#endif
      pager_reset( pPager );
#if SQLITE_OMIT_MEMORYDB
      if ( 1 == MEMDB )
#else
      if ( 1 == pPager.memDb )
#endif
      {
        pager_unlock( pPager );
      }
      else
      {
        /* If it is open, sync the journal file before calling UnlockAndRollback.
        ** If this is not done, then an unsynced portion of the open journal 
        ** file may be played back into the database. If a power failure occurs 
        ** while this is happening, the database could become corrupt.
        **
        ** If an error occurs while trying to sync the journal, shift the pager
        ** into the ERROR state. This causes UnlockAndRollback to unlock the
        ** database and close the journal file without attempting to roll it
        ** back or finalize it. The next database user will have to do hot-journal
        ** rollback before accessing the database file.
        */
        if ( isOpen( pPager.jfd ) )
        {
          pager_error( pPager, pagerSyncHotJournal( pPager ) );
        }
        pagerUnlockAndRollback( pPager );
      }
      sqlite3EndBenignMalloc();
#if SQLITE_TEST
      enable_simulated_io_errors();
#endif

      PAGERTRACE( "CLOSE %d\n", PAGERID( pPager ) );
      IOTRACE( "CLOSE %p\n", pPager );
      sqlite3OsClose( pPager.jfd );
      sqlite3OsClose( pPager.fd );
      //sqlite3_free( ref pTmp );
      sqlite3PcacheClose( pPager.pPCache );

#if SQLITE_HAS_CODEC
      if ( pPager.xCodecFree != null )
        pPager.xCodecFree( ref pPager.pCodec );
#endif
      Debug.Assert( null == pPager.aSavepoint && !pPager.pInJournal );
      Debug.Assert( !isOpen( pPager.jfd ) && !isOpen( pPager.sjfd ) );

      //sqlite3_free( ref pPager );
      return SQLITE_OK;
    }

#if !NDEBUG || SQLITE_TEST
    /*
** Return the page number for page pPg.
*/
    static Pgno sqlite3PagerPagenumber( DbPage pPg )
    {
      return pPg.pgno;
    }
#else
static Pgno sqlite3PagerPagenumber( DbPage pPg )    {      return pPg.pgno;    }
#endif


    /*
** Increment the reference count for page pPg.
*/
    static void sqlite3PagerRef( DbPage pPg )
    {
      sqlite3PcacheRef( pPg );
    }

    /*
    ** Sync the journal. In other words, make sure all the pages that have
    ** been written to the journal have actually reached the surface of the
    ** disk and can be restored in the event of a hot-journal rollback.
    **
    ** If the Pager.noSync flag is set, then this function is a no-op.
    ** Otherwise, the actions required depend on the journal-mode and the 
    ** device characteristics of the the file-system, as follows:
    **
    **   * If the journal file is an in-memory journal file, no action need
    **     be taken.
    **
    **   * Otherwise, if the device does not support the SAFE_APPEND property,
    **     then the nRec field of the most recently written journal header
    **     is updated to contain the number of journal records that have
    **     been written following it. If the pager is operating in full-sync
    **     mode, then the journal file is synced before this field is updated.
    **
    **   * If the device does not support the SEQUENTIAL property, then 
    **     journal file is synced.
    **
    ** Or, in pseudo-code:
    **
    **   if( NOT <in-memory journal> ){
    **     if( NOT SAFE_APPEND ){
    **       if( <full-sync mode> ) xSync(<journal file>);
    **       <update nRec field>
    **     } 
    **     if( NOT SEQUENTIAL ) xSync(<journal file>);
    **   }
    **
    ** If successful, this routine clears the PGHDR_NEED_SYNC flag of every 
    ** page currently held in memory before returning SQLITE_OK. If an IO
    ** error is encountered, then the IO error code is returned to the caller.
    */
    static int syncJournal( Pager pPager, int newHdr )
    {
      int rc = SQLITE_OK;
      Debug.Assert( pPager.eState == PAGER_WRITER_CACHEMOD
      || pPager.eState == PAGER_WRITER_DBMOD
      );
      Debug.Assert( assert_pager_state( pPager ) );
      Debug.Assert( !pagerUseWal( pPager ) );

      rc = sqlite3PagerExclusiveLock( pPager );
      if ( rc != SQLITE_OK )
        return rc;

      if ( !pPager.noSync )
      {
        Debug.Assert( !pPager.tempFile );
        if ( isOpen( pPager.jfd ) && pPager.journalMode != PAGER_JOURNALMODE_MEMORY )
        {
          int iDc = sqlite3OsDeviceCharacteristics( pPager.fd );
          Debug.Assert( isOpen( pPager.jfd ) );

          if ( 0 == ( iDc & SQLITE_IOCAP_SAFE_APPEND ) )
          {
            /* This block deals with an obscure problem. If the last connection
            ** that wrote to this database was operating in persistent-journal
            ** mode, then the journal file may at this point actually be larger
            ** than Pager.journalOff bytes. If the next thing in the journal
            ** file happens to be a journal-header (written as part of the
            ** previous connection's transaction), and a crash or power-failure
            ** occurs after nRec is updated but before this connection writes
            ** anything else to the journal file (or commits/rolls back its
            ** transaction), then SQLite may become confused when doing the
            ** hot-journal rollback following recovery. It may roll back all
            ** of this connections data, then proceed to rolling back the old,
            ** out-of-date data that follows it. Database corruption.
            **
            ** To work around this, if the journal file does appear to contain
            ** a valid header following Pager.journalOff, then write a 0x00
            ** byte to the start of it to prevent it from being recognized.
            **
            ** Variable iNextHdrOffset is set to the offset at which this
            ** problematic header will occur, if it exists. aMagic is used
            ** as a temporary buffer to inspect the first couple of bytes of
            ** the potential journal header.
            */
            i64 iNextHdrOffset;
            u8[] aMagic = new u8[8];
            u8[] zHeader = new u8[aJournalMagic.Length + 4];
            aJournalMagic.CopyTo( zHeader, 0 );// memcpy(zHeader, aJournalMagic, sizeof(aJournalMagic));
            put32bits( zHeader, aJournalMagic.Length, pPager.nRec );
            iNextHdrOffset = journalHdrOffset( pPager );
            rc = sqlite3OsRead( pPager.jfd, aMagic, 8, iNextHdrOffset );
            if ( rc == SQLITE_OK && 0 == memcmp( aMagic, aJournalMagic, 8 ) )
            {
              u8[] zerobyte = new u8[1];
              rc = sqlite3OsWrite( pPager.jfd, zerobyte, 1, iNextHdrOffset );
            }
            if ( rc != SQLITE_OK && rc != SQLITE_IOERR_SHORT_READ )
            {
              return rc;
            }

            /* Write the nRec value into the journal file header. If in
            ** full-synchronous mode, sync the journal first. This ensures that
            ** all data has really hit the disk before nRec is updated to mark
            ** it as a candidate for rollback.
            **
            ** This is not required if the persistent media supports the
            ** SAFE_APPEND property. Because in this case it is not possible
            ** for garbage data to be appended to the file, the nRec field
            ** is populated with 0xFFFFFFFF when the journal header is written
            ** and never needs to be updated.
            */
            if ( pPager.fullSync && 0 == ( iDc & SQLITE_IOCAP_SEQUENTIAL ) )
            {

              PAGERTRACE( "SYNC journal of %d\n", PAGERID( pPager ) );
              IOTRACE( "JSYNC %p\n", pPager );
              rc = sqlite3OsSync( pPager.jfd, pPager.syncFlags );
              if ( rc != SQLITE_OK )
                return rc;
            }
            IOTRACE( "JHDR %p %lld\n", pPager, pPager.journalHdr );
            rc = sqlite3OsWrite(
            pPager.jfd, zHeader, zHeader.Length, pPager.journalHdr
            );
            if ( rc != SQLITE_OK )
              return rc;
          }
          if ( 0 == ( iDc & SQLITE_IOCAP_SEQUENTIAL ) )
          {

            PAGERTRACE( "SYNC journal of %d\n", PAGERID( pPager ) );
            IOTRACE( "JSYNC %p\n", pPager );
            rc = sqlite3OsSync( pPager.jfd, pPager.syncFlags |
            ( pPager.syncFlags == SQLITE_SYNC_FULL ? SQLITE_SYNC_DATAONLY : 0 )
            );
            if ( rc != SQLITE_OK )
              return rc;
          }
          pPager.journalHdr = pPager.journalOff;
          if ( newHdr != 0 && 0 == ( iDc & SQLITE_IOCAP_SAFE_APPEND ) )
          {
            pPager.nRec = 0;
            rc = writeJournalHdr( pPager );
            if ( rc != SQLITE_OK )
              return rc;
          }
        }
        else
        {
          pPager.journalHdr = pPager.journalOff;
        }
      }
      /* Unless the pager is in noSync mode, the journal file was just 
      ** successfully synced. Either way, clear the PGHDR_NEED_SYNC flag on 
      ** all pages.
      */
      sqlite3PcacheClearSyncFlags( pPager.pPCache );
      pPager.eState = PAGER_WRITER_DBMOD;
      Debug.Assert( assert_pager_state( pPager ) );
      return SQLITE_OK;
    }

    /*
    ** The argument is the first in a linked list of dirty pages connected
    ** by the PgHdr.pDirty pointer. This function writes each one of the
    ** in-memory pages in the list to the database file. The argument may
    ** be NULL, representing an empty list. In this case this function is
    ** a no-op.
    **
    ** The pager must hold at least a RESERVED lock when this function
    ** is called. Before writing anything to the database file, this lock
    ** is upgraded to an EXCLUSIVE lock. If the lock cannot be obtained,
    ** SQLITE_BUSY is returned and no data is written to the database file.
    **
    ** If the pager is a temp-file pager and the actual file-system file
    ** is not yet open, it is created and opened before any data is
    ** written out.
    **
    ** Once the lock has been upgraded and, if necessary, the file opened,
    ** the pages are written out to the database file in list order. Writing
    ** a page is skipped if it meets either of the following criteria:
    **
    **   * The page number is greater than Pager.dbSize, or
    **   * The PGHDR_DONT_WRITE flag is set on the page.
    **
    ** If writing out a page causes the database file to grow, Pager.dbFileSize
    ** is updated accordingly. If page 1 is written out, then the value cached
    ** in Pager.dbFileVers[] is updated to match the new value stored in
    ** the database file.
    **
    ** If everything is successful, SQLITE_OK is returned. If an IO error
    ** occurs, an IO error code is returned. Or, if the EXCLUSIVE lock cannot
    ** be obtained, SQLITE_BUSY is returned.
    */
    static int pager_write_pagelist( Pager pPager, PgHdr pList )
    {
      int rc = SQLITE_OK;                  /* Return code */

      /* This function is only called for rollback pagers in WRITER_DBMOD state. */
      Debug.Assert( !pagerUseWal( pPager ) );
      Debug.Assert( pPager.eState == PAGER_WRITER_DBMOD );
      Debug.Assert( pPager.eLock == EXCLUSIVE_LOCK );
      /* If the file is a temp-file has not yet been opened, open it now. It
      ** is not possible for rc to be other than SQLITE_OK if this branch
      ** is taken, as pager_wait_on_lock() is a no-op for temp-files.
      */
      if ( !isOpen( pPager.fd ) )
      {
        Debug.Assert( pPager.tempFile && rc == SQLITE_OK );
        rc = pagerOpentemp( pPager, ref pPager.fd, (int)pPager.vfsFlags );
      }

      /* Before the first write, give the VFS a hint of what the final
      ** file size will be.
      */
      Debug.Assert( rc != SQLITE_OK || isOpen( pPager.fd ) );
      if ( rc == SQLITE_OK && pPager.dbSize > pPager.dbHintSize )
      {
        sqlite3_int64 szFile = pPager.pageSize * (sqlite3_int64)pPager.dbSize;
        sqlite3OsFileControl( pPager.fd, SQLITE_FCNTL_SIZE_HINT, ref szFile );
        pPager.dbHintSize = pPager.dbSize;
      }
      while ( rc == SQLITE_OK && pList )
      {
        Pgno pgno = pList.pgno;

        /* If there are dirty pages in the page cache with page numbers greater
        ** than Pager.dbSize, this means sqlite3PagerTruncateImage() was called to
        ** make the file smaller (presumably by auto-vacuum code). Do not write
        ** any such pages to the file.
        **
        ** Also, do not write out any page that has the PGHDR_DONT_WRITE flag
        ** set (set by sqlite3PagerDontWrite()).
        */
        if ( pList.pgno <= pPager.dbSize && 0 == ( pList.flags & PGHDR_DONT_WRITE ) )
        {
          i64 offset = ( pList.pgno - 1 ) * (i64)pPager.pageSize;      /* Offset to write */
          byte[] pData = null;                                   /* Data to write */

          Debug.Assert( ( pList.flags & PGHDR_NEED_SYNC ) == 0 );
          if ( pList.pgno == 1 )
            pager_write_changecounter( pList );

          /* Encode the database */
          if ( CODEC2( pPager, pList.pData, pgno, SQLITE_ENCRYPT_WRITE_CTX, ref pData ) )
            return SQLITE_NOMEM;//     CODEC2(pPager, pList.pData, pgno, 6, return SQLITE_NOMEM, pData);

          /* Write out the page data. */
          rc = sqlite3OsWrite( pPager.fd, pData, pPager.pageSize, offset );
          /* If page 1 was just written, update Pager.dbFileVers to match
          ** the value now stored in the database file. If writing this
          ** page caused the database file to grow, update dbFileSize.
          */
          if ( pgno == 1 )
          {
            Buffer.BlockCopy( pData, 24, pPager.dbFileVers, 0, pPager.dbFileVers.Length );// memcpy(pPager.dbFileVers, pData[24], pPager.dbFileVers).Length;
          }
          if ( pgno > pPager.dbFileSize )
          {
            pPager.dbFileSize = pgno;
          }
          /* Update any backup objects copying the contents of this pager. */
          sqlite3BackupUpdate( pPager.pBackup, pgno, pList.pData );


          PAGERTRACE( "STORE %d page %d hash(%08x)\n",
          PAGERID( pPager ), pgno, pager_pagehash( pList ) );
          IOTRACE( "PGOUT %p %d\n", pPager, pgno );
#if SQLITE_TEST
#if !TCLSH
          PAGER_INCR( ref sqlite3_pager_writedb_count );
#else
          int iValue;
          iValue = sqlite3_pager_writedb_count.iValue;
          PAGER_INCR( ref iValue );
          sqlite3_pager_writedb_count.iValue = iValue;
#endif

          PAGER_INCR( ref pPager.nWrite );
#endif
        }
        else
        {

          PAGERTRACE( "NOSTORE %d page %d\n", PAGERID( pPager ), pgno );
        }
        pager_set_pagehash( pList );
        pList = pList.pDirty;
      }
      return rc;
    }

    /*
    ** Ensure that the sub-journal file is open. If it is already open, this 
    ** function is a no-op.
    **
    ** SQLITE_OK is returned if everything goes according to plan. An 
    ** SQLITE_IOERR_XXX error code is returned if a call to sqlite3OsOpen() 
    ** fails.
    */
    static int openSubJournal( Pager pPager )
    {
      int rc = SQLITE_OK;
      if ( !isOpen( pPager.sjfd ) )
      {
        if ( pPager.journalMode == PAGER_JOURNALMODE_MEMORY || pPager.subjInMemory != 0 )
        {
          sqlite3MemJournalOpen( pPager.sjfd );
        }
        else
        {
          rc = pagerOpentemp( pPager, ref pPager.sjfd, SQLITE_OPEN_SUBJOURNAL );
        }
      }
      return rc;
    }

    /*
    ** Append a record of the current state of page pPg to the sub-journal.
    ** It is the callers responsibility to use subjRequiresPage() to check
    ** that it is really required before calling this function.
    **
    ** If successful, set the bit corresponding to pPg.pgno in the bitvecs
    ** for all open savepoints before returning.
    **
    ** This function returns SQLITE_OK if everything is successful, an IO
    ** error code if the attempt to write to the sub-journal fails, or
    ** SQLITE_NOMEM if a malloc fails while setting a bit in a savepoint
    ** bitvec.
    */
    static int subjournalPage( PgHdr pPg )
    {
      int rc = SQLITE_OK;
      Pager pPager = pPg.pPager;
      if ( pPager.journalMode != PAGER_JOURNALMODE_OFF )
      {

        /* Open the sub-journal, if it has not already been opened */
        Debug.Assert( pPager.useJournal != 0 );
        Debug.Assert( isOpen( pPager.jfd ) || pagerUseWal( pPager ) );
        Debug.Assert( isOpen( pPager.sjfd ) || pPager.nSubRec == 0 );
        Debug.Assert( pagerUseWal( pPager )
        || pageInJournal( pPg )
        || pPg.pgno > pPager.dbOrigSize
        );
        rc = openSubJournal( pPager );

        /* If the sub-journal was opened successfully (or was already open),
        ** write the journal record into the file.  */
        if ( rc == SQLITE_OK )
        {
          byte[] pData = pPg.pData;
          i64 offset = pPager.nSubRec * ( 4 + pPager.pageSize );
          byte[] pData2 = null;

          if ( CODEC2( pPager, pData, pPg.pgno, SQLITE_ENCRYPT_READ_CTX, ref pData2 ) )
            return SQLITE_NOMEM;//CODEC2(pPager, pData, pPg.pgno, 7, return SQLITE_NOMEM, pData2);
          PAGERTRACE( "STMT-JOURNAL %d page %d\n", PAGERID( pPager ), pPg.pgno );
          rc = write32bits( pPager.sjfd, offset, pPg.pgno );
          if ( rc == SQLITE_OK )
          {
            rc = sqlite3OsWrite( pPager.sjfd, pData2, pPager.pageSize, offset + 4 );
          }
        }
      }
      if ( rc == SQLITE_OK )
      {
        pPager.nSubRec++;
        Debug.Assert( pPager.nSavepoint > 0 );
        rc = addToSavepointBitvecs( pPager, pPg.pgno );
      }
      return rc;
    }

    /*
    ** This function is called by the pcache layer when it has reached some
    ** soft memory limit. The first argument is a pointer to a Pager object
    ** (cast as a void*). The pager is always 'purgeable' (not an in-memory
    ** database). The second argument is a reference to a page that is
    ** currently dirty but has no outstanding references. The page
    ** is always associated with the Pager object passed as the first
    ** argument.
    **
    ** The job of this function is to make pPg clean by writing its contents
    ** out to the database file, if possible. This may involve syncing the
    ** journal file.
    **
    ** If successful, sqlite3PcacheMakeClean() is called on the page and
    ** SQLITE_OK returned. If an IO error occurs while trying to make the
    ** page clean, the IO error code is returned. If the page cannot be
    ** made clean for some other reason, but no error occurs, then SQLITE_OK
    ** is returned by sqlite3PcacheMakeClean() is not called.
    */
    static int pagerStress( object p, PgHdr pPg )
    {
      Pager pPager = (Pager)p;
      int rc = SQLITE_OK;

      Debug.Assert( pPg.pPager == pPager );
      Debug.Assert( ( pPg.flags & PGHDR_DIRTY ) != 0 );

      /* The doNotSyncSpill flag is set during times when doing a sync of
      ** journal (and adding a new header) is not allowed.  This occurs
      ** during calls to sqlite3PagerWrite() while trying to journal multiple
      ** pages belonging to the same sector.
      **
      ** The doNotSpill flag inhibits all cache spilling regardless of whether
      ** or not a sync is required.  This is set during a rollback.
      **
      ** Spilling is also prohibited when in an error state since that could
      ** lead to database corruption.   In the current implementaton it 
      ** is impossible for sqlite3PCacheFetch() to be called with createFlag==1
      ** while in the error state, hence it is impossible for this routine to
      ** be called in the error state.  Nevertheless, we include a NEVER()
      ** test for the error state as a safeguard against future changes.
      */
      if ( NEVER( pPager.errCode != 0 ) )
        return SQLITE_OK;
      if ( pPager.doNotSpill != 0 )
        return SQLITE_OK;
      if ( pPager.doNotSyncSpill != 0 && ( pPg.flags & PGHDR_NEED_SYNC ) != 0 )
      {
        return SQLITE_OK;
      }
      pPg.pDirty = null;
      if ( pagerUseWal( pPager ) )
      {
        /* Write a single frame for this page to the log. */
        if ( subjRequiresPage( pPg ) )
        {
          rc = subjournalPage( pPg );
        }
        if ( rc == SQLITE_OK )
        {
          rc = pagerWalFrames( pPager, pPg, 0, 0, 0 );
        }
      }
      else
      {

        /* Sync the journal file if required. */
        if ( ( pPg.flags & PGHDR_NEED_SYNC ) != 0
        || pPager.eState == PAGER_WRITER_CACHEMOD
        )
        {
          rc = syncJournal( pPager, 1 );
        }

        /* If the page number of this page is larger than the current size of
        ** the database image, it may need to be written to the sub-journal.
        ** This is because the call to pager_write_pagelist() below will not
        ** actually write data to the file in this case.
        **
        ** Consider the following sequence of events:
        **
        **   BEGIN;
        **     <journal page X>
        **     <modify page X>
        **     SAVEPOINT sp;
        **       <shrink database file to Y pages>
        **       pagerStress(page X)
        **     ROLLBACK TO sp;
        **
        ** If (X>Y), then when pagerStress is called page X will not be written
        ** out to the database file, but will be dropped from the cache. Then,
        ** following the "ROLLBACK TO sp" statement, reading page X will read
        ** data from the database file. This will be the copy of page X as it
        ** was when the transaction started, not as it was when "SAVEPOINT sp"
        ** was executed.
        **
        ** The solution is to write the current data for page X into the
        ** sub-journal file now (if it is not already there), so that it will
        ** be restored to its current value when the "ROLLBACK TO sp" is
        ** executed.
        */
        if ( NEVER(
        rc == SQLITE_OK && pPg.pgno > pPager.dbSize && subjRequiresPage( pPg )
        ) )
        {
          rc = subjournalPage( pPg );
        }

        /* Write the contents of the page out to the database file. */
        if ( rc == SQLITE_OK )
        {
          Debug.Assert( ( pPg.flags & PGHDR_NEED_SYNC ) == 0 );
          rc = pager_write_pagelist( pPager, pPg );
        }
      }
      /* Mark the page as clean. */
      if ( rc == SQLITE_OK )
      {
        PAGERTRACE( "STRESS %d page %d\n", PAGERID( pPager ), pPg.pgno );
        sqlite3PcacheMakeClean( pPg );
      }

      return pager_error( pPager, rc );
    }


    /*
    ** Allocate and initialize a new Pager object and put a pointer to it
    ** in *ppPager. The pager should eventually be freed by passing it
    ** to sqlite3PagerClose().
    **
    ** The zFilename argument is the path to the database file to open.
    ** If zFilename is NULL then a randomly-named temporary file is created
    ** and used as the file to be cached. Temporary files are be deleted
    ** automatically when they are closed. If zFilename is ":memory:" then
    ** all information is held in cache. It is never written to disk.
    ** This can be used to implement an in-memory database.
    **
    ** The nExtra parameter specifies the number of bytes of space allocated
    ** along with each page reference. This space is available to the user
    ** via the sqlite3PagerGetExtra() API.
    **
    ** The flags argument is used to specify properties that affect the
    ** operation of the pager. It should be passed some bitwise combination
    ** of the PAGER_OMIT_JOURNAL and PAGER_NO_READLOCK flags.
    **
    ** The vfsFlags parameter is a bitmask to pass to the flags parameter
    ** of the xOpen() method of the supplied VFS when opening files.
    **
    ** If the pager object is allocated and the specified file opened
    ** successfully, SQLITE_OK is returned and *ppPager set to point to
    ** the new pager object. If an error occurs, *ppPager is set to NULL
    ** and error code returned. This function may return SQLITE_NOMEM
    ** (sqlite3Malloc() is used to allocate memory), SQLITE_CANTOPEN or
    ** various SQLITE_IO_XXX errors.
    */
    static int sqlite3PagerOpen(
    sqlite3_vfs pVfs,        /* The virtual file system to use */
    out Pager ppPager,       /* OUT: Return the Pager structure here */
    string zFilename,        /* Name of the database file to open */
    int nExtra,              /* Extra bytes append to each in-memory page */
    int flags,               /* flags controlling this file */
    int vfsFlags,            /* flags passed through to sqlite3_vfs.xOpen() */
    dxReiniter xReinit       /* Function to reinitialize pages */
    )
    {
      u8 pPtr;
      Pager pPager = null;     /* Pager object to allocate and return */
      int rc = SQLITE_OK;      /* Return code */
      u8 tempFile = 0;         /* True for temp files (incl. in-memory files) */ // Needs to be u8 for later tests
      u8 memDb = 0;            /* True if this is an in-memory file */
      bool readOnly = false;   /* True if this is a read-only file */
      int journalFileSize;     /* Bytes to allocate for each journal fd */
      StringBuilder zPathname = null; /* Full path to database file */
      int nPathname = 0;       /* Number of bytes in zPathname */
      bool useJournal = ( flags & PAGER_OMIT_JOURNAL ) == 0; /* False to omit journal */
      bool noReadlock = ( flags & PAGER_NO_READLOCK ) != 0;  /* True to omit read-lock */
      int pcacheSize = sqlite3PcacheSize();       /* Bytes to allocate for PCache */
      u32 szPageDflt = SQLITE_DEFAULT_PAGE_SIZE;  /* Default page size */
      string zUri = null;     /* URI args to copy */
      int nUri = 0;           /* Number of bytes of URI args at *zUri */

      /* Figure out how much space is required for each journal file-handle
      ** (there are two of them, the main journal and the sub-journal). This
      ** is the maximum space required for an in-memory journal file handle
      ** and a regular journal file-handle. Note that a "regular journal-handle"
      ** may be a wrapper capable of caching the first portion of the journal
      ** file in memory to implement the atomic-write optimization (see
      ** source file journal.c).
      */
      if ( sqlite3JournalSize( pVfs ) > sqlite3MemJournalSize() )
      {
        journalFileSize = ROUND8( sqlite3JournalSize( pVfs ) );
      }
      else
      {
        journalFileSize = ROUND8( sqlite3MemJournalSize() );
      }

      /* Set the output variable to NULL in case an error occurs. */
      ppPager = null;

#if !SQLITE_OMIT_MEMORYDB
      if ( ( flags & PAGER_MEMORY ) != 0 )
      {
        memDb = 1;
        zFilename = null;
      }
#endif

      /* Compute and store the full pathname in an allocated buffer pointed
      ** to by zPathname, length nPathname. Or, if this is a temporary file,
      ** leave both nPathname and zPathname set to 0.
      */
      if ( !String.IsNullOrEmpty( zFilename ) )
      {
        nPathname = pVfs.mxPathname + 1;
        zPathname = new StringBuilder( nPathname * 2 );// sqlite3Malloc( nPathname * 2 );
        //if ( zPathname == null )
        //{
        //  return SQLITE_NOMEM;
        //}
        //zPathname[0] = 0; /* Make sure initialized even if FullPathname() fails */
        rc = sqlite3OsFullPathname( pVfs, zFilename, nPathname, zPathname );

        nPathname = sqlite3Strlen30( zPathname );
        zUri = zFilename;
        //string z = zUri = zFilename;//.Substring(sqlite3Strlen30( zFilename ) );
        //while ( *z )
        //{
        //  z += sqlite3Strlen30( z ) + 1;
        //  z += sqlite3Strlen30( z ) + 1;
        //}
        //nUri = zUri.Length;//        &z[1] - zUri;
        if ( rc == SQLITE_OK && nPathname + 8 > pVfs.mxPathname )
        {
          /* This branch is taken when the journal path required by
          ** the database being opened will be more than pVfs.mxPathname
          ** bytes in length. This means the database cannot be opened,
          ** as it will not be possible to open the journal file or even
          ** check for a hot-journal before reading.
          */
          rc = SQLITE_CANTOPEN_BKPT();
        }
        if ( rc != SQLITE_OK )
        {
          //sqlite3_free( ref zPathname );
          return rc;
        }
      }

      /* Allocate memory for the Pager structure, PCache object, the
      ** three file descriptors, the database file name and the journal
      ** file name. The layout in memory is as follows:
      **
      **     Pager object                    (sizeof(Pager) bytes)
      **     PCache object                   (sqlite3PcacheSize() bytes)
      **     Database file handle            (pVfs.szOsFile bytes)
      **     Sub-journal file handle         (journalFileSize bytes)
      **     Main journal file handle        (journalFileSize bytes)
      **     Database file name              (nPathname+1 bytes)
      **     Journal file name               (nPathname+8+1 bytes)
      */
      //pPtr = (u8 *)sqlite3MallocZero(
      //  ROUND8(sizeof(*pPager)) +           /* Pager structure */
      //  ROUND8(pcacheSize)      +           /* PCache object */
      //  ROUND8(pVfs.szOsFile)   +           /* The main db file */
      //  journalFileSize * 2 +       /* The two journal files */
      //  nPathname + 1 + nUri +         /* zFilename */
      //  nPathname + 8 + 1           /* zJournal */
      //#if !SQLITE_OMIT_WAL
      //    + nPathname + 4 + 1              /* zWal */
      //#endif
      //);
      //  Debug.Assert( EIGHT_BYTE_ALIGNMENT(SQLITE_INT_TO_PTR(journalFileSize)));
      //if( !pPtr ){
      //  //sqlite3_free(zPathname);
      //  return SQLITE_NOMEM;
      //}
      pPager = new Pager();//(Pager*)(pPtr);
      pPager.pPCache = new PCache();//(PCache*)(pPtr += ROUND8(sizeof(*pPager)));
      pPager.fd = new sqlite3_file();//(sqlite3_file*)(pPtr += ROUND8(pcacheSize));
      pPager.sjfd = new sqlite3_file();//(sqlite3_file*)(pPtr += ROUND8(pVfs.szOsFile));
      pPager.jfd = new sqlite3_file();//(sqlite3_file*)(pPtr += journalFileSize);
      //pPager.zFilename =    (char*)(pPtr += journalFileSize);
      //Debug.Assert( EIGHT_BYTE_ALIGNMENT(pPager.jfd) );

      /* Fill in the Pager.zFilename and Pager.zJournal buffers, if required. */
      if ( zPathname != null )
      {
        Debug.Assert( nPathname > 0 );
        //pPager.zJournal =   (char*)(pPtr += nPathname + 1 + nUri);
        //memcpy(pPager.zFilename, zPathname, nPathname);
        pPager.zFilename = zPathname.ToString();
        zUri = pPager.zFilename;//.Substring( nPathname + 1 );//memcpy( &pPager.zFilename[nPathname + 1], zUri, nUri );
        //memcpy(pPager.zJournal, zPathname, nPathname);
        //memcpy(&pPager.zJournal[nPathname], "-journal", 8);
        pPager.zJournal = pPager.zFilename + "-journal";
        sqlite3FileSuffix3( pPager.zFilename, pPager.zJournal );
#if !SQLITE_OMIT_WAL
pPager.zWal = &pPager.zJournal[nPathname+8+1];
memcpy(pPager.zWal, zPathname, nPathname);
memcpy(&pPager.zWal[nPathname], "-wal", 4);
        sqlite3FileSuffix3(pPager.zFilename, pPager.zWal);
#endif

        //sqlite3_free( ref zPathname );
      }
      else
      {
        pPager.zFilename = "";
      }
      pPager.pVfs = pVfs;
      pPager.vfsFlags = (u32)vfsFlags;

      /* Open the pager file.
      */
      if ( !String.IsNullOrEmpty( zFilename ) )
      {
        int fout = 0;                    /* VFS flags returned by xOpen() */
        rc = sqlite3OsOpen( pVfs, zFilename, pPager.fd, vfsFlags, ref fout );
        Debug.Assert( 0 == memDb );
        readOnly = ( fout & SQLITE_OPEN_READONLY ) != 0;

        /* If the file was successfully opened for read/write access,
        ** choose a default page size in case we have to create the
        ** database file. The default page size is the maximum of:
        **
        **    + SQLITE_DEFAULT_PAGE_SIZE,
        **    + The value returned by sqlite3OsSectorSize()
        **    + The largest page size that can be written atomically.
        */
        if ( rc == SQLITE_OK && !readOnly )
        {
          setSectorSize( pPager );
          Debug.Assert( SQLITE_DEFAULT_PAGE_SIZE <= SQLITE_MAX_DEFAULT_PAGE_SIZE );
          if ( szPageDflt < pPager.sectorSize )
          {
            if ( pPager.sectorSize > SQLITE_MAX_DEFAULT_PAGE_SIZE )
            {
              szPageDflt = SQLITE_MAX_DEFAULT_PAGE_SIZE;
            }
            else
            {
              szPageDflt = (u32)pPager.sectorSize;
            }
          }
#if SQLITE_ENABLE_ATOMIC_WRITE
{
int iDc = sqlite3OsDeviceCharacteristics(pPager.fd);
int ii;
Debug.Assert(SQLITE_IOCAP_ATOMIC512==(512>>8));
Debug.Assert(SQLITE_IOCAP_ATOMIC64K==(65536>>8));
Debug.Assert(SQLITE_MAX_DEFAULT_PAGE_SIZE<=65536);
for(ii=szPageDflt; ii<=SQLITE_MAX_DEFAULT_PAGE_SIZE; ii=ii*2){
if( iDc&(SQLITE_IOCAP_ATOMIC|(ii>>8)) ){
szPageDflt = ii;
}
}
}
#endif
        }
      }
      else
      {
        /* If a temporary file is requested, it is not opened immediately.
        ** In this case we accept the default page size and delay actually
        ** opening the file until the first call to OsWrite().
        **
        ** This branch is also run for an in-memory database. An in-memory
        ** database is the same as a temp-file that is never written out to
        ** disk and uses an in-memory rollback journal.
        */
        tempFile = 1;
        pPager.eState = PAGER_READER;
        pPager.eLock = EXCLUSIVE_LOCK;
        readOnly = ( vfsFlags & SQLITE_OPEN_READONLY ) != 0;
      }

      /* The following call to PagerSetPagesize() serves to set the value of
      ** Pager.pageSize and to allocate the Pager.pTmpSpace buffer.
      */
      if ( rc == SQLITE_OK )
      {
        Debug.Assert( pPager.memDb == 0 );
        rc = sqlite3PagerSetPagesize( pPager, ref szPageDflt, -1 );
        testcase( rc != SQLITE_OK );
      }

      /* If an error occurred in either of the blocks above, free the
      ** Pager structure and close the file.
      */
      if ( rc != SQLITE_OK )
      {
        Debug.Assert( null == pPager.pTmpSpace );
        sqlite3OsClose( pPager.fd );
        //sqlite3_free( ref pPager );
        return rc;
      }

      /* Initialize the PCache object. */
      Debug.Assert( nExtra < 1000 );
      nExtra = ROUND8( nExtra );
      sqlite3PcacheOpen( (int)szPageDflt, nExtra, 0 == memDb,
      0 == memDb ? (dxStress)pagerStress : null, pPager, pPager.pPCache );

      PAGERTRACE( "OPEN %d %s\n", FILEHANDLEID( pPager.fd ), pPager.zFilename );
      IOTRACE( "OPEN %p %s\n", pPager, pPager.zFilename );
      pPager.useJournal = (u8)( useJournal ? 1 : 0 );
      pPager.noReadlock = (u8)( noReadlock && readOnly ? 1 : 0 );
      /* pPager.stmtOpen = 0; */
      /* pPager.stmtInUse = 0; */
      /* pPager.nRef = 0; */
      /* pPager.stmtSize = 0; */
      /* pPager.stmtJSize = 0; */
      /* pPager.nPage = 0; */
      pPager.mxPgno = SQLITE_MAX_PAGE_COUNT;
      /* pPager.state = PAGER_UNLOCK; */
#if FALSE
Debug.Assert(pPager.state == (tempFile != 0 ? PAGER_EXCLUSIVE : PAGER_UNLOCK));
#endif
      /* pPager.errMask = 0; */
      pPager.tempFile = tempFile != 0;
      Debug.Assert( tempFile == PAGER_LOCKINGMODE_NORMAL
      || tempFile == PAGER_LOCKINGMODE_EXCLUSIVE );
      Debug.Assert( PAGER_LOCKINGMODE_EXCLUSIVE == 1 );
      pPager.exclusiveMode = tempFile != 0;
      pPager.changeCountDone = pPager.tempFile;
      pPager.memDb = memDb;
      pPager.readOnly = readOnly;
      Debug.Assert( useJournal || pPager.tempFile );
      pPager.noSync = pPager.tempFile;
      pPager.fullSync = pPager.noSync;
      pPager.syncFlags = (byte)( pPager.noSync ? 0 : SQLITE_SYNC_NORMAL );
      pPager.ckptSyncFlags = pPager.syncFlags;
      /* pPager.pFirst = 0; */
      /* pPager.pFirstSynced = 0; */
      /* pPager.pLast = 0; */
      pPager.nExtra = (u16)nExtra;
      pPager.journalSizeLimit = SQLITE_DEFAULT_JOURNAL_SIZE_LIMIT;
      Debug.Assert( isOpen( pPager.fd ) || tempFile != 0 );
      setSectorSize( pPager );
      if ( !useJournal )
      {
        pPager.journalMode = PAGER_JOURNALMODE_OFF;
      }
      else if ( memDb != 0 )
      {
        pPager.journalMode = PAGER_JOURNALMODE_MEMORY;
      }
      /* pPager.xBusyHandler = 0; */
      /* pPager.pBusyHandlerArg = 0; */
      pPager.xReiniter = xReinit;
      /* memset(pPager.aHash, 0, sizeof(pPager.aHash)); */
      ppPager = pPager;
      return SQLITE_OK;
    }



    /*
    ** This function is called after transitioning from PAGER_UNLOCK to
    ** PAGER_SHARED state. It tests if there is a hot journal present in
    ** the file-system for the given pager. A hot journal is one that
    ** needs to be played back. According to this function, a hot-journal
    ** file exists if the following criteria are met:
    **
    **   * The journal file exists in the file system, and
    **   * No process holds a RESERVED or greater lock on the database file, and
    **   * The database file itself is greater than 0 bytes in size, and
    **   * The first byte of the journal file exists and is not 0x00.
    **
    ** If the current size of the database file is 0 but a journal file
    ** exists, that is probably an old journal left over from a prior
    ** database with the same name. In this case the journal file is
    ** just deleted using OsDelete, *pExists is set to 0 and SQLITE_OK
    ** is returned.
    **
    ** This routine does not check if there is a master journal filename
    ** at the end of the file. If there is, and that master journal file
    ** does not exist, then the journal file is not really hot. In this
    ** case this routine will return a false-positive. The pager_playback()
    ** routine will discover that the journal file is not really hot and
    ** will not roll it back.
    **
    ** If a hot-journal file is found to exist, *pExists is set to 1 and
    ** SQLITE_OK returned. If no hot-journal file is present, *pExists is
    ** set to 0 and SQLITE_OK returned. If an IO error occurs while trying
    ** to determine whether or not a hot-journal file exists, the IO error
    ** code is returned and the value of *pExists is undefined.
    */
    static int hasHotJournal( Pager pPager, ref int pExists )
    {
      sqlite3_vfs pVfs = pPager.pVfs;
      int rc = SQLITE_OK;           /* Return code */
      int exists = 1;               /* True if a journal file is present */
      int jrnlOpen = isOpen( pPager.jfd ) ? 1 : 0;
      Debug.Assert( pPager.useJournal != 0 );
      Debug.Assert( isOpen( pPager.fd ) );
      Debug.Assert( pPager.eState == PAGER_OPEN );

      Debug.Assert( jrnlOpen == 0 || ( sqlite3OsDeviceCharacteristics( pPager.jfd ) &
      SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN
      ) != 0 );


      pExists = 0;

      if ( 0 == jrnlOpen )
      {
        rc = sqlite3OsAccess( pVfs, pPager.zJournal, SQLITE_ACCESS_EXISTS, ref exists );
      }
      if ( rc == SQLITE_OK && exists != 0 )
      {
        int locked = 0;                 /* True if some process holds a RESERVED lock */

        /* Race condition here:  Another process might have been holding the
        ** the RESERVED lock and have a journal open at the sqlite3OsAccess()
        ** call above, but then delete the journal and drop the lock before
        ** we get to the following sqlite3OsCheckReservedLock() call.  If that
        ** is the case, this routine might think there is a hot journal when
        ** in fact there is none.  This results in a false-positive which will
        ** be dealt with by the playback routine.  Ticket #3883.
        */
        rc = sqlite3OsCheckReservedLock( pPager.fd, ref locked );
        if ( rc == SQLITE_OK && locked == 0 )
        {
          Pgno nPage = 0;                 /* Number of pages in database file */

          /* Check the size of the database file. If it consists of 0 pages,
          ** then delete the journal file. See the header comment above for
          ** the reasoning here.  Delete the obsolete journal file under
          ** a RESERVED lock to avoid race conditions and to avoid violating
          ** [H33020].
          */
          rc = pagerPagecount( pPager, ref nPage );
          if ( rc == SQLITE_OK )
          {
            if ( nPage == 0 )
            {
              sqlite3BeginBenignMalloc();
              if ( pagerLockDb( pPager, RESERVED_LOCK ) == SQLITE_OK )
              {
                sqlite3OsDelete( pVfs, pPager.zJournal, 0 );
                if ( !pPager.exclusiveMode )
                  pagerUnlockDb( pPager, SHARED_LOCK );
              }
              sqlite3EndBenignMalloc();
            }
            else
            {
              /* The journal file exists and no other connection has a reserved
              ** or greater lock on the database file. Now check that there is
              ** at least one non-zero bytes at the start of the journal file.
              ** If there is, then we consider this journal to be hot. If not,
              ** it can be ignored.
              */
              if ( 0 == jrnlOpen )
              {
                int f = SQLITE_OPEN_READONLY | SQLITE_OPEN_MAIN_JOURNAL;
                rc = sqlite3OsOpen( pVfs, pPager.zJournal, pPager.jfd, f, ref f );
              }
              if ( rc == SQLITE_OK )
              {
                u8[] first = new u8[1];
                rc = sqlite3OsRead( pPager.jfd, first, 1, 0 );
                if ( rc == SQLITE_IOERR_SHORT_READ )
                {
                  rc = SQLITE_OK;
                }
                if ( 0 == jrnlOpen )
                {
                  sqlite3OsClose( pPager.jfd );
                }
                pExists = ( first[0] != 0 ) ? 1 : 0;
              }
              else if ( rc == SQLITE_CANTOPEN )
              {
                /* If we cannot open the rollback journal file in order to see if
                ** its has a zero header, that might be due to an I/O error, or
                ** it might be due to the race condition described above and in
                ** ticket #3883.  Either way, assume that the journal is hot.
                ** This might be a false positive.  But if it is, then the
                ** automatic journal playback and recovery mechanism will deal
                ** with it under an EXCLUSIVE lock where we do not need to
                ** worry so much with race conditions.
                */
                pExists = 1;
                rc = SQLITE_OK;
              }
            }
          }
        }
      }
      return rc;
    }



    /*
    ** This function is called to obtain a shared lock on the database file.
    ** It is illegal to call sqlite3PagerAcquire() until after this function
    ** has been successfully called. If a shared-lock is already held when
    ** this function is called, it is a no-op.
    **
    ** The following operations are also performed by this function.
    **
    **   1) If the pager is currently in PAGER_OPEN state (no lock held
    **      on the database file), then an attempt is made to obtain a
    **      SHARED lock on the database file. Immediately after obtaining
    **      the SHARED lock, the file-system is checked for a hot-journal,
    **      which is played back if present. Following any hot-journal 
    **      rollback, the contents of the cache are validated by checking
    **      the 'change-counter' field of the database file header and
    **      discarded if they are found to be invalid.
    **
    **   2) If the pager is running in exclusive-mode, and there are currently
    **      no outstanding references to any pages, and is in the error state,
    **      then an attempt is made to clear the error state by discarding
    **      the contents of the page cache and rolling back any open journal
    **      file.
    **
    ** If everything is successful, SQLITE_OK is returned. If an IO error 
    ** occurs while locking the database, checking for a hot-journal file or 
    ** rolling back a journal file, the IO error code is returned.
    */
    static int sqlite3PagerSharedLock( Pager pPager )
    {
      int rc = SQLITE_OK;                /* Return code */

      /* This routine is only called from b-tree and only when there are no
      ** outstanding pages. This implies that the pager state should either
      ** be OPEN or READER. READER is only possible if the pager is or was in 
      ** exclusive access mode.
      */
      Debug.Assert( sqlite3PcacheRefCount( pPager.pPCache ) == 0 );
      Debug.Assert( assert_pager_state( pPager ) );
      Debug.Assert( pPager.eState == PAGER_OPEN || pPager.eState == PAGER_READER );
      if ( NEVER(
#if SQLITE_OMIT_MEMORYDB
0!=MEMDB
#else
 0 != pPager.memDb
#endif
 && pPager.errCode != 0 ) )
      {
        return pPager.errCode;
      }

      if ( !pagerUseWal( pPager ) && pPager.eState == PAGER_OPEN )
      {
        int bHotJournal = 1;          /* True if there exists a hot journal-file */

        Debug.Assert(
#if SQLITE_OMIT_MEMORYDB
0==MEMDB
#else
 0 == pPager.memDb
#endif
 );
        Debug.Assert( pPager.noReadlock == 0 || pPager.readOnly );

        if ( pPager.noReadlock == 0 )
        {
          rc = pager_wait_on_lock( pPager, SHARED_LOCK );
          if ( rc != SQLITE_OK )
          {
            Debug.Assert( pPager.eLock == NO_LOCK || pPager.eLock == UNKNOWN_LOCK );
            goto failed;
          }
        }

        /* If a journal file exists, and there is no RESERVED lock on the
        ** database file, then it either needs to be played back or deleted.
        */
        if ( pPager.eLock <= SHARED_LOCK )
        {
          rc = hasHotJournal( pPager, ref bHotJournal );
        }
        if ( rc != SQLITE_OK )
        {
          goto failed;
        }
        if ( bHotJournal != 0 )
        {
          /* Get an EXCLUSIVE lock on the database file. At this point it is
          ** important that a RESERVED lock is not obtained on the way to the
          ** EXCLUSIVE lock. If it were, another process might open the
          ** database file, detect the RESERVED lock, and conclude that the
          ** database is safe to read while this process is still rolling the 
          ** hot-journal back.
          ** 
          ** Because the intermediate RESERVED lock is not requested, any
          ** other process attempting to access the database file will get to 
          ** this point in the code and fail to obtain its own EXCLUSIVE lock 
          ** on the database file.
          **
          ** Unless the pager is in locking_mode=exclusive mode, the lock is
          ** downgraded to SHARED_LOCK before this function returns.
          */
          rc = pagerLockDb( pPager, EXCLUSIVE_LOCK );
          if ( rc != SQLITE_OK )
          {
            goto failed;
          }

          /* If it is not already open and the file exists on disk, open the 
          ** journal for read/write access. Write access is required because 
          ** in exclusive-access mode the file descriptor will be kept open 
          ** and possibly used for a transaction later on. Also, write-access 
          ** is usually required to finalize the journal in journal_mode=persist 
          ** mode (and also for journal_mode=truncate on some systems).
          **
          ** If the journal does not exist, it usually means that some 
          ** other connection managed to get in and roll it back before 
          ** this connection obtained the exclusive lock above. Or, it 
          ** may mean that the pager was in the error-state when this
          ** function was called and the journal file does not exist.
          */
          if ( !isOpen( pPager.jfd ) )
          {
            sqlite3_vfs pVfs = pPager.pVfs;
            int bExists = 0;              /* True if journal file exists */
            rc = sqlite3OsAccess(
            pVfs, pPager.zJournal, SQLITE_ACCESS_EXISTS, ref bExists );
            if ( rc == SQLITE_OK && bExists != 0 )
            {
              int fout = 0;
              int f = SQLITE_OPEN_READWRITE | SQLITE_OPEN_MAIN_JOURNAL;
              Debug.Assert( !pPager.tempFile );
              rc = sqlite3OsOpen( pVfs, pPager.zJournal, pPager.jfd, f, ref fout );
              Debug.Assert( rc != SQLITE_OK || isOpen( pPager.jfd ) );
              if ( rc == SQLITE_OK && ( fout & SQLITE_OPEN_READONLY ) != 0 )
              {
                rc = SQLITE_CANTOPEN_BKPT();
                sqlite3OsClose( pPager.jfd );
              }
            }
          }

          /* Playback and delete the journal.  Drop the database write
          ** lock and reacquire the read lock. Purge the cache before
          ** playing back the hot-journal so that we don't end up with
          ** an inconsistent cache.  Sync the hot journal before playing
          ** it back since the process that crashed and left the hot journal
          ** probably did not sync it and we are required to always sync
          ** the journal before playing it back.
          */
          if ( isOpen( pPager.jfd ) )
          {
            Debug.Assert( rc == SQLITE_OK );
            rc = pagerSyncHotJournal( pPager );
            if ( rc == SQLITE_OK )
            {
              rc = pager_playback( pPager, 1 );
              pPager.eState = PAGER_OPEN;
            }
          }
          else if ( !pPager.exclusiveMode )
          {
            pagerUnlockDb( pPager, SHARED_LOCK );
          }

          if ( rc != SQLITE_OK )
          {
            /* This branch is taken if an error occurs while trying to open
            ** or roll back a hot-journal while holding an EXCLUSIVE lock. The
            ** pager_unlock() routine will be called before returning to unlock
            ** the file. If the unlock attempt fails, then Pager.eLock must be
            ** set to UNKNOWN_LOCK (see the comment above the #define for 
            ** UNKNOWN_LOCK above for an explanation). 
            **
            ** In order to get pager_unlock() to do this, set Pager.eState to
            ** PAGER_ERROR now. This is not actually counted as a transition
            ** to ERROR state in the state diagram at the top of this file,
            ** since we know that the same call to pager_unlock() will very
            ** shortly transition the pager object to the OPEN state. Calling
            ** assert_pager_state() would fail now, as it should not be possible
            ** to be in ERROR state when there are zero outstanding page 
            ** references.
            */
            pager_error( pPager, rc );
            goto failed;
          }

          Debug.Assert( pPager.eState == PAGER_OPEN );
          Debug.Assert( ( pPager.eLock == SHARED_LOCK )
          || ( pPager.exclusiveMode && pPager.eLock > SHARED_LOCK )
          );
        }

        if ( !pPager.tempFile
        && ( pPager.pBackup != null || sqlite3PcachePagecount( pPager.pPCache ) > 0 )
        )
        {
          /* The shared-lock has just been acquired on the database file
          ** and there are already pages in the cache (from a previous
          ** read or write transaction).  Check to see if the database
          ** has been modified.  If the database has changed, flush the
          ** cache.
          **
          ** Database changes is detected by looking at 15 bytes beginning
          ** at offset 24 into the file.  The first 4 of these 16 bytes are
          ** a 32-bit counter that is incremented with each change.  The
          ** other bytes change randomly with each file change when
          ** a codec is in use.
          ** 
          ** There is a vanishingly small chance that a change will not be 
          ** detected.  The chance of an undetected change is so small that
          ** it can be neglected.
          */
          Pgno nPage = 0;
          byte[] dbFileVers = new byte[pPager.dbFileVers.Length];

          rc = pagerPagecount( pPager, ref nPage );
          if ( rc != 0 )
            goto failed;

          if ( nPage > 0 )
          {
            IOTRACE( "CKVERS %p %d\n", pPager, dbFileVers.Length );
            rc = sqlite3OsRead( pPager.fd, dbFileVers, dbFileVers.Length, 24 );
            if ( rc != SQLITE_OK )
            {
              goto failed;
            }
          }
          else
          {
            Array.Clear( dbFileVers, 0, dbFileVers.Length );// memset( dbFileVers, 0, sizeof( dbFileVers ) );
          }

          if ( memcmp( pPager.dbFileVers, dbFileVers, dbFileVers.Length ) != 0 )
          {
            pager_reset( pPager );
          }
        }

        /* If there is a WAL file in the file-system, open this database in WAL
        ** mode. Otherwise, the following function call is a no-op.
        */
        rc = pagerOpenWalIfPresent( pPager );
#if !SQLITE_OMIT_WAL
Debug.Assert( pPager.pWal == null || rc == SQLITE_OK );
#endif
      }

      if ( pagerUseWal( pPager ) )
      {
        Debug.Assert( rc == SQLITE_OK );
        rc = pagerBeginReadTransaction( pPager );
      }

      if ( pPager.eState == PAGER_OPEN && rc == SQLITE_OK )
      {
        rc = pagerPagecount( pPager, ref pPager.dbSize );
      }

failed:
      if ( rc != SQLITE_OK )
      {
        Debug.Assert(
#if SQLITE_OMIT_MEMORYDB
0==MEMDB
#else
 0 == pPager.memDb
#endif
 );
        pager_unlock( pPager );
        Debug.Assert( pPager.eState == PAGER_OPEN );
      }
      else
      {
        pPager.eState = PAGER_READER;
      }
      return rc;
    }

    /*
    ** If the reference count has reached zero, rollback any active
    ** transaction and unlock the pager.
    **
    ** Except, in locking_mode=EXCLUSIVE when there is nothing to in
    ** the rollback journal, the unlock is not performed and there is
    ** nothing to rollback, so this routine is a no-op.
    */
    static void pagerUnlockIfUnused( Pager pPager )
    {
      if ( sqlite3PcacheRefCount( pPager.pPCache ) == 0 )
      {
        pagerUnlockAndRollback( pPager );
      }
    }

    /*
    ** Acquire a reference to page number pgno in pager pPager (a page
    ** reference has type DbPage*). If the requested reference is
    ** successfully obtained, it is copied to *ppPage and SQLITE_OK returned.
    **
    ** If the requested page is already in the cache, it is returned.
    ** Otherwise, a new page object is allocated and populated with data
    ** read from the database file. In some cases, the pcache module may
    ** choose not to allocate a new page object and may reuse an existing
    ** object with no outstanding references.
    **
    ** The extra data appended to a page is always initialized to zeros the
    ** first time a page is loaded into memory. If the page requested is
    ** already in the cache when this function is called, then the extra
    ** data is left as it was when the page object was last used.
    **
    ** If the database image is smaller than the requested page or if a
    ** non-zero value is passed as the noContent parameter and the
    ** requested page is not already stored in the cache, then no
    ** actual disk read occurs. In this case the memory image of the
    ** page is initialized to all zeros.
    **
    ** If noContent is true, it means that we do not care about the contents
    ** of the page. This occurs in two seperate scenarios:
    **
    **   a) When reading a free-list leaf page from the database, and
    **
    **   b) When a savepoint is being rolled back and we need to load
    **      a new page into the cache to be filled with the data read
    **      from the savepoint journal.
    **
    ** If noContent is true, then the data returned is zeroed instead of
    ** being read from the database. Additionally, the bits corresponding
    ** to pgno in Pager.pInJournal (bitvec of pages already written to the
    ** journal file) and the PagerSavepoint.pInSavepoint bitvecs of any open
    ** savepoints are set. This means if the page is made writable at any
    ** point in the future, using a call to sqlite3PagerWrite(), its contents
    ** will not be journaled. This saves IO.
    **
    ** The acquisition might fail for several reasons.  In all cases,
    ** an appropriate error code is returned and *ppPage is set to NULL.
    **
    ** See also sqlite3PagerLookup().  Both this routine and Lookup() attempt
    ** to find a page in the in-memory cache first.  If the page is not already
    ** in memory, this routine goes to disk to read it in whereas Lookup()
    ** just returns 0.  This routine acquires a read-lock the first time it
    ** has to go to disk, and could also playback an old journal if necessary.
    ** Since Lookup() never goes to disk, it never has to deal with locks
    ** or journal files.
    */

    // Under C# from the header file
    //#define sqlite3PagerGet(A,B,C) sqlite3PagerAcquire(A,B,C,0)

    static int sqlite3PagerGet(
    Pager pPager,       /* The pager open on the database file */
    u32 pgno,          /* Page number to fetch */
    ref DbPage ppPage   /* Write a pointer to the page here */
    )
    {
      return sqlite3PagerAcquire( pPager, pgno, ref ppPage, 0 );
    }

    static int sqlite3PagerAcquire(
    Pager pPager,      /* The pager open on the database file */
    u32 pgno,          /* Page number to fetch */
    ref DbPage ppPage, /* Write a pointer to the page here */
    u8 noContent     /* Do not bother reading content from disk if true */
    )
    {
      int rc;
      PgHdr pPg = null;

      Debug.Assert( pPager.eState >= PAGER_READER );
      Debug.Assert( assert_pager_state( pPager ) );
      if ( pgno == 0 )
      {
        return SQLITE_CORRUPT_BKPT();
      }

      /* If the pager is in the error state, return an error immediately. 
      ** Otherwise, request the page from the PCache layer. */
      if ( pPager.errCode != SQLITE_OK )
      {
        rc = pPager.errCode;
      }
      else
      {
        rc = sqlite3PcacheFetch( pPager.pPCache, pgno, 1, ref ppPage );
      }

      if ( rc != SQLITE_OK )
      {
        /* Either the call to sqlite3PcacheFetch() returned an error or the
        ** pager was already in the error-state when this function was called.
        ** Set pPg to 0 and jump to the exception handler.  */
        pPg = null;
        goto pager_acquire_err;
      }
      Debug.Assert( ( ppPage ).pgno == pgno );
      Debug.Assert( ( ppPage ).pPager == pPager || ( ppPage ).pPager == null );

      if ( ( ppPage ).pPager != null && 0 == noContent )
      {
        /* In this case the pcache already contains an initialized copy of
        ** the page. Return without further ado.  */
        Debug.Assert( pgno <= PAGER_MAX_PGNO && pgno != PAGER_MJ_PGNO( pPager ) );
        PAGER_INCR( ref pPager.nHit );
        return SQLITE_OK;

      }
      else
      {
        /* The pager cache has created a new page. Its content needs to 
        ** be initialized.  */
#if SQLITE_TEST
        PAGER_INCR( ref pPager.nMiss );
#endif
        pPg = ppPage;
        pPg.pPager = pPager;
        pPg.pExtra = new MemPage();//memset(pPg.pExtra, 0, pPager.nExtra);

        /* The maximum page number is 2^31. Return SQLITE_CORRUPT if a page
        ** number greater than this, or the unused locking-page, is requested. */
        if ( pgno > PAGER_MAX_PGNO || pgno == PAGER_MJ_PGNO( pPager ) )
        {
          rc = SQLITE_CORRUPT_BKPT();
          goto pager_acquire_err;
        }
        if (
#if SQLITE_OMIT_MEMORYDB
1==MEMDB
#else
 pPager.memDb != 0
#endif
 || pPager.dbSize < pgno || noContent != 0 || !isOpen( pPager.fd ) )
        {
          if ( pgno > pPager.mxPgno )
          {
            rc = SQLITE_FULL;
            goto pager_acquire_err;
          }
          if ( noContent != 0 )
          {
            /* Failure to set the bits in the InJournal bit-vectors is benign.
            ** It merely means that we might do some extra work to journal a
            ** page that does not need to be journaled.  Nevertheless, be sure
            ** to test the case where a malloc error occurs while trying to set
            ** a bit in a bit vector.
            */
            sqlite3BeginBenignMalloc();
            if ( pgno <= pPager.dbOrigSize )
            {
#if !NDEBUG || SQLITE_COVERAGE_TEST
              rc = sqlite3BitvecSet( pPager.pInJournal, pgno );          //TESTONLY( rc = ) sqlite3BitvecSet(pPager.pInJournal, pgno);
#else
sqlite3BitvecSet(pPager.pInJournal, pgno);
#endif
              testcase( rc == SQLITE_NOMEM );
            }
#if !NDEBUG || SQLITE_COVERAGE_TEST
            rc = addToSavepointBitvecs( pPager, pgno ); //TESTONLY( rc = ) addToSavepointBitvecs(pPager, pgno);
#else
addToSavepointBitvecs(pPager, pgno);
#endif

            testcase( rc == SQLITE_NOMEM );
            sqlite3EndBenignMalloc();
          }
          //memset(pPg.pData, 0, pPager.pageSize);
          Array.Clear( pPg.pData, 0, pPager.pageSize );
          IOTRACE( "ZERO %p %d\n", pPager, pgno );
        }
        else
        {
          Debug.Assert( pPg.pPager == pPager );
          rc = readDbPage( pPg );
          if ( rc != SQLITE_OK )
          {
            goto pager_acquire_err;
          }
        }

        pager_set_pagehash( pPg );
      }
      return SQLITE_OK;

pager_acquire_err:
      Debug.Assert( rc != SQLITE_OK );
      if ( pPg != null )
      {
        sqlite3PcacheDrop( pPg );
      }
      pagerUnlockIfUnused( pPager );

      ppPage = null;
      return rc;
    }

    /*
    ** Acquire a page if it is already in the in-memory cache.  Do
    ** not read the page from disk.  Return a pointer to the page,
    ** or 0 if the page is not in cache. 
    **
    ** See also sqlite3PagerGet().  The difference between this routine
    ** and sqlite3PagerGet() is that _get() will go to the disk and read
    ** in the page if the page is not already in cache.  This routine
    ** returns NULL if the page is not in cache or if a disk I/O error 
    ** has ever happened.
    */
    static DbPage sqlite3PagerLookup( Pager pPager, u32 pgno )
    {
      PgHdr pPg = null;

      Debug.Assert( pPager != null );
      Debug.Assert( pgno != 0 );
      Debug.Assert( pPager.pPCache != null );
      Debug.Assert( pPager.eState >= PAGER_READER && pPager.eState != PAGER_ERROR );
      sqlite3PcacheFetch( pPager.pPCache, pgno, 0, ref pPg );

      return pPg;
    }

    /*
    ** Release a page reference.
    **
    ** If the number of references to the page drop to zero, then the
    ** page is added to the LRU list.  When all references to all pages
    ** are released, a rollback occurs and the lock on the database is
    ** removed.
    */
    static void sqlite3PagerUnref( DbPage pPg )
    {
      if ( pPg != null )
      {
        Pager pPager = pPg.pPager;
        sqlite3PcacheRelease( pPg );
        pagerUnlockIfUnused( pPager );
      }
    }

    /*
    ** This function is called at the start of every write transaction.
    ** There must already be a RESERVED or EXCLUSIVE lock on the database
    ** file when this routine is called.
    **
    ** Open the journal file for pager pPager and write a journal header
    ** to the start of it. If there are active savepoints, open the sub-journal
    ** as well. This function is only used when the journal file is being
    ** opened to write a rollback log for a transaction. It is not used
    ** when opening a hot journal file to roll it back.
    **
    ** If the journal file is already open (as it may be in exclusive mode),
    ** then this function just writes a journal header to the start of the
    ** already open file.
    **
    ** Whether or not the journal file is opened by this function, the
    ** Pager.pInJournal bitvec structure is allocated.
    **
    ** Return SQLITE_OK if everything is successful. Otherwise, return
    ** SQLITE_NOMEM if the attempt to allocate Pager.pInJournal fails, or
    ** an IO error code if opening or writing the journal file fails.
    */
    static int pager_open_journal( Pager pPager )
    {
      int rc = SQLITE_OK;                        /* Return code */
      sqlite3_vfs pVfs = pPager.pVfs;            /* Local cache of vfs pointer */

      Debug.Assert( pPager.eState == PAGER_WRITER_LOCKED );
      Debug.Assert( assert_pager_state( pPager ) );
      Debug.Assert( pPager.pInJournal == null );

      /* If already in the error state, this function is a no-op.  But on
      ** the other hand, this routine is never called if we are already in
      ** an error state. */
      if ( NEVER( pPager.errCode ) != 0 )
        return pPager.errCode;

      if ( !pagerUseWal( pPager ) && pPager.journalMode != PAGER_JOURNALMODE_OFF )
      {
        pPager.pInJournal = sqlite3BitvecCreate( pPager.dbSize );
        //if (pPager.pInJournal == null)
        //{
        //  return SQLITE_NOMEM;
        //}

        /* Open the journal file if it is not already open. */
        if ( !isOpen( pPager.jfd ) )
        {
          if ( pPager.journalMode == PAGER_JOURNALMODE_MEMORY )
          {
            sqlite3MemJournalOpen( pPager.jfd );
          }
          else
          {
            int flags =                   /* VFS flags to open journal file */
            SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE |
            ( pPager.tempFile ?
            ( SQLITE_OPEN_DELETEONCLOSE | SQLITE_OPEN_TEMP_JOURNAL ) :
            ( SQLITE_OPEN_MAIN_JOURNAL )
            );
#if SQLITE_ENABLE_ATOMIC_WRITE
rc = sqlite3JournalOpen(
pVfs, pPager.zJournal, pPager.jfd, flags, jrnlBufferSize(pPager)
);
#else
            int int0 = 0;
            rc = sqlite3OsOpen( pVfs, pPager.zJournal, pPager.jfd, flags, ref int0 );
#endif
          }
          Debug.Assert( rc != SQLITE_OK || isOpen( pPager.jfd ) );
        }

        /* Write the first journal header to the journal file and open
        ** the sub-journal if necessary.
        */
        if ( rc == SQLITE_OK )
        {
          /* TODO: Check if all of these are really required. */
          pPager.nRec = 0;
          pPager.journalOff = 0;
          pPager.setMaster = 0;
          pPager.journalHdr = 0;
          rc = writeJournalHdr( pPager );
        }
      }
      if ( rc != SQLITE_OK )
      {
        sqlite3BitvecDestroy( ref pPager.pInJournal );
        pPager.pInJournal = null;
      }
      else
      {
        Debug.Assert( pPager.eState == PAGER_WRITER_LOCKED );
        pPager.eState = PAGER_WRITER_CACHEMOD;
      }
      return rc;
    }

    /*
    ** Begin a write-transaction on the specified pager object. If a 
    ** write-transaction has already been opened, this function is a no-op.
    **
    ** If the exFlag argument is false, then acquire at least a RESERVED
    ** lock on the database file. If exFlag is true, then acquire at least
    ** an EXCLUSIVE lock. If such a lock is already held, no locking 
    ** functions need be called.
    **
    ** If the subjInMemory argument is non-zero, then any sub-journal opened
    ** within this transaction will be opened as an in-memory file. This
    ** has no effect if the sub-journal is already opened (as it may be when
    ** running in exclusive mode) or if the transaction does not require a
    ** sub-journal. If the subjInMemory argument is zero, then any required
    ** sub-journal is implemented in-memory if pPager is an in-memory database, 
    ** or using a temporary file otherwise.
    */
    static int sqlite3PagerBegin( Pager pPager, bool exFlag, int subjInMemory )
    {
      int rc = SQLITE_OK;
      if ( pPager.errCode != 0 )
        return pPager.errCode;
      Debug.Assert( pPager.eState >= PAGER_READER && pPager.eState < PAGER_ERROR );
      pPager.subjInMemory = (u8)subjInMemory;
      if ( ALWAYS( pPager.eState == PAGER_READER ) )
      {
        Debug.Assert( pPager.pInJournal == null );
        if ( pagerUseWal( pPager ) )
        {
          /* If the pager is configured to use locking_mode=exclusive, and an
          ** exclusive lock on the database is not already held, obtain it now.
          */
          if ( pPager.exclusiveMode && sqlite3WalExclusiveMode( pPager.pWal, -1 ) )
          {
            rc = pagerLockDb( pPager, EXCLUSIVE_LOCK );
            if ( rc != SQLITE_OK )
            {
              return rc;
            }
            sqlite3WalExclusiveMode( pPager.pWal, 1 );
          }

          /* Grab the write lock on the log file. If successful, upgrade to
          ** PAGER_RESERVED state. Otherwise, return an error code to the caller.
          ** The busy-handler is not invoked if another connection already
          ** holds the write-lock. If possible, the upper layer will call it.
          */
          rc = sqlite3WalBeginWriteTransaction( pPager.pWal );
        }
        else
        {
          /* Obtain a RESERVED lock on the database file. If the exFlag parameter
          ** is true, then immediately upgrade this to an EXCLUSIVE lock. The
          ** busy-handler callback can be used when upgrading to the EXCLUSIVE
          ** lock, but not when obtaining the RESERVED lock.
          */
          rc = pagerLockDb( pPager, RESERVED_LOCK );
          if ( rc == SQLITE_OK && exFlag )
          {
            rc = pager_wait_on_lock( pPager, EXCLUSIVE_LOCK );
          }
        }

        if ( rc == SQLITE_OK )
        {
          /* Change to WRITER_LOCKED state.
          **
          ** WAL mode sets Pager.eState to PAGER_WRITER_LOCKED or CACHEMOD
          ** when it has an open transaction, but never to DBMOD or FINISHED.
          ** This is because in those states the code to roll back savepoint 
          ** transactions may copy data from the sub-journal into the database 
          ** file as well as into the page cache. Which would be incorrect in 
          ** WAL mode.
          */
          pPager.eState = PAGER_WRITER_LOCKED;
          pPager.dbHintSize = pPager.dbSize;
          pPager.dbFileSize = pPager.dbSize;
          pPager.dbOrigSize = pPager.dbSize;
          pPager.journalOff = 0;
        }

        Debug.Assert( rc == SQLITE_OK || pPager.eState == PAGER_READER );
        Debug.Assert( rc != SQLITE_OK || pPager.eState == PAGER_WRITER_LOCKED );
        Debug.Assert( assert_pager_state( pPager ) );
      }

      PAGERTRACE( "TRANSACTION %d\n", PAGERID( pPager ) );
      return rc;
    }


    /*
    ** Mark a single data page as writeable. The page is written into the
    ** main journal or sub-journal as required. If the page is written into
    ** one of the journals, the corresponding bit is set in the
    ** Pager.pInJournal bitvec and the PagerSavepoint.pInSavepoint bitvecs
    ** of any open savepoints as appropriate.
    */
    static int pager_write( PgHdr pPg )
    {
      byte[] pData = pPg.pData;
      Pager pPager = pPg.pPager;
      int rc = SQLITE_OK;

      /* This routine is not called unless a write-transaction has already 
      ** been started. The journal file may or may not be open at this point.
      ** It is never called in the ERROR state.
      */
      Debug.Assert( pPager.eState == PAGER_WRITER_LOCKED
      || pPager.eState == PAGER_WRITER_CACHEMOD
      || pPager.eState == PAGER_WRITER_DBMOD
      );
      Debug.Assert( assert_pager_state( pPager ) );

      /* If an error has been previously detected, report the same error
      ** again. This should not happen, but the check provides robustness. */
      if ( NEVER( pPager.errCode ) != 0 )
        return pPager.errCode;

      /* Higher-level routines never call this function if database is not
      ** writable.  But check anyway, just for robustness. */
      if ( NEVER( pPager.readOnly ) )
        return SQLITE_PERM;

#if SQLITE_CHECK_PAGES
CHECK_PAGE(pPg);
#endif
      /* The journal file needs to be opened. Higher level routines have already
** obtained the necessary locks to begin the write-transaction, but the
** rollback journal might not yet be open. Open it now if this is the case.
**
** This is done before calling sqlite3PcacheMakeDirty() on the page. 
** Otherwise, if it were done after calling sqlite3PcacheMakeDirty(), then
** an error might occur and the pager would end up in WRITER_LOCKED state
** with pages marked as dirty in the cache.
*/
      if ( pPager.eState == PAGER_WRITER_LOCKED )
      {
        rc = pager_open_journal( pPager );
        if ( rc != SQLITE_OK )
          return rc;
      }
      Debug.Assert( pPager.eState >= PAGER_WRITER_CACHEMOD );
      Debug.Assert( assert_pager_state( pPager ) );

      /* Mark the page as dirty.  If the page has already been written
      ** to the journal then we can return right away.
      */
      sqlite3PcacheMakeDirty( pPg );
      if ( pageInJournal( pPg ) && !subjRequiresPage( pPg ) )
      {
        Debug.Assert( !pagerUseWal( pPager ) );
      }
      else
      {
        /* The transaction journal now exists and we have a RESERVED or an
        ** EXCLUSIVE lock on the main database file.  Write the current page to
        ** the transaction journal if it is not there already.
        */
        if ( !pageInJournal( pPg ) && !pagerUseWal( pPager ) )
        {
          Debug.Assert( pagerUseWal( pPager ) == false );
          if ( pPg.pgno <= pPager.dbOrigSize && isOpen( pPager.jfd ) )
          {
            u32 cksum;
            byte[] pData2 = null;
            i64 iOff = pPager.journalOff;

            /* We should never write to the journal file the page that
            ** contains the database locks.  The following Debug.Assert verifies
            ** that we do not. */
            Debug.Assert( pPg.pgno != ( ( PENDING_BYTE / ( pPager.pageSize ) ) + 1 ) );//PAGER_MJ_PGNO(pPager) );

            Debug.Assert( pPager.journalHdr <= pPager.journalOff );
            if ( CODEC2( pPager, pData, pPg.pgno, SQLITE_ENCRYPT_READ_CTX, ref pData2 ) )
              return SQLITE_NOMEM;  // CODEC2(pPager, pData, pPg.pgno, 7, return SQLITE_NOMEM, pData2);
            cksum = pager_cksum( pPager, pData2 );

            /* Even if an IO or diskfull error occurred while journalling the
            ** page in the block above, set the need-sync flag for the page.
            ** Otherwise, when the transaction is rolled back, the logic in
            ** playback_one_page() will think that the page needs to be restored
            ** in the database file. And if an IO error occurs while doing so,
            ** then corruption may follow.
            */

            pPg.flags |= PGHDR_NEED_SYNC;

            rc = write32bits( pPager.jfd, iOff, pPg.pgno );
            if ( rc != SQLITE_OK )
              return rc;
            rc = sqlite3OsWrite( pPager.jfd, pData2, pPager.pageSize, iOff + 4 );
            if ( rc != SQLITE_OK )
              return rc;
            rc = write32bits( pPager.jfd, iOff + pPager.pageSize + 4, cksum );
            if ( rc != SQLITE_OK )
              return rc;

            IOTRACE( "JOUT %p %d %lld %d\n", pPager, pPg.pgno,
            pPager.journalOff, pPager.pageSize );
#if SQLITE_TEST
#if !TCLSH
            PAGER_INCR( ref sqlite3_pager_writej_count );
#else
            int iValue = sqlite3_pager_writej_count.iValue;
            PAGER_INCR( ref iValue );
            sqlite3_pager_writej_count.iValue = iValue;
#endif
#endif
            PAGERTRACE( "JOURNAL %d page %d needSync=%d hash(%08x)\n",
            PAGERID( pPager ), pPg.pgno,
            ( ( pPg.flags & PGHDR_NEED_SYNC ) != 0 ? 1 : 0 ), pager_pagehash( pPg ) );

            pPager.journalOff += 8 + pPager.pageSize;
            pPager.nRec++;
            Debug.Assert( pPager.pInJournal != null );
            rc = sqlite3BitvecSet( pPager.pInJournal, pPg.pgno );
            testcase( rc == SQLITE_NOMEM );
            Debug.Assert( rc == SQLITE_OK || rc == SQLITE_NOMEM );
            rc |= addToSavepointBitvecs( pPager, pPg.pgno );
            if ( rc != SQLITE_OK )
            {
              Debug.Assert( rc == SQLITE_NOMEM );
              return rc;
            }
          }
          else
          {
            if ( pPager.eState != PAGER_WRITER_DBMOD )
            {
              pPg.flags |= PGHDR_NEED_SYNC;
            }
            PAGERTRACE( "APPEND %d page %d needSync=%d\n",
            PAGERID( pPager ), pPg.pgno,
            ( ( pPg.flags & PGHDR_NEED_SYNC ) != 0 ? 1 : 0 ) );
          }
        }

        /* If the statement journal is open and the page is not in it,
        ** then write the current page to the statement journal.  Note that
        ** the statement journal format differs from the standard journal format
        ** in that it omits the checksums and the header.
        */
        if ( subjRequiresPage( pPg ) )
        {
          rc = subjournalPage( pPg );
        }
      }

      /* Update the database size and return.
      */
      if ( pPager.dbSize < pPg.pgno )
      {
        pPager.dbSize = pPg.pgno;
      }
      return rc;
    }

    /*
    ** Mark a data page as writeable. This routine must be called before
    ** making changes to a page. The caller must check the return value
    ** of this function and be careful not to change any page data unless
    ** this routine returns SQLITE_OK.
    **
    ** The difference between this function and pager_write() is that this
    ** function also deals with the special case where 2 or more pages
    ** fit on a single disk sector. In this case all co-resident pages
    ** must have been written to the journal file before returning.
    **
    ** If an error occurs, SQLITE_NOMEM or an IO error code is returned
    ** as appropriate. Otherwise, SQLITE_OK.
    */
    static int sqlite3PagerWrite( DbPage pDbPage )
    {
      int rc = SQLITE_OK;

      PgHdr pPg = pDbPage;
      Pager pPager = pPg.pPager;
      u32 nPagePerSector = (u32)( pPager.sectorSize / pPager.pageSize );

      Debug.Assert( pPager.eState >= PAGER_WRITER_LOCKED );
      Debug.Assert( pPager.eState != PAGER_ERROR );
      Debug.Assert( assert_pager_state( pPager ) );

      if ( nPagePerSector > 1 )
      {
        Pgno nPageCount = 0;     /* Total number of pages in database file */
        Pgno pg1;                /* First page of the sector pPg is located on. */
        Pgno nPage = 0;          /* Number of pages starting at pg1 to journal */
        int ii;                  /* Loop counter */
        bool needSync = false;   /* True if any page has PGHDR_NEED_SYNC */

        /* Set the doNotSyncSpill flag to 1. This is because we cannot allow
        ** a journal header to be written between the pages journaled by
        ** this function.
        */
        Debug.Assert(
#if SQLITE_OMIT_MEMORYDB
0==MEMDB
#else
 0 == pPager.memDb
#endif
 );
        Debug.Assert( pPager.doNotSyncSpill == 0 );
        pPager.doNotSyncSpill++;

        /* This trick assumes that both the page-size and sector-size are
        ** an integer power of 2. It sets variable pg1 to the identifier
        ** of the first page of the sector pPg is located on.
        */
        pg1 = (u32)( ( pPg.pgno - 1 ) & ~( nPagePerSector - 1 ) ) + 1;

        nPageCount = pPager.dbSize;
        if ( pPg.pgno > nPageCount )
        {
          nPage = ( pPg.pgno - pg1 ) + 1;
        }
        else if ( ( pg1 + nPagePerSector - 1 ) > nPageCount )
        {
          nPage = nPageCount + 1 - pg1;
        }
        else
        {
          nPage = nPagePerSector;
        }
        Debug.Assert( nPage > 0 );
        Debug.Assert( pg1 <= pPg.pgno );
        Debug.Assert( ( pg1 + nPage ) > pPg.pgno );

        for ( ii = 0; ii < nPage && rc == SQLITE_OK; ii++ )
        {
          u32 pg = (u32)( pg1 + ii );
          PgHdr pPage = new PgHdr();
          if ( pg == pPg.pgno || sqlite3BitvecTest( pPager.pInJournal, pg ) == 0 )
          {
            if ( pg != ( ( PENDING_BYTE / ( pPager.pageSize ) ) + 1 ) ) //PAGER_MJ_PGNO(pPager))
            {
              rc = sqlite3PagerGet( pPager, pg, ref pPage );
              if ( rc == SQLITE_OK )
              {
                rc = pager_write( pPage );
                if ( ( pPage.flags & PGHDR_NEED_SYNC ) != 0 )
                {
                  needSync = true;
                }
                sqlite3PagerUnref( pPage );
              }
            }
          }
          else if ( ( pPage = pager_lookup( pPager, pg ) ) != null )
          {
            if ( ( pPage.flags & PGHDR_NEED_SYNC ) != 0 )
            {
              needSync = true;
            }
            sqlite3PagerUnref( pPage );
          }
        }

        /* If the PGHDR_NEED_SYNC flag is set for any of the nPage pages
        ** starting at pg1, then it needs to be set for all of them. Because
        ** writing to any of these nPage pages may damage the others, the
        ** journal file must contain sync()ed copies of all of them
        ** before any of them can be written out to the database file.
        */
        if ( rc == SQLITE_OK && needSync )
        {
          Debug.Assert(
#if SQLITE_OMIT_MEMORYDB
0==MEMDB
#else
 0 == pPager.memDb
#endif
 );
          for ( ii = 0; ii < nPage; ii++ )
          {
            PgHdr pPage = pager_lookup( pPager, (u32)( pg1 + ii ) );
            if ( pPage != null )
            {
              pPage.flags |= PGHDR_NEED_SYNC;
              sqlite3PagerUnref( pPage );
            }
          }
        }

        Debug.Assert( pPager.doNotSyncSpill == 1 );
        pPager.doNotSyncSpill--;
      }
      else
      {
        rc = pager_write( pDbPage );
      }
      return rc;
    }

    /*
    ** Return TRUE if the page given in the argument was previously passed
    ** to sqlite3PagerWrite().  In other words, return TRUE if it is ok
    ** to change the content of the page.
    */
#if !NDEBUG
    static bool sqlite3PagerIswriteable( DbPage pPg )
    {
      return ( pPg.flags & PGHDR_DIRTY ) != 0;
    }
#else
static bool sqlite3PagerIswriteable( DbPage pPg ) { return true; }
#endif

    /*
** A call to this routine tells the pager that it is not necessary to
** write the information on page pPg back to the disk, even though
** that page might be marked as dirty.  This happens, for example, when
** the page has been added as a leaf of the freelist and so its
** content no longer matters.
**
** The overlying software layer calls this routine when all of the data
** on the given page is unused. The pager marks the page as clean so
** that it does not get written to disk.
**
** Tests show that this optimization can quadruple the speed of large
** DELETE operations.
*/
    static void sqlite3PagerDontWrite( PgHdr pPg )
    {
      Pager pPager = pPg.pPager;

      if ( ( pPg.flags & PGHDR_DIRTY ) != 0 && pPager.nSavepoint == 0 )
      {
        PAGERTRACE( "DONT_WRITE page %d of %d\n", pPg.pgno, PAGERID( pPager ) );
        IOTRACE( "CLEAN %p %d\n", pPager, pPg.pgno );
        pPg.flags |= PGHDR_DONT_WRITE;
        pager_set_pagehash( pPg );
      }
    }

    /*
    ** This routine is called to increment the value of the database file 
    ** change-counter, stored as a 4-byte big-endian integer starting at 
    ** byte offset 24 of the pager file.  The secondary change counter at
    ** 92 is also updated, as is the SQLite version number at offset 96.
    **
    ** But this only happens if the pPager.changeCountDone flag is false.
    ** To avoid excess churning of page 1, the update only happens once.
    ** See also the pager_write_changecounter() routine that does an 
    ** unconditional update of the change counters.
    **
    ** If the isDirectMode flag is zero, then this is done by calling 
    ** sqlite3PagerWrite() on page 1, then modifying the contents of the
    ** page data. In this case the file will be updated when the current
    ** transaction is committed.
    **
    ** The isDirectMode flag may only be non-zero if the library was compiled
    ** with the SQLITE_ENABLE_ATOMIC_WRITE macro defined. In this case,
    ** if isDirect is non-zero, then the database file is updated directly
    ** by writing an updated version of page 1 using a call to the 
    ** sqlite3OsWrite() function.
    */
    static int pager_incr_changecounter( Pager pPager, bool isDirectMode )
    {
      int rc = SQLITE_OK;

      Debug.Assert( pPager.eState == PAGER_WRITER_CACHEMOD
      || pPager.eState == PAGER_WRITER_DBMOD
      );
      Debug.Assert( assert_pager_state( pPager ) );

      /* Declare and initialize constant integer 'isDirect'. If the
      ** atomic-write optimization is enabled in this build, then isDirect
      ** is initialized to the value passed as the isDirectMode parameter
      ** to this function. Otherwise, it is always set to zero.
      **
      ** The idea is that if the atomic-write optimization is not
      ** enabled at compile time, the compiler can omit the tests of
      ** 'isDirect' below, as well as the block enclosed in the
      ** "if( isDirect )" condition.
      */
#if !SQLITE_ENABLE_ATOMIC_WRITE
      //# define DIRECT_MODE 0
      bool DIRECT_MODE = false;
      Debug.Assert( isDirectMode == false );
      UNUSED_PARAMETER( isDirectMode );
#else
//# define DIRECT_MODE isDirectMode
int DIRECT_MODE = isDirectMode;
#endif

      if ( !pPager.changeCountDone && pPager.dbSize > 0 )
      {
        PgHdr pPgHdr = null;            /* Reference to page 1 */

        Debug.Assert( !pPager.tempFile && isOpen( pPager.fd ) );

        /* Open page 1 of the file for writing. */
        rc = sqlite3PagerGet( pPager, 1, ref pPgHdr );
        Debug.Assert( pPgHdr == null || rc == SQLITE_OK );

        /* If page one was fetched successfully, and this function is not
        ** operating in direct-mode, make page 1 writable.  When not in 
        ** direct mode, page 1 is always held in cache and hence the PagerGet()
        ** above is always successful - hence the ALWAYS on rc==SQLITE_OK.
        */
        if ( !DIRECT_MODE && ALWAYS( rc == SQLITE_OK ) )
        {
          rc = sqlite3PagerWrite( pPgHdr );
        }

        if ( rc == SQLITE_OK )
        {
          /* Actually do the update of the change counter */
          pager_write_changecounter( pPgHdr );

          /* If running in direct mode, write the contents of page 1 to the file. */
          if ( DIRECT_MODE )
          {
            u8[] zBuf = null;
            Debug.Assert( pPager.dbFileSize > 0 );
            if ( CODEC2( pPager, pPgHdr.pData, 1, SQLITE_ENCRYPT_WRITE_CTX, ref zBuf ) )
              return rc = SQLITE_NOMEM;//CODEC2(pPager, pPgHdr.pData, 1, 6, rc=SQLITE_NOMEM, zBuf);
            if ( rc == SQLITE_OK )
            {
              rc = sqlite3OsWrite( pPager.fd, zBuf, pPager.pageSize, 0 );
            }

            if ( rc == SQLITE_OK )
            {
              pPager.changeCountDone = true;
            }
          }
          else
          {
            pPager.changeCountDone = true;
          }
        }

        /* Release the page reference. */
        sqlite3PagerUnref( pPgHdr );
      }
      return rc;
    }

    /*
    ** Sync the database file to disk. This is a no-op for in-memory databases
    ** or pages with the Pager.noSync flag set.
    **
    ** If successful, or if called on a pager for which it is a no-op, this
    ** function returns SQLITE_OK. Otherwise, an IO error code is returned.
    */
    static int sqlite3PagerSync( Pager pPager )
    {
      long rc = SQLITE_OK;
      if ( !pPager.noSync )
      {
        Debug.Assert(
#if SQLITE_OMIT_MEMORYDB
0 == MEMDB
#else
 0 == pPager.memDb
#endif
 );
        rc = sqlite3OsSync( pPager.fd, pPager.syncFlags );
      }
      else if ( isOpen( pPager.fd ) )
      {
        Debug.Assert(
#if SQLITE_OMIT_MEMORYDB
0 == MEMDB
#else
 0 == pPager.memDb
#endif
 );
        sqlite3OsFileControl( pPager.fd, SQLITE_FCNTL_SYNC_OMITTED, ref rc );
      }
      return (int)rc;
    }

    /*
    ** This function may only be called while a write-transaction is active in
    ** rollback. If the connection is in WAL mode, this call is a no-op. 
    ** Otherwise, if the connection does not already have an EXCLUSIVE lock on 
    ** the database file, an attempt is made to obtain one.
    **
    ** If the EXCLUSIVE lock is already held or the attempt to obtain it is
    ** successful, or the connection is in WAL mode, SQLITE_OK is returned.
    ** Otherwise, either SQLITE_BUSY or an SQLITE_IOERR_XXX error code is 
    ** returned.
    */
    static int sqlite3PagerExclusiveLock( Pager pPager )
    {
      int rc = SQLITE_OK;
      Debug.Assert( pPager.eState == PAGER_WRITER_CACHEMOD
      || pPager.eState == PAGER_WRITER_DBMOD
      || pPager.eState == PAGER_WRITER_LOCKED
      );
      Debug.Assert( assert_pager_state( pPager ) );
      if ( false == pagerUseWal( pPager ) )
      {
        rc = pager_wait_on_lock( pPager, EXCLUSIVE_LOCK );
      }
      return rc;
    }

    /*
    ** Sync the database file for the pager pPager. zMaster points to the name
    ** of a master journal file that should be written into the individual
    ** journal file. zMaster may be NULL, which is interpreted as no master
    ** journal (a single database transaction).
    **
    ** This routine ensures that:
    **
    **   * The database file change-counter is updated,
    **   * the journal is synced (unless the atomic-write optimization is used),
    **   * all dirty pages are written to the database file,
    **   * the database file is truncated (if required), and
    **   * the database file synced.
    **
    ** The only thing that remains to commit the transaction is to finalize
    ** (delete, truncate or zero the first part of) the journal file (or
    ** delete the master journal file if specified).
    **
    ** Note that if zMaster==NULL, this does not overwrite a previous value
    ** passed to an sqlite3PagerCommitPhaseOne() call.
    **
    ** If the final parameter - noSync - is true, then the database file itself
    ** is not synced. The caller must call sqlite3PagerSync() directly to
    ** sync the database file before calling CommitPhaseTwo() to delete the
    ** journal file in this case.
    */
    static int sqlite3PagerCommitPhaseOne(
    Pager pPager,         /* Pager object */
    string zMaster,       /* If not NULL, the master journal name */
    bool noSync           /* True to omit the xSync on the db file */
    )
    {
      int rc = SQLITE_OK;             /* Return code */

      Debug.Assert( pPager.eState == PAGER_WRITER_LOCKED
      || pPager.eState == PAGER_WRITER_CACHEMOD
      || pPager.eState == PAGER_WRITER_DBMOD
      || pPager.eState == PAGER_ERROR
      );
      Debug.Assert( assert_pager_state( pPager ) );

      /* If a prior error occurred, report that error again. */
      if ( NEVER( pPager.errCode != 0 ) )
        return pPager.errCode;

      PAGERTRACE( "DATABASE SYNC: File=%s zMaster=%s nSize=%d\n",
      pPager.zFilename, zMaster, pPager.dbSize );

      /* If no database changes have been made, return early. */
      if ( pPager.eState < PAGER_WRITER_CACHEMOD )
        return SQLITE_OK;

      if (
#if SQLITE_OMIT_MEMORYDB
0 != MEMDB
#else
 0 != pPager.memDb
#endif
 )
      {
        /* If this is an in-memory db, or no pages have been written to, or this
        ** function has already been called, it is mostly a no-op.  However, any
        ** backup in progress needs to be restarted.
        */
        sqlite3BackupRestart( pPager.pBackup );
      }
      else
      {
        if ( pagerUseWal( pPager ) )
        {
          PgHdr pList = sqlite3PcacheDirtyList( pPager.pPCache );
          PgHdr pPageOne = null;
          if ( pList == null )
          {
            /* Must have at least one page for the WAL commit flag.
            ** Ticket [2d1a5c67dfc2363e44f29d9bbd57f] 2null11-null5-18 */
            rc = sqlite3PagerGet( pPager, 1, ref pPageOne );
            pList = pPageOne;
            pList.pDirty = null;
          }
          Debug.Assert( rc == SQLITE_OK );
          if ( ALWAYS( pList ) )
          {

            rc = pagerWalFrames( pPager, pList, pPager.dbSize, 1,
            ( pPager.fullSync ? pPager.syncFlags : (byte)0 )
            );
          }
          sqlite3PagerUnref( pPageOne );
          if ( rc == SQLITE_OK )
          {
            sqlite3PcacheCleanAll( pPager.pPCache );
          }
        }
        else
        {

          /* The following block updates the change-counter. Exactly how it
          ** does this depends on whether or not the atomic-update optimization
          ** was enabled at compile time, and if this transaction meets the
          ** runtime criteria to use the operation:
          **
          **    * The file-system supports the atomic-write property for
          **      blocks of size page-size, and
          **    * This commit is not part of a multi-file transaction, and
          **    * Exactly one page has been modified and store in the journal file.
          **
          ** If the optimization was not enabled at compile time, then the
          ** pager_incr_changecounter() function is called to update the change
          ** counter in 'indirect-mode'. If the optimization is compiled in but
          ** is not applicable to this transaction, call sqlite3JournalCreate()
          ** to make sure the journal file has actually been created, then call
          ** pager_incr_changecounter() to update the change-counter in indirect
          ** mode.
          **
          ** Otherwise, if the optimization is both enabled and applicable,
          ** then call pager_incr_changecounter() to update the change-counter
          ** in 'direct' mode. In this case the journal file will never be
          ** created for this transaction.
          */
#if SQLITE_ENABLE_ATOMIC_WRITE
PgHdr *pPg;
Debug.Assert( isOpen(pPager.jfd) 
|| pPager.journalMode==PAGER_JOURNALMODE_OFF 
|| pPager.journalMode==PAGER_JOURNALMODE_WAL 
);
if( !zMaster && isOpen(pPager.jfd)
&& pPager.journalOff==jrnlBufferSize(pPager)
&& pPager.dbSize>=pPager.dbOrigSize
&& (0==(pPg = sqlite3PcacheDirtyList(pPager.pPCache)) || 0==pPg.pDirty)
){
/* Update the db file change counter via the direct-write method. The
** following call will modify the in-memory representation of page 1
** to include the updated change counter and then write page 1
** directly to the database file. Because of the atomic-write
** property of the host file-system, this is safe.
*/
rc = pager_incr_changecounter(pPager, 1);
}else{
rc = sqlite3JournalCreate(pPager.jfd);
if( rc==SQLITE_OK ){
rc = pager_incr_changecounter(pPager, 0);
}
}
#else
          rc = pager_incr_changecounter( pPager, false );
#endif
          if ( rc != SQLITE_OK )
            goto commit_phase_one_exit;

          /* If this transaction has made the database smaller, then all pages
          ** being discarded by the truncation must be written to the journal
          ** file. This can only happen in auto-vacuum mode.
          **
          ** Before reading the pages with page numbers larger than the
          ** current value of Pager.dbSize, set dbSize back to the value
          ** that it took at the start of the transaction. Otherwise, the
          ** calls to sqlite3PagerGet() return zeroed pages instead of
          ** reading data from the database file.
          */
#if !SQLITE_OMIT_AUTOVACUUM
          if ( pPager.dbSize < pPager.dbOrigSize
          && pPager.journalMode != PAGER_JOURNALMODE_OFF
          )
          {
            Pgno i;                                   /* Iterator variable */
            Pgno iSkip = PAGER_MJ_PGNO( pPager ); /* Pending lock page */
            Pgno dbSize = pPager.dbSize;       /* Database image size */
            pPager.dbSize = pPager.dbOrigSize;
            for ( i = dbSize + 1; i <= pPager.dbOrigSize; i++ )
            {
              if ( 0 == sqlite3BitvecTest( pPager.pInJournal, i ) && i != iSkip )
              {
                PgHdr pPage = null;             /* Page to journal */
                rc = sqlite3PagerGet( pPager, i, ref pPage );
                if ( rc != SQLITE_OK )
                  goto commit_phase_one_exit;
                rc = sqlite3PagerWrite( pPage );
                sqlite3PagerUnref( pPage );
                if ( rc != SQLITE_OK )
                  goto commit_phase_one_exit;
              }
            }
            pPager.dbSize = dbSize;
          }
#endif

          /* Write the master journal name into the journal file. If a master
** journal file name has already been written to the journal file,
** or if zMaster is NULL (no master journal), then this call is a no-op.
*/
          rc = writeMasterJournal( pPager, zMaster );
          if ( rc != SQLITE_OK )
            goto commit_phase_one_exit;

          /* Sync the journal file and write all dirty pages to the database.
          ** If the atomic-update optimization is being used, this sync will not 
          ** create the journal file or perform any real IO.
          **
          ** Because the change-counter page was just modified, unless the
          ** atomic-update optimization is used it is almost certain that the
          ** journal requires a sync here. However, in locking_mode=exclusive
          ** on a system under memory pressure it is just possible that this is 
          ** not the case. In this case it is likely enough that the redundant
          ** xSync() call will be changed to a no-op by the OS anyhow. 
          */
          rc = syncJournal( pPager, 0 );
          if ( rc != SQLITE_OK )
            goto commit_phase_one_exit;

          rc = pager_write_pagelist( pPager, sqlite3PcacheDirtyList( pPager.pPCache ) );
          if ( rc != SQLITE_OK )
          {
            Debug.Assert( rc != SQLITE_IOERR_BLOCKED );
            goto commit_phase_one_exit;
          }
          sqlite3PcacheCleanAll( pPager.pPCache );

          /* If the file on disk is not the same size as the database image,
          ** then use pager_truncate to grow or shrink the file here.
          */
          if ( pPager.dbSize != pPager.dbFileSize )
          {
            Pgno nNew = (Pgno)( pPager.dbSize - ( pPager.dbSize == PAGER_MJ_PGNO( pPager ) ? 1 : 0 ) );
            Debug.Assert( pPager.eState >= PAGER_WRITER_DBMOD );
            rc = pager_truncate( pPager, nNew );
            if ( rc != SQLITE_OK )
              goto commit_phase_one_exit;
          }

          /* Finally, sync the database file. */
          if ( !noSync )
          {
            rc = sqlite3PagerSync( pPager );
          }
          IOTRACE( "DBSYNC %p\n", pPager );
        }
      }

commit_phase_one_exit:
      if ( rc == SQLITE_OK && !pagerUseWal( pPager ) )
      {
        pPager.eState = PAGER_WRITER_FINISHED;
      }
      return rc;
    }


    /*
    ** When this function is called, the database file has been completely
    ** updated to reflect the changes made by the current transaction and
    ** synced to disk. The journal file still exists in the file-system
    ** though, and if a failure occurs at this point it will eventually
    ** be used as a hot-journal and the current transaction rolled back.
    **
    ** This function finalizes the journal file, either by deleting,
    ** truncating or partially zeroing it, so that it cannot be used
    ** for hot-journal rollback. Once this is done the transaction is
    ** irrevocably committed.
    **
    ** If an error occurs, an IO error code is returned and the pager
    ** moves into the error state. Otherwise, SQLITE_OK is returned.
    */
    static int sqlite3PagerCommitPhaseTwo( Pager pPager )
    {
      int rc = SQLITE_OK;                 /* Return code */

      /* This routine should not be called if a prior error has occurred.
      ** But if (due to a coding error elsewhere in the system) it does get
      ** called, just return the same error code without doing anything. */
      if ( NEVER( pPager.errCode ) != 0 )
        return pPager.errCode;

      Debug.Assert( pPager.eState == PAGER_WRITER_LOCKED
      || pPager.eState == PAGER_WRITER_FINISHED
      || ( pagerUseWal( pPager ) && pPager.eState == PAGER_WRITER_CACHEMOD )
      );
      Debug.Assert( assert_pager_state( pPager ) );

      /* An optimization. If the database was not actually modified during
      ** this transaction, the pager is running in exclusive-mode and is
      ** using persistent journals, then this function is a no-op.
      **
      ** The start of the journal file currently contains a single journal
      ** header with the nRec field set to 0. If such a journal is used as
      ** a hot-journal during hot-journal rollback, 0 changes will be made
      ** to the database file. So there is no need to zero the journal
      ** header. Since the pager is in exclusive mode, there is no need
      ** to drop any locks either.
      */
      if ( pPager.eState == PAGER_WRITER_LOCKED
      && pPager.exclusiveMode
      && pPager.journalMode == PAGER_JOURNALMODE_PERSIST
      )
      {
        Debug.Assert( pPager.journalOff == JOURNAL_HDR_SZ( pPager ) || 0 == pPager.journalOff );
        pPager.eState = PAGER_READER;
        return SQLITE_OK;
      }

      PAGERTRACE( "COMMIT %d\n", PAGERID( pPager ) );
      rc = pager_end_transaction( pPager, pPager.setMaster );
      return pager_error( pPager, rc );
    }

    /*
    ** If a write transaction is open, then all changes made within the 
    ** transaction are reverted and the current write-transaction is closed.
    ** The pager falls back to PAGER_READER state if successful, or PAGER_ERROR
    ** state if an error occurs.
    **
    ** If the pager is already in PAGER_ERROR state when this function is called,
    ** it returns Pager.errCode immediately. No work is performed in this case.
    **
    ** Otherwise, in rollback mode, this function performs two functions:
    **
    **   1) It rolls back the journal file, restoring all database file and 
    **      in-memory cache pages to the state they were in when the transaction
    **      was opened, and
    **
    **   2) It finalizes the journal file, so that it is not used for hot
    **      rollback at any point in the future.
    **
    ** Finalization of the journal file (task 2) is only performed if the 
    ** rollback is successful.
    **
    ** In WAL mode, all cache-entries containing data modified within the
    ** current transaction are either expelled from the cache or reverted to
    ** their pre-transaction state by re-reading data from the database or
    ** WAL files. The WAL transaction is then closed.
    */
    static int sqlite3PagerRollback( Pager pPager )
    {
      int rc = SQLITE_OK;                  /* Return code */
      PAGERTRACE( "ROLLBACK %d\n", PAGERID( pPager ) );

      /* PagerRollback() is a no-op if called in READER or OPEN state. If
      ** the pager is already in the ERROR state, the rollback is not 
      ** attempted here. Instead, the error code is returned to the caller.
      */
      Debug.Assert( assert_pager_state( pPager ) );
      if ( pPager.eState == PAGER_ERROR )
        return pPager.errCode;
      if ( pPager.eState <= PAGER_READER )
        return SQLITE_OK;

      if ( pagerUseWal( pPager ) )
      {
        int rc2;

        rc = sqlite3PagerSavepoint( pPager, SAVEPOINT_ROLLBACK, -1 );
        rc2 = pager_end_transaction( pPager, pPager.setMaster );
        if ( rc == SQLITE_OK )
          rc = rc2;
        rc = pager_error( pPager, rc );
      }
      else if ( !isOpen( pPager.jfd ) || pPager.eState == PAGER_WRITER_LOCKED )
      {
        int eState = pPager.eState;
        rc = pager_end_transaction( pPager, 0 );
        if (
#if SQLITE_OMIT_MEMORYDB
0==MEMDB
#else
 0 == pPager.memDb
#endif
 && eState > PAGER_WRITER_LOCKED )
        {
          /* This can happen using journal_mode=off. Move the pager to the error 
          ** state to indicate that the contents of the cache may not be trusted.
          ** Any active readers will get SQLITE_ABORT.
          */
          pPager.errCode = SQLITE_ABORT;
          pPager.eState = PAGER_ERROR;
          return rc;
        }
      }
      else
      {
        rc = pager_playback( pPager, 0 );
      }

      Debug.Assert( pPager.eState == PAGER_READER || rc != SQLITE_OK );
      Debug.Assert( rc == SQLITE_OK || rc == SQLITE_FULL || ( rc & 0xFF ) == SQLITE_IOERR );

      /* If an error occurs during a ROLLBACK, we can no longer trust the pager
      ** cache. So call pager_error() on the way out to make any error persistent.
      */
      return pager_error( pPager, rc );
    }


    /*
    ** Return TRUE if the database file is opened read-only.  Return FALSE
    ** if the database is (in theory) writable.
    */
    static bool sqlite3PagerIsreadonly( Pager pPager )
    {
      return pPager.readOnly;
    }

    /*
    ** Return the number of references to the pager.
    */
    static int sqlite3PagerRefcount( Pager pPager )
    {
      return sqlite3PcacheRefCount( pPager.pPCache );
    }

    /*
    ** Return the approximate number of bytes of memory currently
    ** used by the pager and its associated cache.
    */
    static int sqlite3PagerMemUsed( Pager pPager )
    {
      int perPageSize = pPager.pageSize + pPager.nExtra + 20; //+ sizeof(PgHdr) + 5*sizeof(void*);
      return perPageSize * sqlite3PcachePagecount( pPager.pPCache )
      + 0// Not readily available under C#// sqlite3MallocSize(pPager);
      + pPager.pageSize;
    }

    /*
    ** Return the number of references to the specified page.
    */
    static int sqlite3PagerPageRefcount( DbPage pPage )
    {
      return sqlite3PcachePageRefcount( pPage );
    }


#if SQLITE_TEST
    /*
** This routine is used for testing and analysis only.
*/
    static int[] sqlite3PagerStats( Pager pPager )
    {
      int[] a = new int[11];
      a[0] = sqlite3PcacheRefCount( pPager.pPCache );
      a[1] = sqlite3PcachePagecount( pPager.pPCache );
      a[2] = sqlite3PcacheGetCachesize( pPager.pPCache );
      a[3] = pPager.eState == PAGER_OPEN ? -1 : (int)pPager.dbSize;
      a[4] = pPager.eState;
      a[5] = pPager.errCode;
      a[6] = pPager.nHit;
      a[7] = pPager.nMiss;
      a[8] = 0;  /* Used to be pPager.nOvfl */
      a[9] = pPager.nRead;
      a[10] = pPager.nWrite;
      return a;
    }
#endif

    /*
** Return true if this is an in-memory pager.
*/
    static bool sqlite3PagerIsMemdb( Pager pPager )
    {
#if SQLITE_OMIT_MEMORYDB
return MEMDB != 0;
#else
      return pPager.memDb != 0;
#endif
    }

    /*
    ** Check that there are at least nSavepoint savepoints open. If there are
    ** currently less than nSavepoints open, then open one or more savepoints
    ** to make up the difference. If the number of savepoints is already
    ** equal to nSavepoint, then this function is a no-op.
    **
    ** If a memory allocation fails, SQLITE_NOMEM is returned. If an error
    ** occurs while opening the sub-journal file, then an IO error code is
    ** returned. Otherwise, SQLITE_OK.
    */
    static int sqlite3PagerOpenSavepoint( Pager pPager, int nSavepoint )
    {
      int rc = SQLITE_OK;                      /* Return code */
      int nCurrent = pPager.nSavepoint;        /* Current number of savepoints */

      Debug.Assert( pPager.eState >= PAGER_WRITER_LOCKED );
      Debug.Assert( assert_pager_state( pPager ) );

      if ( nSavepoint > nCurrent && pPager.useJournal != 0 )
      {
        int ii;                 /* Iterator variable */
        PagerSavepoint[] aNew;  /* New Pager.aSavepoint array */

        /* Grow the Pager.aSavepoint array using realloc(). Return SQLITE_NOMEM
        ** if the allocation fails. Otherwise, zero the new portion in case a 
        ** malloc failure occurs while populating it in the for(...) loop below.
        */
        //aNew = (PagerSavepoint *)sqlite3Realloc(
        //    pPager.aSavepoint, sizeof(PagerSavepoint)*nSavepoint
        //);
        Array.Resize( ref pPager.aSavepoint, nSavepoint );
        aNew = pPager.aSavepoint;
        //if( null==aNew ){
        //  return SQLITE_NOMEM;
        //}
        // memset(&aNew[nCurrent], 0, (nSavepoint-nCurrent) * sizeof(PagerSavepoint));
        // pPager.aSavepoint = aNew;

        /* Populate the PagerSavepoint structures just allocated. */
        for ( ii = nCurrent; ii < nSavepoint; ii++ )
        {
          aNew[ii] = new PagerSavepoint();
          aNew[ii].nOrig = pPager.dbSize;
          if ( isOpen( pPager.jfd ) && pPager.journalOff > 0 )
          {
            aNew[ii].iOffset = pPager.journalOff;
          }
          else
          {
            aNew[ii].iOffset = (int)JOURNAL_HDR_SZ( pPager );
          }
          aNew[ii].iSubRec = pPager.nSubRec;
          aNew[ii].pInSavepoint = sqlite3BitvecCreate( pPager.dbSize );
          //if ( null == aNew[ii].pInSavepoint )
          //{
          //  return SQLITE_NOMEM;
          //}
          if ( pagerUseWal( pPager ) )
          {
            sqlite3WalSavepoint( pPager.pWal, aNew[ii].aWalData );
          }
          pPager.nSavepoint = ii + 1;
        }
        Debug.Assert( pPager.nSavepoint == nSavepoint );
        assertTruncateConstraint( pPager );
      }

      return rc;
    }

    /*
    ** This function is called to rollback or release (commit) a savepoint.
    ** The savepoint to release or rollback need not be the most recently
    ** created savepoint.
    **
    ** Parameter op is always either SAVEPOINT_ROLLBACK or SAVEPOINT_RELEASE.
    ** If it is SAVEPOINT_RELEASE, then release and destroy the savepoint with
    ** index iSavepoint. If it is SAVEPOINT_ROLLBACK, then rollback all changes
    ** that have occurred since the specified savepoint was created.
    **
    ** The savepoint to rollback or release is identified by parameter
    ** iSavepoint. A value of 0 means to operate on the outermost savepoint
    ** (the first created). A value of (Pager.nSavepoint-1) means operate
    ** on the most recently created savepoint. If iSavepoint is greater than
    ** (Pager.nSavepoint-1), then this function is a no-op.
    **
    ** If a negative value is passed to this function, then the current
    ** transaction is rolled back. This is different to calling
    ** sqlite3PagerRollback() because this function does not terminate
    ** the transaction or unlock the database, it just restores the
    ** contents of the database to its original state.
    **
    ** In any case, all savepoints with an index greater than iSavepoint
    ** are destroyed. If this is a release operation (op==SAVEPOINT_RELEASE),
    ** then savepoint iSavepoint is also destroyed.
    **
    ** This function may return SQLITE_NOMEM if a memory allocation fails,
    ** or an IO error code if an IO error occurs while rolling back a
    ** savepoint. If no errors occur, SQLITE_OK is returned.
    */
    static int sqlite3PagerSavepoint( Pager pPager, int op, int iSavepoint )
    {
      int rc = pPager.errCode;       /* Return code */

      Debug.Assert( op == SAVEPOINT_RELEASE || op == SAVEPOINT_ROLLBACK );
      Debug.Assert( iSavepoint >= 0 || op == SAVEPOINT_ROLLBACK );

      if ( rc == SQLITE_OK && iSavepoint < pPager.nSavepoint )
      {
        int ii;        /* Iterator variable */
        int nNew;      /* Number of remaining savepoints after this op. */

        /* Figure out how many savepoints will still be active after this
        ** operation. Store this value in nNew. Then free resources associated
        ** with any savepoints that are destroyed by this operation.
        */
        nNew = iSavepoint + ( ( op == SAVEPOINT_RELEASE ) ? 0 : 1 );
        for ( ii = nNew; ii < pPager.nSavepoint; ii++ )
        {
          sqlite3BitvecDestroy( ref pPager.aSavepoint[ii].pInSavepoint );
        }
        pPager.nSavepoint = nNew;

        /* If this is a release of the outermost savepoint, truncate 
        ** the sub-journal to zero bytes in size. */
        if ( op == SAVEPOINT_RELEASE )
        {
          if ( nNew == 0 && isOpen( pPager.sjfd ) )
          {
            /* Only truncate if it is an in-memory sub-journal. */
            if ( sqlite3IsMemJournal( pPager.sjfd ) )
            {
              rc = sqlite3OsTruncate( pPager.sjfd, 0 );
              Debug.Assert( rc == SQLITE_OK );
            }
            pPager.nSubRec = 0;
          }
        }
        /* Else this is a rollback operation, playback the specified savepoint.
        ** If this is a temp-file, it is possible that the journal file has
        ** not yet been opened. In this case there have been no changes to
        ** the database file, so the playback operation can be skipped.
        */
        else if ( pagerUseWal( pPager ) || isOpen( pPager.jfd ) )
        {
          PagerSavepoint pSavepoint = ( nNew == 0 ) ? null : pPager.aSavepoint[nNew - 1];
          rc = pagerPlaybackSavepoint( pPager, pSavepoint );
          Debug.Assert( rc != SQLITE_DONE );
        }
      }
      return rc;
    }

    /*
    ** Return the full pathname of the database file.
    */
    static string sqlite3PagerFilename( Pager pPager )
    {
      return pPager.zFilename;
    }

    /*
    ** Return the VFS structure for the pager.
    */
    static sqlite3_vfs sqlite3PagerVfs( Pager pPager )
    {
      return pPager.pVfs;
    }

    /*
    ** Return the file handle for the database file associated
    ** with the pager.  This might return NULL if the file has
    ** not yet been opened.
    */
    static sqlite3_file sqlite3PagerFile( Pager pPager )
    {
      return pPager.fd;
    }

    /*
    ** Return the full pathname of the journal file.
    */
    static string sqlite3PagerJournalname( Pager pPager )
    {
      return pPager.zJournal;
    }

    /*
    ** Return true if fsync() calls are disabled for this pager.  Return FALSE
    ** if fsync()s are executed normally.
    */
    static bool sqlite3PagerNosync( Pager pPager )
    {
      return pPager.noSync;
    }

#if SQLITE_HAS_CODEC
    /*
** Set or retrieve the codec for this pager
*/
    static void sqlite3PagerSetCodec(
    Pager pPager,
    dxCodec xCodec,                 //void *(*xCodec)(void*,void*,Pgno,int),
    dxCodecSizeChng xCodecSizeChng, //void (*xCodecSizeChng)(void*,int,int),
    dxCodecFree xCodecFree,         //void (*xCodecFree)(void*),
    codec_ctx pCodec
    )
    {
      if ( pPager.xCodecFree != null )
        pPager.xCodecFree( ref pPager.pCodec );
      pPager.xCodec = ( pPager.memDb != 0 ) ? null : xCodec;
      pPager.xCodecSizeChng = xCodecSizeChng;
      pPager.xCodecFree = xCodecFree;
      pPager.pCodec = pCodec;
      pagerReportSize( pPager );
    }

    static object sqlite3PagerGetCodec( Pager pPager )
    {
      return pPager.pCodec;
    }
#endif

#if !SQLITE_OMIT_AUTOVACUUM
    /*
** Move the page pPg to location pgno in the file.
**
** There must be no references to the page previously located at
** pgno (which we call pPgOld) though that page is allowed to be
** in cache.  If the page previously located at pgno is not already
** in the rollback journal, it is not put there by by this routine.
**
** References to the page pPg remain valid. Updating any
** meta-data associated with pPg (i.e. data stored in the nExtra bytes
** allocated along with the page) is the responsibility of the caller.
**
** A transaction must be active when this routine is called. It used to be
** required that a statement transaction was not active, but this restriction
** has been removed (CREATE INDEX needs to move a page when a statement
** transaction is active).
**
** If the fourth argument, isCommit, is non-zero, then this page is being
** moved as part of a database reorganization just before the transaction
** is being committed. In this case, it is guaranteed that the database page
** pPg refers to will not be written to again within this transaction.
**
** This function may return SQLITE_NOMEM or an IO error code if an error
** occurs. Otherwise, it returns SQLITE_OK.
*/
    static int sqlite3PagerMovepage( Pager pPager, DbPage pPg, u32 pgno, int isCommit )
    {
      PgHdr pPgOld;                /* The page being overwritten. */
      u32 needSyncPgno = 0;        /* Old value of pPg.pgno, if sync is required */
      int rc;                      /* Return code */
      Pgno origPgno;               /* The original page number */

      Debug.Assert( pPg.nRef > 0 );
      Debug.Assert( pPager.eState == PAGER_WRITER_CACHEMOD
      || pPager.eState == PAGER_WRITER_DBMOD
      );
      Debug.Assert( assert_pager_state( pPager ) );

      /* In order to be able to rollback, an in-memory database must journal
      ** the page we are moving from.
      */
      if (
#if SQLITE_OMIT_MEMORYDB
1==MEMDB
#else
 pPager.memDb != 0
#endif
 )
      {
        rc = sqlite3PagerWrite( pPg );
        if ( rc != 0 )
          return rc;
      }

      /* If the page being moved is dirty and has not been saved by the latest
      ** savepoint, then save the current contents of the page into the
      ** sub-journal now. This is required to handle the following scenario:
      **
      **   BEGIN;
      **     <journal page X, then modify it in memory>
      **     SAVEPOINT one;
      **       <Move page X to location Y>
      **     ROLLBACK TO one;
      **
      ** If page X were not written to the sub-journal here, it would not
      ** be possible to restore its contents when the "ROLLBACK TO one"
      ** statement were is processed.
      **
      ** subjournalPage() may need to allocate space to store pPg.pgno into
      ** one or more savepoint bitvecs. This is the reason this function
      ** may return SQLITE_NOMEM.
      */
      if ( ( pPg.flags & PGHDR_DIRTY ) != 0
      && subjRequiresPage( pPg )
      && SQLITE_OK != ( rc = subjournalPage( pPg ) )
      )
      {
        return rc;
      }

      PAGERTRACE( "MOVE %d page %d (needSync=%d) moves to %d\n",
      PAGERID( pPager ), pPg.pgno, ( pPg.flags & PGHDR_NEED_SYNC ) != 0 ? 1 : 0, pgno );
      IOTRACE( "MOVE %p %d %d\n", pPager, pPg.pgno, pgno );

      /* If the journal needs to be sync()ed before page pPg.pgno can
      ** be written to, store pPg.pgno in local variable needSyncPgno.
      **
      ** If the isCommit flag is set, there is no need to remember that
      ** the journal needs to be sync()ed before database page pPg.pgno
      ** can be written to. The caller has already promised not to write to it.
      */
      if ( ( ( pPg.flags & PGHDR_NEED_SYNC ) != 0 ) && 0 == isCommit )
      {
        needSyncPgno = pPg.pgno;
        Debug.Assert( pageInJournal( pPg ) || pPg.pgno > pPager.dbOrigSize );
        Debug.Assert( ( pPg.flags & PGHDR_DIRTY ) != 0 );
      }

      /* If the cache contains a page with page-number pgno, remove it
      ** from its hash chain. Also, if the PGHDR_NEED_SYNC was set for
      ** page pgno before the 'move' operation, it needs to be retained
      ** for the page moved there.
      */
      pPg.flags &= ~PGHDR_NEED_SYNC;
      pPgOld = pager_lookup( pPager, pgno );
      Debug.Assert( null == pPgOld || pPgOld.nRef == 1 );
      if ( pPgOld != null )
      {
        pPg.flags |= ( pPgOld.flags & PGHDR_NEED_SYNC );
        if (
#if SQLITE_OMIT_MEMORYDB
1==MEMDB
#else
 pPager.memDb != 0
#endif
 )
        {
          /* Do not discard pages from an in-memory database since we might
          ** need to rollback later.  Just move the page out of the way. */
          sqlite3PcacheMove( pPgOld, pPager.dbSize + 1 );
        }
        else
        {
          sqlite3PcacheDrop( pPgOld );
        }
      }
      origPgno = pPg.pgno;
      sqlite3PcacheMove( pPg, pgno );
      sqlite3PcacheMakeDirty( pPg );

      /* For an in-memory database, make sure the original page continues
      ** to exist, in case the transaction needs to roll back.  Use pPgOld
      ** as the original page since it has already been allocated.
      */
      if (
#if SQLITE_OMIT_MEMORYDB
0!=MEMDB
#else
 0 != pPager.memDb
#endif
 )
      {
        Debug.Assert( pPgOld );
        sqlite3PcacheMove( pPgOld, origPgno );
        sqlite3PagerUnref( pPgOld );
      }

      if ( needSyncPgno != 0 )
      {
        /* If needSyncPgno is non-zero, then the journal file needs to be
        ** sync()ed before any data is written to database file page needSyncPgno.
        ** Currently, no such page exists in the page-cache and the
        ** "is journaled" bitvec flag has been set. This needs to be remedied by
        ** loading the page into the pager-cache and setting the PGHDR_NEED_SYNC
        ** flag.
        **
        ** If the attempt to load the page into the page-cache fails, (due
        ** to a malloc() or IO failure), clear the bit in the pInJournal[]
        ** array. Otherwise, if the page is loaded and written again in
        ** this transaction, it may be written to the database file before
        ** it is synced into the journal file. This way, it may end up in
        ** the journal file twice, but that is not a problem.
        */
        PgHdr pPgHdr = null;
        rc = sqlite3PagerGet( pPager, needSyncPgno, ref pPgHdr );
        if ( rc != SQLITE_OK )
        {
          if ( needSyncPgno <= pPager.dbOrigSize )
          {
            Debug.Assert( pPager.pTmpSpace != null );
            u32[] pTemp = new u32[pPager.pTmpSpace.Length];
            sqlite3BitvecClear( pPager.pInJournal, needSyncPgno, pTemp );//pPager.pTmpSpace );
          }
          return rc;
        }
        pPgHdr.flags |= PGHDR_NEED_SYNC;
        sqlite3PcacheMakeDirty( pPgHdr );
        sqlite3PagerUnref( pPgHdr );
      }
      return SQLITE_OK;
    }
#endif

    /*
** Return a pointer to the data for the specified page.
*/
    static byte[] sqlite3PagerGetData( DbPage pPg )
    {
      Debug.Assert( pPg.nRef > 0 || pPg.pPager.memDb != 0 );
      return pPg.pData;
    }

    /*
    ** Return a pointer to the Pager.nExtra bytes of "extra" space
    ** allocated along with the specified page.
    */
    static MemPage sqlite3PagerGetExtra( DbPage pPg )
    {
      return pPg.pExtra;
    }

    /*
    ** Get/set the locking-mode for this pager. Parameter eMode must be one
    ** of PAGER_LOCKINGMODE_QUERY, PAGER_LOCKINGMODE_NORMAL or
    ** PAGER_LOCKINGMODE_EXCLUSIVE. If the parameter is not _QUERY, then
    ** the locking-mode is set to the value specified.
    **
    ** The returned value is either PAGER_LOCKINGMODE_NORMAL or
    ** PAGER_LOCKINGMODE_EXCLUSIVE, indicating the current (possibly updated)
    ** locking-mode.
    */
    static bool sqlite3PagerLockingMode( Pager pPager, int eMode )
    {
      Debug.Assert( eMode == PAGER_LOCKINGMODE_QUERY
      || eMode == PAGER_LOCKINGMODE_NORMAL
      || eMode == PAGER_LOCKINGMODE_EXCLUSIVE );
      Debug.Assert( PAGER_LOCKINGMODE_QUERY < 0 );
      Debug.Assert( PAGER_LOCKINGMODE_NORMAL >= 0 && PAGER_LOCKINGMODE_EXCLUSIVE >= 0 );
      Debug.Assert( pPager.exclusiveMode || false == sqlite3WalHeapMemory( pPager.pWal ) );
      if ( eMode >= 0 && !pPager.tempFile && !sqlite3WalHeapMemory( pPager.pWal ) )
      {
        pPager.exclusiveMode = eMode != 0;
      }
      return pPager.exclusiveMode;
    }

    /*
    ** Set the journal-mode for this pager. Parameter eMode must be one of:
    **
    **    PAGER_JOURNALMODE_DELETE
    **    PAGER_JOURNALMODE_TRUNCATE
    **    PAGER_JOURNALMODE_PERSIST
    **    PAGER_JOURNALMODE_OFF
    **    PAGER_JOURNALMODE_MEMORY
    **    PAGER_JOURNALMODE_WAL
    **
    ** The journalmode is set to the value specified if the change is allowed.
    ** The change may be disallowed for the following reasons:
    **
    **   *  An in-memory database can only have its journal_mode set to _OFF
    **      or _MEMORY.
    **
    **   *  Temporary databases cannot have _WAL journalmode.
    **
    ** The returned indicate the current (possibly updated) journal-mode.
    */
    static int sqlite3PagerSetJournalMode( Pager pPager, int eMode )
    {
      u8 eOld = pPager.journalMode;    /* Prior journalmode */

#if SQLITE_DEBUG
      /* The print_pager_state() routine is intended to be used by the debugger
** only.  We invoke it once here to suppress a compiler warning. */
      print_pager_state( pPager );
#endif

      /* The eMode parameter is always valid */
      Debug.Assert( eMode == PAGER_JOURNALMODE_DELETE
      || eMode == PAGER_JOURNALMODE_TRUNCATE
      || eMode == PAGER_JOURNALMODE_PERSIST
      || eMode == PAGER_JOURNALMODE_OFF
      || eMode == PAGER_JOURNALMODE_WAL
      || eMode == PAGER_JOURNALMODE_MEMORY );

      /* This routine is only called from the OP_JournalMode opcode, and
      ** the logic there will never allow a temporary file to be changed
      ** to WAL mode.
      */
      Debug.Assert( pPager.tempFile == false || eMode != PAGER_JOURNALMODE_WAL );

      /* Do allow the journalmode of an in-memory database to be set to
      ** anything other than MEMORY or OFF
      */
      if (
#if SQLITE_OMIT_MEMORYDB
1==MEMDB
#else
 1 == pPager.memDb
#endif
 )
      {
        Debug.Assert( eOld == PAGER_JOURNALMODE_MEMORY || eOld == PAGER_JOURNALMODE_OFF );
        if ( eMode != PAGER_JOURNALMODE_MEMORY && eMode != PAGER_JOURNALMODE_OFF )
        {
          eMode = eOld;
        }
      }

      if ( eMode != eOld )
      {
        /* Change the journal mode. */
        Debug.Assert( pPager.eState != PAGER_ERROR );
        pPager.journalMode = (u8)eMode;

        /* When transistioning from TRUNCATE or PERSIST to any other journal
        ** mode except WAL, unless the pager is in locking_mode=exclusive mode,
        ** delete the journal file.
        */
        Debug.Assert( ( PAGER_JOURNALMODE_TRUNCATE & 5 ) == 1 );
        Debug.Assert( ( PAGER_JOURNALMODE_PERSIST & 5 ) == 1 );
        Debug.Assert( ( PAGER_JOURNALMODE_DELETE & 5 ) == 0 );
        Debug.Assert( ( PAGER_JOURNALMODE_MEMORY & 5 ) == 4 );
        Debug.Assert( ( PAGER_JOURNALMODE_OFF & 5 ) == 0 );
        Debug.Assert( ( PAGER_JOURNALMODE_WAL & 5 ) == 5 );

        Debug.Assert( isOpen( pPager.fd ) || pPager.exclusiveMode );
        if ( !pPager.exclusiveMode && ( eOld & 5 ) == 1 && ( eMode & 1 ) == 0 )
        {

          /* In this case we would like to delete the journal file. If it is
          ** not possible, then that is not a problem. Deleting the journal file
          ** here is an optimization only.
          **
          ** Before deleting the journal file, obtain a RESERVED lock on the
          ** database file. This ensures that the journal file is not deleted
          ** while it is in use by some other client.
          */
          sqlite3OsClose( pPager.jfd );
          if ( pPager.eLock >= RESERVED_LOCK )
          {
            sqlite3OsDelete( pPager.pVfs, pPager.zJournal, 0 );
          }
          else
          {
            int rc = SQLITE_OK;
            int state = pPager.eState;
            Debug.Assert( state == PAGER_OPEN || state == PAGER_READER );
            if ( state == PAGER_OPEN )
            {
              rc = sqlite3PagerSharedLock( pPager );
            }
            if ( pPager.eState == PAGER_READER )
            {
              Debug.Assert( rc == SQLITE_OK );
              rc = pagerLockDb( pPager, RESERVED_LOCK );
            }
            if ( rc == SQLITE_OK )
            {
              sqlite3OsDelete( pPager.pVfs, pPager.zJournal, 0 );
            }
            if ( rc == SQLITE_OK && state == PAGER_READER )
            {
              pagerUnlockDb( pPager, SHARED_LOCK );
            }
            else if ( state == PAGER_OPEN )
            {
              pager_unlock( pPager );
            }
            Debug.Assert( state == pPager.eState );
          }
        }
      }

      /* Return the new journal mode */
      return (int)pPager.journalMode;
    }

    /*
    ** Return the current journal mode.
    */
    static int sqlite3PagerGetJournalMode( Pager pPager )
    {
      return (int)pPager.journalMode;
    }

    /*
    ** Return TRUE if the pager is in a state where it is OK to change the
    ** journalmode.  Journalmode changes can only happen when the database
    ** is unmodified.
    */
    static int sqlite3PagerOkToChangeJournalMode( Pager pPager )
    {
      Debug.Assert( assert_pager_state( pPager ) );
      if ( pPager.eState >= PAGER_WRITER_CACHEMOD )
        return 0;
      if ( NEVER( isOpen( pPager.jfd ) && pPager.journalOff > 0 ) )
        return 0;
      return 1;
    }


    /*
    ** Get/set the size-limit used for persistent journal files.
    **
    ** Setting the size limit to -1 means no limit is enforced.
    ** An attempt to set a limit smaller than -1 is a no-op.
    */
    static i64 sqlite3PagerJournalSizeLimit( Pager pPager, i64 iLimit )
    {
      if ( iLimit >= -1 )
      {
        pPager.journalSizeLimit = iLimit;
        sqlite3WalLimit( pPager.pWal, iLimit );
      }
      return pPager.journalSizeLimit;
    }

    /*
    ** Return a pointer to the pPager.pBackup variable. The backup module
    ** in backup.c maintains the content of this variable. This module
    ** uses it opaquely as an argument to sqlite3BackupRestart() and
    ** sqlite3BackupUpdate() only.
    */
    static sqlite3_backup sqlite3PagerBackupPtr( Pager pPager )
    {
      return pPager.pBackup;
    }

#if !SQLITE_OMIT_WAL
/*
** This function is called when the user invokes "PRAGMA wal_checkpoint",
** "PRAGMA wal_blocking_checkpoint" or calls the sqlite3_wal_checkpoint()
** or wal_blocking_checkpoint() API functions.
**
** Parameter eMode is one of SQLITE_CHECKPOINT_PASSIVE, FULL or RESTART.
*/
int sqlite3PagerCheckpoint(Pager *pPager, int eMode, int *pnLog, int *pnCkpt){
  int rc = SQLITE_OK;
  if( pPager.pWal ){
    rc = sqlite3WalCheckpoint(pPager.pWal, eMode,
        pPager.xBusyHandler, pPager.pBusyHandlerArg,
        pPager.ckptSyncFlags, pPager.pageSize, (u8 *)pPager.pTmpSpace,
        pnLog, pnCkpt
    );
  }
  return rc;
}

    int sqlite3PagerWalCallback(Pager *pPager){
return sqlite3WalCallback(pPager.pWal);
}

/*
** Return true if the underlying VFS for the given pager supports the
** primitives necessary for write-ahead logging.
*/
int sqlite3PagerWalSupported(Pager *pPager){
const sqlite3_io_methods *pMethods = pPager.fd->pMethods;
return pPager.exclusiveMode || (pMethods->iVersion>=2 && pMethods->xShmMap);
}

/*
** Attempt to take an exclusive lock on the database file. If a PENDING lock
** is obtained instead, immediately release it.
*/
static int pagerExclusiveLock(Pager *pPager){
int rc;                         /* Return code */

assert( pPager.eLock==SHARED_LOCK || pPager.eLock==EXCLUSIVE_LOCK );
rc = pagerLockDb(pPager, EXCLUSIVE_LOCK);
if( rc!=SQLITE_OK ){
/* If the attempt to grab the exclusive lock failed, release the
** pending lock that may have been obtained instead.  */
pagerUnlockDb(pPager, SHARED_LOCK);
}

return rc;
}

/*
** Call sqlite3WalOpen() to open the WAL handle. If the pager is in 
** exclusive-locking mode when this function is called, take an EXCLUSIVE
** lock on the database file and use heap-memory to store the wal-index
** in. Otherwise, use the normal shared-memory.
*/
static int pagerOpenWal(Pager *pPager){
int rc = SQLITE_OK;

assert( pPager.pWal==0 && pPager.tempFile==0 );
assert( pPager.eLock==SHARED_LOCK || pPager.eLock==EXCLUSIVE_LOCK || pPager.noReadlock);

/* If the pager is already in exclusive-mode, the WAL module will use 
** heap-memory for the wal-index instead of the VFS shared-memory 
** implementation. Take the exclusive lock now, before opening the WAL
** file, to make sure this is safe.
*/
if( pPager.exclusiveMode ){
rc = pagerExclusiveLock(pPager);
}

/* Open the connection to the log file. If this operation fails, 
** (e.g. due to malloc() failure), return an error code.
*/
if( rc==SQLITE_OK ){
rc = sqlite3WalOpen(pPager.pVfs, 
pPager.fd, pPager.zWal, pPager.exclusiveMode, &pPager.pWal
        pPager.journalSizeLimit, &pPager.pWal
);
}

return rc;
}


/*
** The caller must be holding a SHARED lock on the database file to call
** this function.
**
** If the pager passed as the first argument is open on a real database
** file (not a temp file or an in-memory database), and the WAL file
** is not already open, make an attempt to open it now. If successful,
** return SQLITE_OK. If an error occurs or the VFS used by the pager does 
** not support the xShmXXX() methods, return an error code. *pbOpen is
** not modified in either case.
**
** If the pager is open on a temp-file (or in-memory database), or if
** the WAL file is already open, set *pbOpen to 1 and return SQLITE_OK
** without doing anything.
*/
int sqlite3PagerOpenWal(
Pager *pPager,                  /* Pager object */
int *pbOpen                     /* OUT: Set to true if call is a no-op */
){
int rc = SQLITE_OK;             /* Return code */

assert( assert_pager_state(pPager) );
assert( pPager.eState==PAGER_OPEN   || pbOpen );
assert( pPager.eState==PAGER_READER || !pbOpen );
assert( pbOpen==0 || *pbOpen==0 );
assert( pbOpen!=0 || (!pPager.tempFile && !pPager.pWal) );

if( !pPager.tempFile && !pPager.pWal ){
if( !sqlite3PagerWalSupported(pPager) ) return SQLITE_CANTOPEN;

/* Close any rollback journal previously open */
sqlite3OsClose(pPager.jfd);

rc = pagerOpenWal(pPager);
if( rc==SQLITE_OK ){
pPager.journalMode = PAGER_JOURNALMODE_WAL;
pPager.eState = PAGER_OPEN;
}
}else{
*pbOpen = 1;
}

return rc;
}

/*
** This function is called to close the connection to the log file prior
** to switching from WAL to rollback mode.
**
** Before closing the log file, this function attempts to take an 
** EXCLUSIVE lock on the database file. If this cannot be obtained, an
** error (SQLITE_BUSY) is returned and the log connection is not closed.
** If successful, the EXCLUSIVE lock is not released before returning.
*/
int sqlite3PagerCloseWal(Pager *pPager){
int rc = SQLITE_OK;

assert( pPager.journalMode==PAGER_JOURNALMODE_WAL );

/* If the log file is not already open, but does exist in the file-system,
** it may need to be checkpointed before the connection can switch to
** rollback mode. Open it now so this can happen.
*/
if( !pPager.pWal ){
int logexists = 0;
rc = pagerLockDb(pPager, SHARED_LOCK);
if( rc==SQLITE_OK ){
rc = sqlite3OsAccess(
pPager.pVfs, pPager.zWal, SQLITE_ACCESS_EXISTS, &logexists
);
}
if( rc==SQLITE_OK && logexists ){
rc = pagerOpenWal(pPager);
}
}

/* Checkpoint and close the log. Because an EXCLUSIVE lock is held on
** the database file, the log and log-summary files will be deleted.
*/
if( rc==SQLITE_OK && pPager.pWal ){
rc = pagerExclusiveLock(pPager);
if( rc==SQLITE_OK ){
rc = sqlite3WalClose(pPager.pWal, pPager.ckptSyncFlags,
           pPager.pageSize, (u8*)pPager.pTmpSpace);
pPager.pWal = 0;
}
}
return rc;
}

#if SQLITE_HAS_CODEC
/*
** This function is called by the wal module when writing page content
** into the log file.
**
** This function returns a pointer to a buffer containing the encrypted
** page content. If a malloc fails, this function may return NULL.
*/
void sqlite3PagerCodec(PgHdr *pPg){
voidaData = 0;
CODEC2(pPg->pPager, pPg->pData, pPg->pgno, 6, return 0, aData);
return aData;
}
#endif //* SQLITE_HAS_CODEC */

#endif //* !SQLITE_OMIT_WAL */

#endif // * SQLITE_OMIT_DISKIO */
  }
}