/* Copyright 2014 Google Inc. All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#if !defined(__has_feature) || !__has_feature(objc_arc)
#error "This file requires ARC support."
#endif

#import "GTMSessionFetcher.h"

#import <sys/utsname.h>

GTM_ASSUME_NONNULL_BEGIN

NSString *const kGTMSessionFetcherStartedNotification           = @"kGTMSessionFetcherStartedNotification";
NSString *const kGTMSessionFetcherStoppedNotification           = @"kGTMSessionFetcherStoppedNotification";
NSString *const kGTMSessionFetcherRetryDelayStartedNotification = @"kGTMSessionFetcherRetryDelayStartedNotification";
NSString *const kGTMSessionFetcherRetryDelayStoppedNotification = @"kGTMSessionFetcherRetryDelayStoppedNotification";

NSString *const kGTMSessionFetcherCompletionInvokedNotification = @"kGTMSessionFetcherCompletionInvokedNotification";
NSString *const kGTMSessionFetcherCompletionDataKey = @"data";
NSString *const kGTMSessionFetcherCompletionErrorKey = @"error";

NSString *const kGTMSessionFetcherErrorDomain       = @"com.google.GTMSessionFetcher";
NSString *const kGTMSessionFetcherStatusDomain      = @"com.google.HTTPStatus";
NSString *const kGTMSessionFetcherStatusDataKey     = @"data";  // data returned with a kGTMSessionFetcherStatusDomain error

NSString *const kGTMSessionFetcherNumberOfRetriesDoneKey        = @"kGTMSessionFetcherNumberOfRetriesDoneKey";
NSString *const kGTMSessionFetcherElapsedIntervalWithRetriesKey = @"kGTMSessionFetcherElapsedIntervalWithRetriesKey";

static NSString *const kGTMSessionIdentifierPrefix = @"com.google.GTMSessionFetcher";
static NSString *const kGTMSessionIdentifierDestinationFileURLMetadataKey = @"_destURL";
static NSString *const kGTMSessionIdentifierBodyFileURLMetadataKey        = @"_bodyURL";

// The default max retry interview is 10 minutes for uploads (POST/PUT/PATCH),
// 1 minute for downloads.
static const NSTimeInterval kUnsetMaxRetryInterval = -1.0;
static const NSTimeInterval kDefaultMaxDownloadRetryInterval = 60.0;
static const NSTimeInterval kDefaultMaxUploadRetryInterval = 60.0 * 10.;

#ifdef GTMSESSION_PERSISTED_DESTINATION_KEY
// Projects using unique class names should also define a unique persisted destination key.
static NSString * const kGTMSessionFetcherPersistedDestinationKey =
    GTMSESSION_PERSISTED_DESTINATION_KEY;
#else
static NSString * const kGTMSessionFetcherPersistedDestinationKey =
    @"com.google.GTMSessionFetcher.downloads";
#endif

GTM_ASSUME_NONNULL_END

//
// GTMSessionFetcher
//

#if 0
#define GTM_LOG_BACKGROUND_SESSION(...) GTMSESSION_LOG_DEBUG(__VA_ARGS__)
#else
#define GTM_LOG_BACKGROUND_SESSION(...)
#endif

#ifndef GTM_TARGET_SUPPORTS_APP_TRANSPORT_SECURITY
  #if (!TARGET_OS_IPHONE && defined(MAC_OS_X_VERSION_10_11) && MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_11) \
      || (TARGET_OS_IPHONE && defined(__IPHONE_9_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_9_0)
    #define GTM_TARGET_SUPPORTS_APP_TRANSPORT_SECURITY 1
  #endif
#endif

@interface GTMSessionFetcher ()

@property(atomic, strong, readwrite, GTM_NULLABLE) NSData *downloadedData;
@property(atomic, strong, readwrite, GTM_NULLABLE) NSData *downloadResumeData;

#if GTM_BACKGROUND_TASK_FETCHING
@property(assign, atomic) UIBackgroundTaskIdentifier backgroundTaskIdentifier;
#endif

@property(atomic, readwrite, getter=isUsingBackgroundSession) BOOL usingBackgroundSession;

@end

#if !GTMSESSION_BUILD_COMBINED_SOURCES
@interface GTMSessionFetcher (GTMSessionFetcherLoggingInternal)
- (void)logFetchWithError:(NSError *)error;
- (void)logNowWithError:(GTM_NULLABLE NSError *)error;
- (NSInputStream *)loggedInputStreamForInputStream:(NSInputStream *)inputStream;
- (GTMSessionFetcherBodyStreamProvider)loggedStreamProviderForStreamProvider:
    (GTMSessionFetcherBodyStreamProvider)streamProvider;
@end
#endif  // !GTMSESSION_BUILD_COMBINED_SOURCES

GTM_ASSUME_NONNULL_BEGIN

static NSTimeInterval InitialMinRetryInterval(void) {
  return 1.0 + ((double)(arc4random_uniform(0x0FFFF)) / (double) 0x0FFFF);
}

static BOOL IsLocalhost(NSString * GTM_NULLABLE_TYPE host) {
  // We check if there's host, and then make the comparisons.
  if (host == nil) return NO;
  return ([host caseInsensitiveCompare:@"localhost"] == NSOrderedSame
          || [host isEqual:@"::1"]
          || [host isEqual:@"127.0.0.1"]);
}

static GTMSessionFetcherTestBlock GTM_NULLABLE_TYPE gGlobalTestBlock;

@implementation GTMSessionFetcher {
  NSMutableURLRequest *_request; // after beginFetch, changed only in delegate callbacks
  BOOL _useUploadTask;           // immutable after beginFetch
  NSURL *_bodyFileURL;           // immutable after beginFetch
  GTMSessionFetcherBodyStreamProvider _bodyStreamProvider;  // immutable after beginFetch
  NSURLSession *_session;
  BOOL _shouldInvalidateSession;  // immutable after beginFetch
  NSURLSession *_sessionNeedingInvalidation;
  NSURLSessionConfiguration *_configuration;
  NSURLSessionTask *_sessionTask;
  NSString *_taskDescription;
  float _taskPriority;
  NSURLResponse *_response;
  NSString *_sessionIdentifier;
  BOOL _wasCreatedFromBackgroundSession;
  BOOL _didCreateSessionIdentifier;
  NSString *_sessionIdentifierUUID;
  BOOL _userRequestedBackgroundSession;
  BOOL _usingBackgroundSession;
  NSMutableData * GTM_NULLABLE_TYPE _downloadedData;
  NSError *_downloadFinishedError;
  NSData *_downloadResumeData;  // immutable after construction
  NSURL *_destinationFileURL;
  int64_t _downloadedLength;
  NSURLCredential *_credential;     // username & password
  NSURLCredential *_proxyCredential; // credential supplied to proxy servers
  BOOL _isStopNotificationNeeded;   // set when start notification has been sent
  BOOL _isUsingTestBlock;  // set when a test block was provided (remains set when the block is released)
  id _userData;                      // retained, if set by caller
  NSMutableDictionary *_properties;  // more data retained for caller
  dispatch_queue_t _callbackQueue;
  dispatch_group_t _callbackGroup;   // read-only after creation
  NSOperationQueue *_delegateQueue;  // immutable after beginFetch

  id<GTMFetcherAuthorizationProtocol> _authorizer;  // immutable after beginFetch

  // The service object that created and monitors this fetcher, if any.
  id<GTMSessionFetcherServiceProtocol> _service;  // immutable; set by the fetcher service upon creation
  NSString *_serviceHost;
  NSInteger _servicePriority;       // immutable after beginFetch
  BOOL _hasStoppedFetching;         // counterpart to _initialBeginFetchDate
  BOOL _userStoppedFetching;

  BOOL _isRetryEnabled;             // user wants auto-retry
  NSTimer *_retryTimer;
  NSUInteger _retryCount;
  NSTimeInterval _maxRetryInterval; // default 60 (download) or 600 (upload) seconds
  NSTimeInterval _minRetryInterval; // random between 1 and 2 seconds
  NSTimeInterval _retryFactor;      // default interval multiplier is 2
  NSTimeInterval _lastRetryInterval;
  NSDate *_initialBeginFetchDate;   // date that beginFetch was first invoked; immutable after initial beginFetch
  NSDate *_initialRequestDate;      // date of first request to the target server (ignoring auth)
  BOOL _hasAttemptedAuthRefresh;    // accessed only in shouldRetryNowForStatus:

  NSString *_comment;               // comment for log
  NSString *_log;
#if !STRIP_GTM_FETCH_LOGGING
  NSMutableData *_loggedStreamData;
  NSURL *_redirectedFromURL;
  NSString *_logRequestBody;
  NSString *_logResponseBody;
  BOOL _hasLoggedError;
  BOOL _deferResponseBodyLogging;
#endif
}

#if !GTMSESSION_UNIT_TESTING
+ (void)load {
  [self fetchersForBackgroundSessions];
}
#endif

+ (instancetype)fetcherWithRequest:(GTM_NULLABLE NSURLRequest *)request {
  return [[self alloc] initWithRequest:request configuration:nil];
}

+ (instancetype)fetcherWithURL:(NSURL *)requestURL {
  return [self fetcherWithRequest:[NSURLRequest requestWithURL:requestURL]];
}

+ (instancetype)fetcherWithURLString:(NSString *)requestURLString {
  return [self fetcherWithURL:(NSURL *)[NSURL URLWithString:requestURLString]];
}

+ (instancetype)fetcherWithDownloadResumeData:(NSData *)resumeData {
  GTMSessionFetcher *fetcher = [self fetcherWithRequest:nil];
  fetcher.comment = @"Resuming download";
  fetcher.downloadResumeData = resumeData;
  return fetcher;
}

+ (GTM_NULLABLE instancetype)fetcherWithSessionIdentifier:(NSString *)sessionIdentifier {
  GTMSESSION_ASSERT_DEBUG(sessionIdentifier != nil, @"Invalid session identifier");
  NSMapTable *sessionIdentifierToFetcherMap = [self sessionIdentifierToFetcherMap];
  GTMSessionFetcher *fetcher = [sessionIdentifierToFetcherMap objectForKey:sessionIdentifier];
  if (!fetcher && [sessionIdentifier hasPrefix:kGTMSessionIdentifierPrefix]) {
    fetcher = [self fetcherWithRequest:nil];
    [fetcher setSessionIdentifier:sessionIdentifier];
    [sessionIdentifierToFetcherMap setObject:fetcher forKey:sessionIdentifier];
    fetcher->_wasCreatedFromBackgroundSession = YES;
    [fetcher setCommentWithFormat:@"Resuming %@",
     fetcher && fetcher->_sessionIdentifierUUID ? fetcher->_sessionIdentifierUUID : @"?"];
  }
  return fetcher;
}

+ (NSMapTable *)sessionIdentifierToFetcherMap {
  // TODO: What if a service is involved in creating the fetcher? Currently, when re-creating
  // fetchers, if a service was involved, it is not re-created. Should the service maintain a map?
  static NSMapTable *gSessionIdentifierToFetcherMap = nil;

  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    gSessionIdentifierToFetcherMap = [NSMapTable strongToWeakObjectsMapTable];
  });
  return gSessionIdentifierToFetcherMap;
}

#if !GTM_ALLOW_INSECURE_REQUESTS
+ (BOOL)appAllowsInsecureRequests {
  // If the main bundle Info.plist key NSAppTransportSecurity is present, and it specifies
  // NSAllowsArbitraryLoads, then we need to explicitly enforce secure schemes.
#if GTM_TARGET_SUPPORTS_APP_TRANSPORT_SECURITY
  static BOOL allowsInsecureRequests;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    NSBundle *mainBundle = [NSBundle mainBundle];
    NSDictionary *appTransportSecurity =
        [mainBundle objectForInfoDictionaryKey:@"NSAppTransportSecurity"];
    allowsInsecureRequests =
        [[appTransportSecurity objectForKey:@"NSAllowsArbitraryLoads"] boolValue];
  });
  return allowsInsecureRequests;
#else
  // For builds targeting iOS 8 or 10.10 and earlier, we want to require fetcher
  // security checks.
  return YES;
#endif  // GTM_TARGET_SUPPORTS_APP_TRANSPORT_SECURITY
}
#else  // GTM_ALLOW_INSECURE_REQUESTS
+ (BOOL)appAllowsInsecureRequests {
  return YES;
}
#endif  // !GTM_ALLOW_INSECURE_REQUESTS


- (instancetype)init {
  return [self initWithRequest:nil configuration:nil];
}

- (instancetype)initWithRequest:(NSURLRequest *)request  {
  return [self initWithRequest:request configuration:nil];
}

- (instancetype)initWithRequest:(GTM_NULLABLE NSURLRequest *)request
                  configuration:(GTM_NULLABLE NSURLSessionConfiguration *)configuration {
  self = [super init];
  if (self) {
    if (![NSURLSession class]) {
      Class oldFetcherClass = NSClassFromString(@"GTMHTTPFetcher");
      if (oldFetcherClass && request) {
        self = [[oldFetcherClass alloc] initWithRequest:(NSURLRequest *)request];
      } else {
        self = nil;
      }
      return self;
    }
#if GTM_BACKGROUND_TASK_FETCHING
    _backgroundTaskIdentifier = UIBackgroundTaskInvalid;
#endif
    _request = [request mutableCopy];
    _configuration = configuration;

    NSData *bodyData = request.HTTPBody;
    if (bodyData) {
      _bodyLength = (int64_t)bodyData.length;
    } else {
      _bodyLength = NSURLSessionTransferSizeUnknown;
    }

    _callbackQueue = dispatch_get_main_queue();
    _callbackGroup = dispatch_group_create();
    _delegateQueue = [NSOperationQueue mainQueue];

    _minRetryInterval = InitialMinRetryInterval();
    _maxRetryInterval = kUnsetMaxRetryInterval;

    _taskPriority = -1.0f;  // Valid values if set are 0.0...1.0.

#if !STRIP_GTM_FETCH_LOGGING
    // Encourage developers to set the comment property or use
    // setCommentWithFormat: by providing a default string.
    _comment = @"(No fetcher comment set)";
#endif
  }
  return self;
}

- (id)copyWithZone:(NSZone *)zone {
  // disallow use of fetchers in a copy property
  [self doesNotRecognizeSelector:_cmd];
  return nil;
}

- (NSString *)description {
  NSString *requestStr = self.request.URL.description;
  if (requestStr.length == 0) {
    if (self.downloadResumeData.length > 0) {
      requestStr = @"<download resume data>";
    } else if (_wasCreatedFromBackgroundSession) {
      requestStr = @"<from bg session>";
    } else {
      requestStr = @"<no request>";
    }
  }
  return [NSString stringWithFormat:@"%@ %p (%@)", [self class], self, requestStr];
}

- (void)dealloc {
  GTMSESSION_ASSERT_DEBUG(!_isStopNotificationNeeded,
                          @"unbalanced fetcher notification for %@", _request.URL);
  [self forgetSessionIdentifierForFetcherWithoutSyncCheck];

  // Note: if a session task or a retry timer was pending, then this instance
  // would be retained by those so it wouldn't be getting dealloc'd,
  // hence we don't need to stopFetch here
}

#pragma mark -

// Begin fetching the URL (or begin a retry fetch).  The delegate is retained
// for the duration of the fetch connection.

- (void)beginFetchWithCompletionHandler:(GTM_NULLABLE GTMSessionFetcherCompletionHandler)handler {
  GTMSessionCheckNotSynchronized(self);

  _completionHandler = [handler copy];

  // The user may have called setDelegate: earlier if they want to use other
  // delegate-style callbacks during the fetch; otherwise, the delegate is nil,
  // which is fine.
  [self beginFetchMayDelay:YES mayAuthorize:YES];
}

- (GTMSessionFetcherCompletionHandler)completionHandlerWithTarget:(GTM_NULLABLE_TYPE id)target
                                                didFinishSelector:(GTM_NULLABLE_TYPE SEL)finishedSelector {
  GTMSessionFetcherAssertValidSelector(target, finishedSelector, @encode(GTMSessionFetcher *),
                                       @encode(NSData *), @encode(NSError *), 0);
  GTMSessionFetcherCompletionHandler completionHandler = ^(NSData *data, NSError *error) {
      if (target && finishedSelector) {
        id selfArg = self;  // Placate ARC.
        NSMethodSignature *sig = [target methodSignatureForSelector:finishedSelector];
        NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:sig];
        [invocation setSelector:(SEL)finishedSelector];
        [invocation setTarget:target];
        [invocation setArgument:&selfArg atIndex:2];
        [invocation setArgument:&data atIndex:3];
        [invocation setArgument:&error atIndex:4];
        [invocation invoke];
      }
  };
  return completionHandler;
}

- (void)beginFetchWithDelegate:(GTM_NULLABLE_TYPE id)target
             didFinishSelector:(GTM_NULLABLE_TYPE SEL)finishedSelector {
  GTMSessionCheckNotSynchronized(self);

  GTMSessionFetcherCompletionHandler handler =  [self completionHandlerWithTarget:target
                                                                didFinishSelector:finishedSelector];
  [self beginFetchWithCompletionHandler:handler];
}

- (void)beginFetchMayDelay:(BOOL)mayDelay
              mayAuthorize:(BOOL)mayAuthorize {
  // This is the internal entry point for re-starting fetches.
  GTMSessionCheckNotSynchronized(self);

  NSMutableURLRequest *fetchRequest = _request;  // The request property is now externally immutable.
  NSURL *fetchRequestURL = fetchRequest.URL;
  NSString *priorSessionIdentifier = self.sessionIdentifier;

  // A utility block for creating error objects when we fail to start the fetch.
  NSError *(^beginFailureError)(NSInteger) = ^(NSInteger code){
    NSString *urlString = fetchRequestURL.absoluteString;
    NSDictionary *userInfo = @{
      NSURLErrorFailingURLStringErrorKey : (urlString ? urlString : @"(missing URL)")
    };
    return [NSError errorWithDomain:kGTMSessionFetcherErrorDomain
                               code:code
                           userInfo:userInfo];
  };

  // Catch delegate queue maxConcurrentOperationCount values other than 1, particularly
  // NSOperationQueueDefaultMaxConcurrentOperationCount (-1), to avoid the additional complexity
  // of simultaneous or out-of-order delegate callbacks.
  GTMSESSION_ASSERT_DEBUG(_delegateQueue.maxConcurrentOperationCount == 1,
                          @"delegate queue %@ should support one concurrent operation, not %zd",
                          _delegateQueue.name, _delegateQueue.maxConcurrentOperationCount);

  if (!_initialBeginFetchDate) {
    // This ivar is set only here on the initial beginFetch so need not be synchronized.
    _initialBeginFetchDate = [[NSDate alloc] init];
  }

  if (self.sessionTask != nil) {
    // If cached fetcher returned through fetcherWithSessionIdentifier:, then it's
    // already begun, but don't consider this a failure, since the user need not know this.
    if (self.sessionIdentifier != nil) {
      return;
    }
    GTMSESSION_ASSERT_DEBUG(NO, @"Fetch object %@ being reused; this should never happen", self);
    [self failToBeginFetchWithError:beginFailureError(GTMSessionFetcherErrorDownloadFailed)];
    return;
  }

  if (fetchRequestURL == nil && !_downloadResumeData && !priorSessionIdentifier) {
    GTMSESSION_ASSERT_DEBUG(NO, @"Beginning a fetch requires a request with a URL");
    [self failToBeginFetchWithError:beginFailureError(GTMSessionFetcherErrorDownloadFailed)];
    return;
  }

  // We'll respect the user's request for a background session (unless this is
  // an upload fetcher, which does its initial request foreground.)
  self.usingBackgroundSession = self.useBackgroundSession && [self canFetchWithBackgroundSession];

  NSURL *bodyFileURL = self.bodyFileURL;
  if (bodyFileURL) {
    NSError *fileCheckError;
    if (![bodyFileURL checkResourceIsReachableAndReturnError:&fileCheckError]) {
      // This assert fires when the file being uploaded no longer exists once
      // the fetcher is ready to start the upload.
      GTMSESSION_ASSERT_DEBUG_OR_LOG(0, @"Body file is unreachable: %@\n  %@",
                                     bodyFileURL.path, fileCheckError);
      [self failToBeginFetchWithError:fileCheckError];
      return;
    }
  }

  NSString *requestScheme = fetchRequestURL.scheme;
  BOOL isDataRequest = [requestScheme isEqual:@"data"];
  if (isDataRequest) {
    // NSURLSession does not support data URLs in background sessions.
#if DEBUG
    if (priorSessionIdentifier || self.sessionIdentifier) {
      GTMSESSION_LOG_DEBUG(@"Converting background to foreground session for %@",
                           fetchRequest);
    }
#endif
    [self setSessionIdentifierInternal:nil];
    self.useBackgroundSession = NO;
  }

#if GTM_ALLOW_INSECURE_REQUESTS
  BOOL shouldCheckSecurity = NO;
#else
  BOOL shouldCheckSecurity = (fetchRequestURL != nil
                              && !isDataRequest
                              && [[self class] appAllowsInsecureRequests]);
#endif

  if (shouldCheckSecurity) {
    // Allow https only for requests, unless overridden by the client.
    //
    // Non-https requests may too easily be snooped, so we disallow them by default.
    //
    // file: and data: schemes are usually safe if they are hardcoded in the client or provided
    // by a trusted source, but since it's fairly rare to need them, it's safest to make clients
    // explicitly whitelist them.
    BOOL isSecure =
        requestScheme != nil && [requestScheme caseInsensitiveCompare:@"https"] == NSOrderedSame;
    if (!isSecure) {
      BOOL allowRequest = NO;
      NSString *host = fetchRequestURL.host;

      // Check schemes first.  A file scheme request may be allowed here, or as a localhost request.
      for (NSString *allowedScheme in _allowedInsecureSchemes) {
        if (requestScheme != nil &&
            [requestScheme caseInsensitiveCompare:allowedScheme] == NSOrderedSame) {
          allowRequest = YES;
          break;
        }
      }
      if (!allowRequest) {
        // Check for localhost requests.  Security checks only occur for non-https requests, so
        // this check won't happen for an https request to localhost.
        BOOL isLocalhostRequest = (host.length == 0 && [fetchRequestURL isFileURL]) || IsLocalhost(host);
        if (isLocalhostRequest) {
          if (self.allowLocalhostRequest) {
            allowRequest = YES;
          } else {
            GTMSESSION_ASSERT_DEBUG(NO, @"Fetch request for localhost but fetcher"
                                        @" allowLocalhostRequest is not set: %@", fetchRequestURL);
          }
        } else {
          GTMSESSION_ASSERT_DEBUG(NO, @"Insecure fetch request has a scheme (%@)"
                                      @" not found in fetcher allowedInsecureSchemes (%@): %@",
                                  requestScheme, _allowedInsecureSchemes ?: @" @[] ", fetchRequestURL);
        }
      }

      if (!allowRequest) {
#if !DEBUG
        NSLog(@"Insecure fetch disallowed for %@", fetchRequestURL.description ?: @"nil request URL");
#endif
        [self failToBeginFetchWithError:beginFailureError(GTMSessionFetcherErrorInsecureRequest)];
        return;
      }
    }  // !isSecure
  }  // (requestURL != nil) && !isDataRequest

  if (self.cookieStorage == nil) {
    self.cookieStorage = [[self class] staticCookieStorage];
  }

  BOOL isRecreatingSession = (self.sessionIdentifier != nil) && (fetchRequest == nil);

  self.canShareSession = !isRecreatingSession && !self.usingBackgroundSession;

  if (!self.session && self.canShareSession) {
    self.session = [_service sessionForFetcherCreation];
    // If _session is nil, then the service's session creation semaphore will block
    // until this fetcher invokes fetcherDidCreateSession: below, so this *must* invoke
    // that method, even if the session fails to be created.
  }

  if (!self.session) {
    // Create a session.
    if (!_configuration) {
      if (priorSessionIdentifier || self.usingBackgroundSession) {
        NSString *sessionIdentifier = priorSessionIdentifier;
        if (!sessionIdentifier) {
          sessionIdentifier = [self createSessionIdentifierWithMetadata:nil];
        }
        NSMapTable *sessionIdentifierToFetcherMap = [[self class] sessionIdentifierToFetcherMap];
        [sessionIdentifierToFetcherMap setObject:self forKey:self.sessionIdentifier];

#if (!TARGET_OS_IPHONE && defined(MAC_OS_X_VERSION_10_10) && MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_10) \
    || (TARGET_OS_IPHONE && defined(__IPHONE_8_0) && __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_8_0)
        // iOS 8/10.10 builds require the new backgroundSessionConfiguration method name.
        _configuration =
            [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:sessionIdentifier];
#elif (!TARGET_OS_IPHONE && defined(MAC_OS_X_VERSION_10_10) && MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_10) \
    || (TARGET_OS_IPHONE && defined(__IPHONE_8_0) && __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_8_0)
        // Do a runtime check to avoid a deprecation warning about using
        // +backgroundSessionConfiguration: on iOS 8.
        if ([NSURLSessionConfiguration respondsToSelector:@selector(backgroundSessionConfigurationWithIdentifier:)]) {
          // Running on iOS 8+/OS X 10.10+.
          _configuration =
              [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:sessionIdentifier];
        } else {
          // Running on iOS 7/OS X 10.9.
          _configuration =
              [NSURLSessionConfiguration backgroundSessionConfiguration:sessionIdentifier];
        }
#else
        // Building with an SDK earlier than iOS 8/OS X 10.10.
        _configuration =
            [NSURLSessionConfiguration backgroundSessionConfiguration:sessionIdentifier];
#endif
        self.usingBackgroundSession = YES;
        self.canShareSession = NO;
      } else {
        _configuration = [NSURLSessionConfiguration ephemeralSessionConfiguration];
      }
#if !GTM_ALLOW_INSECURE_REQUESTS
      _configuration.TLSMinimumSupportedProtocol = kTLSProtocol12;
#endif
    }  // !_configuration
    _configuration.HTTPCookieStorage = self.cookieStorage;

    if (_configurationBlock) {
      _configurationBlock(self, _configuration);
    }

    id<NSURLSessionDelegate> delegate = [_service sessionDelegate];
    if (!delegate || !self.canShareSession) {
      delegate = self;
    }
    self.session = [NSURLSession sessionWithConfiguration:_configuration
                                                 delegate:delegate
                                            delegateQueue:self.sessionDelegateQueue];
    GTMSESSION_ASSERT_DEBUG(self.session, @"Couldn't create session");

    // Tell the service about the session created by this fetcher.  This also signals the
    // service's semaphore to allow other fetchers to request this session.
    [_service fetcherDidCreateSession:self];

    // If this assertion fires, the client probably tried to use a session identifier that was
    // already used. The solution is to make the client use a unique identifier (or better yet let
    // the session fetcher assign the identifier).
    GTMSESSION_ASSERT_DEBUG(self.session.delegate == delegate, @"Couldn't assign delegate.");

    if (self.session) {
      BOOL isUsingSharedDelegate = (delegate != self);
      if (!isUsingSharedDelegate) {
        _shouldInvalidateSession = YES;
      }
    }
  }

  if (isRecreatingSession) {
    _shouldInvalidateSession = YES;

    // Let's make sure there are tasks still running or if not that we get a callback from a
    // completed one; otherwise, we assume the tasks failed.
    // This is the observed behavior perhaps 25% of the time within the Simulator running 7.0.3 on
    // exiting the app after starting an upload and relaunching the app if we manage to relaunch
    // after the task has completed, but before the system relaunches us in the background.
    [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks,
                                                  NSArray *downloadTasks) {
      if (dataTasks.count == 0 && uploadTasks.count == 0 && downloadTasks.count == 0) {
        double const kDelayInSeconds = 1.0;  // We should get progress indication or completion soon
        dispatch_time_t checkForFeedbackDelay =
            dispatch_time(DISPATCH_TIME_NOW, (int64_t)(kDelayInSeconds * NSEC_PER_SEC));
        dispatch_after(checkForFeedbackDelay, dispatch_get_main_queue(), ^{
          if (!self.sessionTask && !fetchRequest) {
            // If our task and/or request haven't been restored, then we assume task feedback lost.
            [self removePersistedBackgroundSessionFromDefaults];
            NSError *sessionError =
                [NSError errorWithDomain:kGTMSessionFetcherErrorDomain
                                    code:GTMSessionFetcherErrorBackgroundFetchFailed
                                userInfo:nil];
            [self failToBeginFetchWithError:sessionError];
          }
        });
      }
    }];
    return;
  }

  self.downloadedData = nil;
  self.downloadedLength = 0;

  if (_servicePriority == NSIntegerMin) {
    mayDelay = NO;
  }
  if (mayDelay && _service) {
    BOOL shouldFetchNow = [_service fetcherShouldBeginFetching:self];
    if (!shouldFetchNow) {
      // The fetch is deferred, but will happen later.
      //
      // If this session is held by the fetcher service, clear the session now so that we don't
      // assume it's still valid after the fetcher is restarted.
      if (self.canShareSession) {
        self.session = nil;
      }
      return;
    }
  }

  NSString *effectiveHTTPMethod = [fetchRequest valueForHTTPHeaderField:@"X-HTTP-Method-Override"];
  if (effectiveHTTPMethod == nil) {
    effectiveHTTPMethod = fetchRequest.HTTPMethod;
  }
  BOOL isEffectiveHTTPGet = (effectiveHTTPMethod == nil
                             || [effectiveHTTPMethod isEqual:@"GET"]);

  BOOL needsUploadTask = (self.useUploadTask || self.bodyFileURL || self.bodyStreamProvider);
  if (_bodyData || self.bodyStreamProvider || fetchRequest.HTTPBodyStream) {
    if (isEffectiveHTTPGet) {
      fetchRequest.HTTPMethod = @"POST";
      isEffectiveHTTPGet = NO;
    }

    if (_bodyData) {
      if (!needsUploadTask) {
        fetchRequest.HTTPBody = _bodyData;
      }
#if !STRIP_GTM_FETCH_LOGGING
    } else if (fetchRequest.HTTPBodyStream) {
      if ([self respondsToSelector:@selector(loggedInputStreamForInputStream:)]) {
        fetchRequest.HTTPBodyStream =
            [self performSelector:@selector(loggedInputStreamForInputStream:)
                       withObject:fetchRequest.HTTPBodyStream];
      }
#endif
    }
  }

  // We authorize after setting up the http method and body in the request
  // because OAuth 1 may need to sign the request body
  if (mayAuthorize && _authorizer && !isDataRequest) {
    BOOL isAuthorized = [_authorizer isAuthorizedRequest:fetchRequest];
    if (!isAuthorized) {
      // Authorization needed.
      //
      // If this session is held by the fetcher service, clear the session now so that we don't
      // assume it's still valid after authorization completes.
      if (self.canShareSession) {
        self.session = nil;
      }

      // Authorizing the request will recursively call this beginFetch:mayDelay:
      // or failToBeginFetchWithError:.
      [self authorizeRequest];
      return;
    }
  }

  // set the default upload or download retry interval, if necessary
  if ([self isRetryEnabled] && self.maxRetryInterval <= 0) {
    if (isEffectiveHTTPGet || [effectiveHTTPMethod isEqual:@"HEAD"]) {
      [self setMaxRetryInterval:kDefaultMaxDownloadRetryInterval];
    } else {
      [self setMaxRetryInterval:kDefaultMaxUploadRetryInterval];
    }
  }

  // finally, start the connection
  NSURLSessionTask *newSessionTask;
  BOOL needsDataAccumulator = NO;
  if (_downloadResumeData) {
    newSessionTask = [_session downloadTaskWithResumeData:_downloadResumeData];
    GTMSESSION_ASSERT_DEBUG_OR_LOG(newSessionTask,
        @"Failed downloadTaskWithResumeData for %@, resume data %tu bytes",
        _session, _downloadResumeData.length);
  } else if (_destinationFileURL && !isDataRequest) {
    newSessionTask = [_session downloadTaskWithRequest:fetchRequest];
    GTMSESSION_ASSERT_DEBUG_OR_LOG(newSessionTask, @"Failed downloadTaskWithRequest for %@, %@",
                                   _session, fetchRequest);
  } else if (needsUploadTask) {
    if (bodyFileURL) {
      newSessionTask = [_session uploadTaskWithRequest:fetchRequest
                                              fromFile:bodyFileURL];
      GTMSESSION_ASSERT_DEBUG_OR_LOG(newSessionTask,
                                     @"Failed uploadTaskWithRequest for %@, %@, file %@",
                                     _session, fetchRequest, bodyFileURL.path);
    } else if (self.bodyStreamProvider) {
      newSessionTask = [_session uploadTaskWithStreamedRequest:fetchRequest];
      GTMSESSION_ASSERT_DEBUG_OR_LOG(newSessionTask,
                                     @"Failed uploadTaskWithStreamedRequest for %@, %@",
                                     _session, fetchRequest);
    } else {
      GTMSESSION_ASSERT_DEBUG_OR_LOG(_bodyData != nil,
                                     @"Upload task needs body data, %@", fetchRequest);
      newSessionTask = [_session uploadTaskWithRequest:fetchRequest
                                            fromData:(NSData * GTM_NONNULL_TYPE)_bodyData];
      GTMSESSION_ASSERT_DEBUG_OR_LOG(newSessionTask,
          @"Failed uploadTaskWithRequest for %@, %@, body data %tu bytes",
          _session, fetchRequest, _bodyData.length);
    }
    needsDataAccumulator = YES;
  } else {
    newSessionTask = [_session dataTaskWithRequest:fetchRequest];
    needsDataAccumulator = YES;
    GTMSESSION_ASSERT_DEBUG_OR_LOG(newSessionTask, @"Failed dataTaskWithRequest for %@, %@",
                                   _session, fetchRequest);
  }
  self.sessionTask = newSessionTask;

  if (!newSessionTask) {
    // We shouldn't get here; if we're here, an earlier assertion should have fired to explain
    // which session task creation failed.
    [self failToBeginFetchWithError:beginFailureError(GTMSessionFetcherErrorTaskCreationFailed)];
    return;
  }

  if (needsDataAccumulator && _accumulateDataBlock == nil) {
    self.downloadedData = [NSMutableData data];
  }
  if (_taskDescription) {
    newSessionTask.taskDescription = _taskDescription;
  }
  if (_taskPriority >= 0) {
#if (!TARGET_OS_IPHONE && defined(MAC_OS_X_VERSION_10_10) && MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_10) \
    || (TARGET_OS_IPHONE && defined(__IPHONE_8_0) && __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_8_0)
    BOOL hasTaskPriority = YES;
#else
    BOOL hasTaskPriority = [newSessionTask respondsToSelector:@selector(setPriority:)];
#endif
    if (hasTaskPriority) {
      newSessionTask.priority = _taskPriority;
    }
  }

#if GTM_DISABLE_FETCHER_TEST_BLOCK
  GTMSESSION_ASSERT_DEBUG(_testBlock == nil && gGlobalTestBlock == nil, @"test blocks disabled");
  _testBlock = nil;
#else
  if (!_testBlock) {
    if (gGlobalTestBlock) {
      // Note that the test block may pass nil for all of its response parameters,
      // indicating that the fetch should actually proceed. This is useful when the
      // global test block has been set, and the app is only testing a specific
      // fetcher.  The block simulation code will then resume the task.
      _testBlock = gGlobalTestBlock;
    }
  }
  _isUsingTestBlock = (_testBlock != nil);
#endif  // GTM_DISABLE_FETCHER_TEST_BLOCK

#if GTM_BACKGROUND_TASK_FETCHING
  // Background tasks seem to interfere with out-of-process uploads and downloads.
  if (!self.skipBackgroundTask && !self.useBackgroundSession) {
    // Tell UIApplication that we want to continue even when the app is in the
    // background.
    id<GTMUIApplicationProtocol> app = [[self class] fetcherUIApplication];
#if DEBUG
    NSString *bgTaskName = [NSString stringWithFormat:@"%@-%@",
                            [self class], fetchRequest.URL.host];
#else
    NSString *bgTaskName = @"GTMSessionFetcher";
#endif
    __block UIBackgroundTaskIdentifier bgTaskID = [app beginBackgroundTaskWithName:bgTaskName
                                                                 expirationHandler:^{
      // Background task expiration callback - this block is always invoked by
      // UIApplication on the main thread.
      if (bgTaskID != UIBackgroundTaskInvalid) {
        if (bgTaskID == self.backgroundTaskIdentifier) {
          self.backgroundTaskIdentifier = UIBackgroundTaskInvalid;
        }
        [app endBackgroundTask:bgTaskID];
      }
    }];
    self.backgroundTaskIdentifier = bgTaskID;
  }
#endif

  if (!_initialRequestDate) {
    _initialRequestDate = [[NSDate alloc] init];
  }

  // We don't expect to reach here even on retry or auth until a stop notification has been sent
  // for the previous task, but we should ensure that we don't unbalance that.
  GTMSESSION_ASSERT_DEBUG(!_isStopNotificationNeeded, @"Start notification without a prior stop");
  [self sendStopNotificationIfNeeded];

  [self addPersistedBackgroundSessionToDefaults];

  [self setStopNotificationNeeded:YES];

  [self postNotificationOnMainThreadWithName:kGTMSessionFetcherStartedNotification
                                    userInfo:nil
                                requireAsync:NO];

  // The service needs to know our task if it is serving as NSURLSession delegate.
  [_service fetcherDidBeginFetching:self];

  if (_testBlock) {
#if !GTM_DISABLE_FETCHER_TEST_BLOCK
    [self simulateFetchForTestBlock];
#endif
  } else {
    // We resume the session task after posting the notification since the
    // delegate callbacks may happen immediately if the fetch is started off
    // the main thread or the session delegate queue is on a background thread,
    // and we don't want to post a start notification after a premature finish
    // of the session task.
    [newSessionTask resume];
  }
}

NSData * GTM_NULLABLE_TYPE GTMDataFromInputStream(NSInputStream *inputStream, NSError **outError) {
  NSMutableData *data = [NSMutableData data];

  [inputStream open];
  NSInteger numberOfBytesRead = 0;
  while ([inputStream hasBytesAvailable]) {
    uint8_t buffer[512];
    numberOfBytesRead = [inputStream read:buffer maxLength:sizeof(buffer)];
    if (numberOfBytesRead > 0) {
      [data appendBytes:buffer length:(NSUInteger)numberOfBytesRead];
    } else {
      break;
    }
  }
  [inputStream close];
  NSError *streamError = inputStream.streamError;

  if (streamError) {
    data = nil;
  }
  if (outError) {
    *outError = streamError;
  }
  return data;
}

#if !GTM_DISABLE_FETCHER_TEST_BLOCK

- (void)simulateFetchForTestBlock {
  // This is invoked on the same thread as the beginFetch method was.
  //
  // Callbacks will all occur on the callback queue.
  _testBlock(self, ^(NSURLResponse *response, NSData *responseData, NSError *error) {
      // Callback from test block.
      if (response == nil && responseData == nil && error == nil) {
        // Assume the fetcher should execute rather than be tested.
        _testBlock = nil;
        _isUsingTestBlock = NO;
        [_sessionTask resume];
        return;
      }

      GTMSessionFetcherBodyStreamProvider bodyStreamProvider = self.bodyStreamProvider;
      if (bodyStreamProvider) {
        bodyStreamProvider(^(NSInputStream *bodyStream){
          // Read from the input stream into an NSData buffer.  We'll drain the stream
          // explicitly on a background queue.
          [self invokeOnCallbackQueue:dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)
                     afterUserStopped:NO
                                block:^{
            NSError *streamError;
            NSData *streamedData = GTMDataFromInputStream(bodyStream, &streamError);

            dispatch_async(dispatch_get_main_queue(), ^{
              // Continue callbacks on the main thread, since serial behavior
              // is more reliable for tests.
              [self simulateDataCallbacksForTestBlockWithBodyData:streamedData
                                                         response:response
                                                     responseData:responseData
                                                            error:(error ?: streamError)];
            });
          }];
        });
      } else {
        // No input stream; use the supplied data or file URL.
        NSURL *bodyFileURL = self.bodyFileURL;
        if (bodyFileURL) {
          NSError *readError;
          _bodyData = [NSData dataWithContentsOfURL:bodyFileURL
                                            options:NSDataReadingMappedIfSafe
                                              error:&readError];
          error = readError;
        }

        // No stream provider.

        // In real fetches, nothing happens until the run loop spins, so apps have leeway to
        // set callbacks after they call beginFetch. We'll mirror that fetcher behavior by
        // delaying callbacks here at least to the next spin of the run loop.  That keeps
        // immediate, synchronous setting of callback blocks after beginFetch working in tests.
        dispatch_async(dispatch_get_main_queue(), ^{
          [self simulateDataCallbacksForTestBlockWithBodyData:_bodyData
                                                     response:response
                                                 responseData:responseData
                                                        error:error];
        });
      }
    });
}

- (void)simulateByteTransferReportWithDataLength:(int64_t)totalDataLength
                                           block:(GTMSessionFetcherSendProgressBlock)block {
  // This utility method simulates transfer progress with up to three callbacks.
  // It is used to call back to any of the progress blocks.
  int64_t sendReportSize = totalDataLength / 3 + 1;
  int64_t totalSent = 0;
  while (totalSent < totalDataLength) {
    int64_t bytesRemaining = totalDataLength - totalSent;
    sendReportSize = MIN(sendReportSize, bytesRemaining);
    totalSent += sendReportSize;
    [self invokeOnCallbackQueueUnlessStopped:^{
        block(sendReportSize, totalSent, totalDataLength);
    }];
  }
}

- (void)simulateDataCallbacksForTestBlockWithBodyData:(NSData * GTM_NULLABLE_TYPE)bodyData
                                             response:(NSURLResponse *)response
                                         responseData:(NSData *)suppliedData
                                                error:(NSError *)suppliedError {
  __block NSData *responseData = suppliedData;
  __block NSError *responseError = suppliedError;

  // This method does the test simulation of callbacks once the upload
  // and download data are known.
  @synchronized(self) {
    GTMSessionMonitorSynchronized(self);

    // Get copies of ivars we'll access in async invocations.  This simulation assumes
    // they won't change during fetcher execution.
    NSURL *destinationFileURL = _destinationFileURL;
    GTMSessionFetcherWillRedirectBlock willRedirectBlock = _willRedirectBlock;
    GTMSessionFetcherDidReceiveResponseBlock didReceiveResponseBlock = _didReceiveResponseBlock;
    GTMSessionFetcherSendProgressBlock sendProgressBlock = _sendProgressBlock;
    GTMSessionFetcherDownloadProgressBlock downloadProgressBlock = _downloadProgressBlock;
    GTMSessionFetcherAccumulateDataBlock accumulateDataBlock = _accumulateDataBlock;
    GTMSessionFetcherReceivedProgressBlock receivedProgressBlock = _receivedProgressBlock;
    GTMSessionFetcherWillCacheURLResponseBlock willCacheURLResponseBlock =
        _willCacheURLResponseBlock;

    // Simulate receipt of redirection.
    if (willRedirectBlock) {
      [self invokeOnCallbackUnsynchronizedQueueAfterUserStopped:YES
                                                          block:^{
          willRedirectBlock((NSHTTPURLResponse *)response, _request,
                             ^(NSURLRequest *redirectRequest) {
              // For simulation, we'll assume the app will just continue.
          });
      }];
    }

    // If the fetcher has a challenge block, simulate a challenge.
    //
    // It might be nice to eventually let the user determine which testBlock
    // fetches get challenged rather than always executing the supplied
    // challenge block.
    if (_challengeBlock) {
      [self invokeOnCallbackUnsynchronizedQueueAfterUserStopped:YES
                                                          block:^{
        if (_challengeBlock) {
          NSURL *requestURL = _request.URL;
          NSString *host = requestURL.host;
          NSURLProtectionSpace *pspace =
              [[NSURLProtectionSpace alloc] initWithHost:host
                                                    port:requestURL.port.integerValue
                                                protocol:requestURL.scheme
                                                   realm:nil
                                    authenticationMethod:NSURLAuthenticationMethodHTTPBasic];
          id<NSURLAuthenticationChallengeSender> unusedSender =
              (id<NSURLAuthenticationChallengeSender>)[NSNull null];
          NSURLAuthenticationChallenge *challenge =
              [[NSURLAuthenticationChallenge alloc] initWithProtectionSpace:pspace
                                                         proposedCredential:nil
                                                       previousFailureCount:0
                                                            failureResponse:nil
                                                                      error:nil
                                                                     sender:unusedSender];
          _challengeBlock(self, challenge, ^(NSURLSessionAuthChallengeDisposition disposition,
                                             NSURLCredential * GTM_NULLABLE_TYPE credential){
            // We could change the responseData and responseError based on the disposition,
            // but it's easier for apps to just supply the expected data and error
            // directly to the test block. So this simulation ignores the disposition.
          });
        }
      }];
    }

    // Simulate receipt of an initial response.
    if (didReceiveResponseBlock) {
      [self invokeOnCallbackUnsynchronizedQueueAfterUserStopped:YES
                                                          block:^{
          didReceiveResponseBlock(response, ^(NSURLSessionResponseDisposition desiredDisposition) {
            // For simulation, we'll assume the disposition is to continue.
          });
      }];
    }

    // Simulate reporting send progress.
    if (sendProgressBlock) {
      [self simulateByteTransferReportWithDataLength:(int64_t)bodyData.length
                                               block:^(int64_t bytesSent,
                                                       int64_t totalBytesSent,
                                                       int64_t totalBytesExpectedToSend) {
          // This is invoked on the callback queue unless stopped.
          sendProgressBlock(bytesSent, totalBytesSent, totalBytesExpectedToSend);
      }];
    }

    if (destinationFileURL) {
      // Simulate download to file progress.
      if (downloadProgressBlock) {
        [self simulateByteTransferReportWithDataLength:(int64_t)responseData.length
                                                 block:^(int64_t bytesDownloaded,
                                                         int64_t totalBytesDownloaded,
                                                         int64_t totalBytesExpectedToDownload) {
            // This is invoked on the callback queue unless stopped.
            downloadProgressBlock(bytesDownloaded, totalBytesDownloaded,
                                  totalBytesExpectedToDownload);
        }];
      }

      NSError *writeError;
      [responseData writeToURL:destinationFileURL
                       options:NSDataWritingAtomic
                         error:&writeError];
      if (writeError) {
        // Tell the test code that writing failed.
        responseError = writeError;
      }
    } else {
      // Simulate download to NSData progress.
      if (accumulateDataBlock) {
        if (responseData) {
          [self invokeOnCallbackQueueUnlessStopped:^{
            accumulateDataBlock(responseData);
          }];
        }
      } else {
        _downloadedData = [responseData mutableCopy];
      }

      if (receivedProgressBlock) {
        [self simulateByteTransferReportWithDataLength:(int64_t)responseData.length
                                                 block:^(int64_t bytesReceived,
                                                         int64_t totalBytesReceived,
                                                         int64_t totalBytesExpectedToReceive) {
            // This is invoked on the callback queue unless stopped.
            receivedProgressBlock(bytesReceived, totalBytesReceived);
         }];
      }

      if (willCacheURLResponseBlock) {
        // Simulate letting the client inspect and alter the cached response.
        NSData *cachedData = responseData ?: [[NSData alloc] init];  // Always have non-nil data.
        NSCachedURLResponse *cachedResponse =
            [[NSCachedURLResponse alloc] initWithResponse:response
                                                     data:cachedData];
        [self invokeOnCallbackUnsynchronizedQueueAfterUserStopped:YES
                                                            block:^{
            willCacheURLResponseBlock(cachedResponse, ^(NSCachedURLResponse *responseToCache){
                // The app may provide an alternative response, or nil to defeat caching.
            });
        }];
      }
    }
    _response = response;
  }  // @synchronized(self)

  NSOperationQueue *queue = self.sessionDelegateQueue;
  [queue addOperationWithBlock:^{
    // Rather than invoke failToBeginFetchWithError: we want to simulate completion of
    // a connection that started and ended, so we'll call down to finishWithError:
    NSInteger status = responseError ? responseError.code : 200;
    if (status >= 200 && status <= 399) {
      [self finishWithError:nil shouldRetry:NO];
    } else {
      [self shouldRetryNowForStatus:status
                              error:responseError
                   forceAssumeRetry:NO
                           response:^(BOOL shouldRetry) {
          [self finishWithError:responseError shouldRetry:shouldRetry];
      }];
    }
  }];
}

#endif  // !GTM_DISABLE_FETCHER_TEST_BLOCK

- (void)setSessionTask:(NSURLSessionTask *)sessionTask {
  @synchronized(self) {
    GTMSessionMonitorSynchronized(self);

    if (_sessionTask != sessionTask) {
      _sessionTask = sessionTask;
      if (_sessionTask) {
        // Request could be nil on restoring this fetcher from a background session.
        if (!_request) {
          _request = [_sessionTask.originalRequest mutableCopy];
        }
      }
    }
  }  // @synchronized(self)
}

- (NSURLSessionTask * GTM_NULLABLE_TYPE)sessionTask {
  @synchronized(self) {
    GTMSessionMonitorSynchronize…