/os/port/chan.c

#include	"u.h"
#include	"../port/lib.h"
#include	"mem.h"
#include	"dat.h"
#include	"fns.h"
#include	"../port/error.h"

char*
channame(Chan *c)		/* DEBUGGING */
{
	if(c == nil)
		return "<nil chan>";
	if(c->name == nil)
		return "<nil name>";
	if(c->name->s == nil)
		return "<nil name.s>";
	return c->name->s;
}

enum
{
	CNAMESLOP	= 20
};

struct
{
	Lock;
	int	fid;
	Chan	*free;
	Chan	*list;
}chanalloc;

typedef struct Elemlist Elemlist;

struct Elemlist
{
	char	*name;	/* copy of name, so '/' can be overwritten */
	int	nelems;
	char	**elems;
	int	*off;
	int	mustbedir;
};

#define SEP(c) ((c) == 0 || (c) == '/')
void cleancname(Cname*);

int
isdotdot(char *p)
{
	return p[0]=='.' && p[1]=='.' && p[2]=='\0';
}

int
incref(Ref *r)
{
	int x;

	lock(&r->l);
	x = ++r->ref;
	unlock(&r->l);
	return x;
}

int
decref(Ref *r)
{
	int x;

	lock(&r->l);
	x = --r->ref;
	unlock(&r->l);
	if(x < 0)
		panic("decref, pc=0x%lux", getcallerpc(&r));

	return x;
}

/*
 * Rather than strncpy, which zeros the rest of the buffer, kstrcpy
 * truncates if necessary, always zero terminates, does not zero fill,
 * and puts ... at the end of the string if it's too long.  Usually used to
 * save a string in up->genbuf;
 */
void
kstrcpy(char *s, char *t, int ns)
{
	int nt;

	nt = strlen(t);
	if(nt+1 <= ns){
		memmove(s, t, nt+1);
		return;
	}
	/* too long */
	if(ns < 4){
		/* but very short! */
		strncpy(s, t, ns);
		return;
	}
	/* truncate with ... at character boundary (very rare case) */
	memmove(s, t, ns-4);
	ns -= 4;
	s[ns] = '\0';
	/* look for first byte of UTF-8 sequence by skipping continuation bytes */
	while(ns>0 && (s[--ns]&0xC0)==0x80)
		;
	strcpy(s+ns, "...");
}

int
emptystr(char *s)
{
	if(s == nil)
		return 1;
	if(s[0] == '\0')
		return 1;
	return 0;
}

/*
 * Atomically replace *p with copy of s
 */
void
kstrdup(char **p, char *s)
{
	int n;
	char *t, *prev;

	n = strlen(s)+1;
	/* if it's a user, we can wait for memory; if not, something's very wrong */
	if(up){
		t = smalloc(n);
		setmalloctag(t, getcallerpc(&p));
	}else{
		t = malloc(n);
		if(t == nil)
			panic("kstrdup: no memory");
	}
	memmove(t, s, n);
	prev = *p;
	*p = t;
	free(prev);
}

void
chandevreset(void)
{
	int i;

	for(i=0; devtab[i] != nil; i++)
		devtab[i]->reset();
}

void
chandevinit(void)
{
	int i;

	for(i=0; devtab[i] != nil; i++)
		devtab[i]->init();
}

void
chandevshutdown(void)
{
	int i;
	
	/* shutdown in reverse order */
	for(i=0; devtab[i] != nil; i++)
		;
	for(i--; i >= 0; i--)
		devtab[i]->shutdown();
}

Chan*
newchan(void)
{
	Chan *c;

	lock(&chanalloc);
	c = chanalloc.free;
	if(c != 0)
		chanalloc.free = c->next;
	unlock(&chanalloc);

	if(c == nil) {
		c = smalloc(sizeof(Chan));
		lock(&chanalloc);
		c->fid = ++chanalloc.fid;
		c->link = chanalloc.list;
		chanalloc.list = c;
		unlock(&chanalloc);
	}

	/* if you get an error before associating with a dev,
	   close calls rootclose, a nop */
	c->type = 0;
	c->flag = 0;
	c->ref = 1;
	c->dev = 0;
	c->offset = 0;
	c->iounit = 0;
	c->umh = 0;
	c->uri = 0;
	c->dri = 0;
	c->aux = 0;
	c->mchan = 0;
	c->mcp = 0;
	c->mux = 0;
	c->mqid.path = 0;
	c->mqid.vers = 0;
	c->mqid.type = 0;
	c->name = 0;
	return c;
}

static Ref ncname;

Cname*
newcname(char *s)
{
	Cname *n;
	int i;

	n = smalloc(sizeof(Cname));
	i = strlen(s);
	n->len = i;
	n->alen = i+CNAMESLOP;
	n->s = smalloc(n->alen);
	memmove(n->s, s, i+1);
	n->ref = 1;
	incref(&ncname);
	return n;
}

void
cnameclose(Cname *n)
{
	if(n == nil)
		return;
	if(decref(n))
		return;
	decref(&ncname);
	free(n->s);
	free(n);
}

Cname*
addelem(Cname *n, char *s)
{
	int i, a;
	char *t;
	Cname *new;

	if(s[0]=='.' && s[1]=='\0')
		return n;

	if(n->ref > 1){
		/* copy on write */
		new = newcname(n->s);
		cnameclose(n);
		n = new;
	}

	i = strlen(s);
	if(n->len+1+i+1 > n->alen){
		a = n->len+1+i+1 + CNAMESLOP;
		t = smalloc(a);
		memmove(t, n->s, n->len+1);
		free(n->s);
		n->s = t;
		n->alen = a;
	}
	if(n->len>0 && n->s[n->len-1]!='/' && s[0]!='/')	/* don't insert extra slash if one is present */
		n->s[n->len++] = '/';
	memmove(n->s+n->len, s, i+1);
	n->len += i;
	if(isdotdot(s))
		cleancname(n);
	return n;
}

void
chanfree(Chan *c)
{
	c->flag = CFREE;

	if(c->umh != nil){
		putmhead(c->umh);
		c->umh = nil;
	}
	if(c->umc != nil){
		cclose(c->umc);
		c->umc = nil;
	}
	if(c->mux != nil){
		muxclose(c->mux);
		c->mux = nil;
	}
	if(c->mchan != nil){
		cclose(c->mchan);
		c->mchan = nil;
	}

	cnameclose(c->name);

	lock(&chanalloc);
	c->next = chanalloc.free;
	chanalloc.free = c;
	unlock(&chanalloc);
}

void
cclose(Chan *c)
{
	if(c == 0)
		return;

	if(c->flag&CFREE)
		panic("cclose %lux", getcallerpc(&c));

	if(decref(c))
		return;

	if(!waserror()){
		devtab[c->type]->close(c);
		poperror();
	}
	chanfree(c);
}

/*
 * Make sure we have the only copy of c.  (Copy on write.)
 */
Chan*
cunique(Chan *c)
{
	Chan *nc;

	if(c->ref != 1) {
		nc = cclone(c);
		cclose(c);
		c = nc;
	}

	return c;
}

int
eqqid(Qid a, Qid b)
{
	return a.path==b.path && a.vers==b.vers;
}

int
eqchan(Chan *a, Chan *b, int pathonly)
{
	if(a->qid.path != b->qid.path)
		return 0;
	if(!pathonly && a->qid.vers!=b->qid.vers)
		return 0;
	if(a->type != b->type)
		return 0;
	if(a->dev != b->dev)
		return 0;
	return 1;
}

int
eqchantdqid(Chan *a, int type, int dev, Qid qid, int pathonly)
{
	if(a->qid.path != qid.path)
		return 0;
	if(!pathonly && a->qid.vers!=qid.vers)
		return 0;
	if(a->type != type)
		return 0;
	if(a->dev != dev)
		return 0;
	return 1;
}

Mhead*
newmhead(Chan *from)
{
	Mhead *mh;

	mh = smalloc(sizeof(Mhead));
	mh->ref = 1;
	mh->from = from;
	incref(from);

/*
	n = from->name->len;
	if(n >= sizeof(mh->fromname))
		n = sizeof(mh->fromname)-1;
	memmove(mh->fromname, from->name->s, n);
	mh->fromname[n] = 0;
*/
	return mh;
}

int
cmount(Chan *new, Chan *old, int flag, char *spec)
{
	Pgrp *pg;
	int order, flg;
	Mhead *m, **l, *mh;
	Mount *nm, *f, *um, **h;

	if(QTDIR & (old->qid.type^new->qid.type))
		error(Emount);

if(old->umh)
	print("cmount old extra umh\n");

	order = flag&MORDER;

	if((old->qid.type&QTDIR)==0 && order != MREPL)
		error(Emount);

	mh = new->umh;

	/*
	 * Not allowed to bind when the old directory
	 * is itself a union.  (Maybe it should be allowed, but I don't see
	 * what the semantics would be.)
	 *
	 * We need to check mh->mount->next to tell unions apart from
	 * simple mount points, so that things like
	 *	mount -c fd /root
	 *	bind -c /root /
	 * work.  The check of mount->mflag catches things like
	 *	mount fd /root
	 *	bind -c /root /
	 * 
	 * This is far more complicated than it should be, but I don't
	 * see an easier way at the moment.		-rsc
	 */
	if((flag&MCREATE) && mh && mh->mount
	&& (mh->mount->next || !(mh->mount->mflag&MCREATE)))
		error(Emount);

	pg = up->env->pgrp;
	wlock(&pg->ns);

	l = &MOUNTH(pg, old->qid);
	for(m = *l; m; m = m->hash) {
		if(eqchan(m->from, old, 1))
			break;
		l = &m->hash;
	}

	if(m == nil) {
		/*
		 *  nothing mounted here yet.  create a mount
		 *  head and add to the hash table.
		 */
		m = newmhead(old);
		*l = m;

		/*
		 *  if this is a union mount, add the old
		 *  node to the mount chain.
		 */
		if(order != MREPL)
			m->mount = newmount(m, old, 0, 0);
	}
	wlock(&m->lock);
	if(waserror()){
		wunlock(&m->lock);
		nexterror();
	}
	wunlock(&pg->ns);

	nm = newmount(m, new, flag, spec);
	if(mh != nil && mh->mount != nil) {
		/*
		 *  copy a union when binding it onto a directory
		 */
		flg = order;
		if(order == MREPL)
			flg = MAFTER;
		h = &nm->next;
		um = mh->mount;
		for(um = um->next; um; um = um->next) {
			f = newmount(m, um->to, flg, um->spec);
			*h = f;
			h = &f->next;
		}
	}

	if(m->mount && order == MREPL) {
		mountfree(m->mount);
		m->mount = 0;
	}

	if(flag & MCREATE)
		nm->mflag |= MCREATE;

	if(m->mount && order == MAFTER) {
		for(f = m->mount; f->next; f = f->next)
			;
		f->next = nm;
	}
	else {
		for(f = nm; f->next; f = f->next)
			;
		f->next = m->mount;
		m->mount = nm;
	}

	wunlock(&m->lock);
	poperror();
	return nm->mountid;
}

void
cunmount(Chan *mnt, Chan *mounted)
{
	Pgrp *pg;
	Mhead *m, **l;
	Mount *f, **p;

	if(mnt->umh)	/* should not happen */
		print("cunmount newp extra umh %p has %p\n", mnt, mnt->umh);

	/*
	 * It _can_ happen that mounted->umh is non-nil, 
	 * because mounted is the result of namec(Aopen)
	 * (see sysfile.c:/^sysunmount).
	 * If we open a union directory, it will have a umh.
	 * Although surprising, this is okay, since the
	 * cclose will take care of freeing the umh.
	 */

	pg = up->env->pgrp;
	wlock(&pg->ns);

	l = &MOUNTH(pg, mnt->qid);
	for(m = *l; m; m = m->hash) {
		if(eqchan(m->from, mnt, 1))
			break;
		l = &m->hash;
	}

	if(m == 0) {
		wunlock(&pg->ns);
		error(Eunmount);
	}

	wlock(&m->lock);
	if(mounted == 0) {
		*l = m->hash;
		wunlock(&pg->ns);
		mountfree(m->mount);
		m->mount = nil;
		cclose(m->from);
		wunlock(&m->lock);
		putmhead(m);
		return;
	}

	p = &m->mount;
	for(f = *p; f; f = f->next) {
		/* BUG: Needs to be 2 pass */
		if(eqchan(f->to, mounted, 1) ||
		  (f->to->mchan && eqchan(f->to->mchan, mounted, 1))) {
			*p = f->next;
			f->next = 0;
			mountfree(f);
			if(m->mount == nil) {
				*l = m->hash;
				cclose(m->from);
				wunlock(&m->lock);
				wunlock(&pg->ns);
				putmhead(m);
				return;
			}
			wunlock(&m->lock);
			wunlock(&pg->ns);
			return;
		}
		p = &f->next;
	}
	wunlock(&m->lock);
	wunlock(&pg->ns);
	error(Eunion);
}

Chan*
cclone(Chan *c)
{
	Chan *nc;
	Walkqid *wq;

	wq = devtab[c->type]->walk(c, nil, nil, 0);
	if(wq == nil)
		error("clone failed");
	nc = wq->clone;
	free(wq);
	nc->name = c->name;
	if(c->name)
		incref(c->name);
	return nc;
}

int
findmount(Chan **cp, Mhead **mp, int type, int dev, Qid qid)
{
	Pgrp *pg;
	Mhead *m;

	pg = up->env->pgrp;
	rlock(&pg->ns);
	for(m = MOUNTH(pg, qid); m; m = m->hash){
		rlock(&m->lock);
if(m->from == nil){
	print("m %p m->from 0\n", m);
	runlock(&m->lock);
	continue;
}
		if(eqchantdqid(m->from, type, dev, qid, 1)) {
			runlock(&pg->ns);
			if(mp != nil){
				incref(m);
				if(*mp != nil)
					putmhead(*mp);
				*mp = m;
			}
			if(*cp != nil)
				cclose(*cp);
			incref(m->mount->to);
			*cp = m->mount->to;
			runlock(&m->lock);
			return 1;
		}
		runlock(&m->lock);
	}

	runlock(&pg->ns);
	return 0;
}

int
domount(Chan **cp, Mhead **mp)
{
	return findmount(cp, mp, (*cp)->type, (*cp)->dev, (*cp)->qid);
}

Chan*
undomount(Chan *c, Cname *name)
{
	Chan *nc;
	Pgrp *pg;
	Mount *t;
	Mhead **h, **he, *f;

	pg = up->env->pgrp;
	rlock(&pg->ns);
	if(waserror()) {
		runlock(&pg->ns);
		nexterror();
	}

	he = &pg->mnthash[MNTHASH];
	for(h = pg->mnthash; h < he; h++) {
		for(f = *h; f; f = f->hash) {
			if(strcmp(f->from->name->s, name->s) != 0)
				continue;
			for(t = f->mount; t; t = t->next) {
				if(eqchan(c, t->to, 1)) {
					/*
					 * We want to come out on the left hand side of the mount
					 * point using the element of the union that we entered on.
					 * To do this, find the element that has a from name of
					 * c->name->s.
					 */
					if(strcmp(t->head->from->name->s, name->s) != 0)
						continue;
					nc = t->head->from;
					incref(nc);
					cclose(c);
					c = nc;
					break;
				}
			}
		}
	}
	poperror();
	runlock(&pg->ns);
	return c;
}

/*
 * Either walks all the way or not at all.  No partial results in *cp.
 * *nerror is the number of names to display in an error message.
 */
static char Edoesnotexist[] = "does not exist";
int
walk(Chan **cp, char **names, int nnames, int nomount, int *nerror)
{
	int dev, dotdot, i, n, nhave, ntry, type;
	Chan *c, *nc;
	Cname *cname;
	Mount *f;
	Mhead *mh, *nmh;
	Walkqid *wq;

	c = *cp;
	incref(c);
	cname = c->name;
	incref(cname);
	mh = nil;

	/*
	 * While we haven't gotten all the way down the path:
	 *    1. step through a mount point, if any
	 *    2. send a walk request for initial dotdot or initial prefix without dotdot
	 *    3. move to the first mountpoint along the way.
	 *    4. repeat.
	 *
	 * An invariant is that each time through the loop, c is on the undomount
	 * side of the mount point, and c's name is cname.
	 */
	for(nhave=0; nhave<nnames; nhave+=n){
		if((c->qid.type&QTDIR)==0){
			if(nerror)
				*nerror = nhave;
			cnameclose(cname);
			cclose(c);
			strcpy(up->env->errstr, Enotdir);
			if(mh != nil)
				putmhead(mh);
			return -1;
		}
		ntry = nnames - nhave;
		if(ntry > MAXWELEM)
			ntry = MAXWELEM;
		dotdot = 0;
		for(i=0; i<ntry; i++){
			if(isdotdot(names[nhave+i])){
				if(i==0) {
					dotdot = 1;
					ntry = 1;
				} else
					ntry = i;
				break;
			}
		}

		if(!dotdot && !nomount)
			domount(&c, &mh);

		type = c->type;
		dev = c->dev;

		if((wq = devtab[type]->walk(c, nil, names+nhave, ntry)) == nil){
			/* try a union mount, if any */
			if(mh && !nomount){
				/*
				 * mh->mount == c, so start at mh->mount->next
				 */
				rlock(&mh->lock);
				for(f = mh->mount->next; f; f = f->next)
					if((wq = devtab[f->to->type]->walk(f->to, nil, names+nhave, ntry)) != nil)
						break;
				runlock(&mh->lock);
				if(f != nil){
					type = f->to->type;
					dev = f->to->dev;
				}
			}
			if(wq == nil){
				cclose(c);
				cnameclose(cname);
				if(nerror)
					*nerror = nhave+1;
				if(mh != nil)
					putmhead(mh);
				return -1;
			}
		}

		nmh = nil;
		if(dotdot) {
			assert(wq->nqid == 1);
			assert(wq->clone != nil);

			cname = addelem(cname, "..");
			nc = undomount(wq->clone, cname);
			n = 1;
		} else {
			nc = nil;
			if(!nomount)
				for(i=0; i<wq->nqid && i<ntry-1; i++)
					if(findmount(&nc, &nmh, type, dev, wq->qid[i]))
						break;
			if(nc == nil){	/* no mount points along path */
				if(wq->clone == nil){
					cclose(c);
					cnameclose(cname);
					if(wq->nqid==0 || (wq->qid[wq->nqid-1].type&QTDIR)){
						if(nerror)
							*nerror = nhave+wq->nqid+1;
						strcpy(up->env->errstr, Edoesnotexist);
					}else{
						if(nerror)
							*nerror = nhave+wq->nqid;
						strcpy(up->env->errstr, Enotdir);
					}
					free(wq);
					if(mh != nil)
						putmhead(mh);
					return -1;
				}
				n = wq->nqid;
				nc = wq->clone;
			}else{		/* stopped early, at a mount point */
				if(wq->clone != nil){
					cclose(wq->clone);
					wq->clone = nil;
				}
				n = i+1;
			}
			for(i=0; i<n; i++)
				cname = addelem(cname, names[nhave+i]);
		}
		cclose(c);
		c = nc;
		putmhead(mh);
		mh = nmh;
		free(wq);
	}

	putmhead(mh);

	c = cunique(c);

	if(c->umh != nil){	//BUG
		print("walk umh\n");
		putmhead(c->umh);
		c->umh = nil;
	}

	cnameclose(c->name);
	c->name = cname;

	cclose(*cp);
	*cp = c;
	if(nerror)
		*nerror = 0;
	return 0;
}

/*
 * c is a mounted non-creatable directory.  find a creatable one.
 */
Chan*
createdir(Chan *c, Mhead *m)
{
	Chan *nc;
	Mount *f;

	rlock(&m->lock);
	if(waserror()) {
		runlock(&m->lock);
		nexterror();
	}
	for(f = m->mount; f; f = f->next) {
		if(f->mflag&MCREATE) {
			nc = cclone(f->to);
			runlock(&m->lock);
			poperror();
			cclose(c);
			return nc;
		}
	}
	error(Enocreate);
	return 0;
}

void
saveregisters(void)
{
}

/*
 * In place, rewrite name to compress multiple /, eliminate ., and process ..
 */
void
cleancname(Cname *n)
{
	char *p;

	if(n->s[0] == '#'){
		p = strchr(n->s, '/');
		if(p == nil)
			return;
		cleanname(p);

		/*
		 * The correct name is #i rather than #i/,
		 * but the correct name of #/ is #/.
		 */
		if(strcmp(p, "/")==0 && n->s[1] != '/')
			*p = '\0';
	}else
		cleanname(n->s);
	n->len = strlen(n->s);
}

static void
growparse(Elemlist *e)
{
	char **new;
	int *inew;
	enum { Delta = 8 };

	if(e->nelems % Delta == 0){
		new = smalloc((e->nelems+Delta) * sizeof(char*));
		memmove(new, e->elems, e->nelems*sizeof(char*));
		free(e->elems);
		e->elems = new;
		inew = smalloc((e->nelems+Delta+1) * sizeof(int));
		memmove(inew, e->off, e->nelems*sizeof(int));
		free(e->off);
		e->off = inew;
	}
}

/*
 * The name is known to be valid.
 * Copy the name so slashes can be overwritten.
 * An empty string will set nelem=0.
 * A path ending in / or /. or /.//./ etc. will have
 * e.mustbedir = 1, so that we correctly
 * reject, e.g., "/adm/users/." when /adm/users is a file
 * rather than a directory.
 */
static void
parsename(char *name, Elemlist *e)
{
	char *slash;

	kstrdup(&e->name, name);
	name = e->name;
	e->nelems = 0;
	e->elems = nil;
	e->off = smalloc(sizeof(int));
	e->off[0] = skipslash(name) - name;
	for(;;){
		name = skipslash(name);
		if(*name=='\0'){
			e->mustbedir = 1;
			break;
		}
		growparse(e);
		
		e->elems[e->nelems++] = name;
		slash = utfrune(name, '/');
		if(slash == nil){
			e->off[e->nelems] = name+strlen(name) - e->name;
			e->mustbedir = 0;
			break;
		}
		e->off[e->nelems] = slash - e->name;
		*slash++ = '\0';
		name = slash;
	}
}

void*
memrchr(void *va, int c, long n)
{
	uchar *a, *e;

	a = va;
	for(e=a+n-1; e>a; e--)
		if(*e == c)
			return e;
	return nil;
}

/*
 * Turn a name into a channel.
 * &name[0] is known to be a valid address.  It may be a kernel address.
 *
 * Opening with amode Aopen, Acreate, or Aremove guarantees
 * that the result will be the only reference to that particular fid.
 * This is necessary since we might pass the result to
 * devtab[]->remove().
 *
 * Opening Atodir, Amount, or Aaccess does not guarantee this.
 *
 * Opening Aaccess can, under certain conditions, return a
 * correct Chan* but with an incorrect Cname attached.
 * Since the functions that open Aaccess (sysstat, syswstat, sys_stat)
 * do not use the Cname*, this avoids an unnecessary clone.
 */
Chan*
namec(char *aname, int amode, int omode, ulong perm)
{
	int n, prefix, len, t, nomount, npath;
	Chan *c, *cnew;
	Cname *cname;
	Elemlist e;
	Rune r;
	Mhead *m;
	char *createerr, tmperrbuf[ERRMAX];
	char *name;

	name = aname;
	if(name[0] == '\0')
		error("empty file name");
	validname(name, 1);

	/*
	 * Find the starting off point (the current slash, the root of
	 * a device tree, or the current dot) as well as the name to
	 * evaluate starting there.
	 */
	nomount = 0;
	switch(name[0]){
	case '/':
		c = up->env->pgrp->slash;
		incref(c);
		break;
	
	case '#':
		nomount = 1;
		up->genbuf[0] = '\0';
		n = 0;
		while(*name!='\0' && (*name != '/' || n < 2)){
			if(n >= sizeof(up->genbuf)-1)
				error(Efilename);
			up->genbuf[n++] = *name++;
		}
		up->genbuf[n] = '\0';
		n = chartorune(&r, up->genbuf+1)+1;
		if(r == 'M')
			error(Enoattach);
		/*
		 *  the nodevs exceptions are
		 *	|  it only gives access to pipes you create
		 *	e  this process's environment
		 *	s  private file2chan creation space
		 *	D private secure sockets name space
		 *	a private TLS name space
		 */
		if(up->env->pgrp->nodevs &&
		   (utfrune("|esDa", r) == nil || r == 's' && up->genbuf[n]!='\0'))
			error(Enoattach);
		t = devno(r, 1);
		if(t == -1)
			error(Ebadsharp);
		c = devtab[t]->attach(up->genbuf+n);
		break;

	default:
		c = up->env->pgrp->dot;
		incref(c);
		break;
	}
	prefix = name - aname;

	e.name = nil;
	e.elems = nil;
	e.off = nil;
	e.nelems = 0;
	if(waserror()){
		cclose(c);
		free(e.name);
		free(e.elems);
		free(e.off);
//dumpmount();
		nexterror();
	}

	/*
	 * Build a list of elements in the path.
	 */
	parsename(name, &e);

	/*
	 * On create, ....
	 */
	if(amode == Acreate){
		/* perm must have DMDIR if last element is / or /. */
		if(e.mustbedir && !(perm&DMDIR)){
			npath = e.nelems;
			strcpy(tmperrbuf, "create without DMDIR");
			goto NameError;
		}

		/* don't try to walk the last path element just yet. */
		if(e.nelems == 0)
			error(Eexist);
		e.nelems--;
	}

	if(walk(&c, e.elems, e.nelems, nomount, &npath) < 0){
		if(npath < 0 || npath > e.nelems){
			print("namec %s walk error npath=%d\n", aname, npath);
			nexterror();
		}
		strcpy(tmperrbuf, up->env->errstr);
	NameError:
		len = prefix+e.off[npath];
		if(len < ERRMAX/3 || (name=memrchr(aname, '/', len))==nil || name==aname)
			snprint(up->genbuf, sizeof up->genbuf, "%.*s", len, aname);
		else
			snprint(up->genbuf, sizeof up->genbuf, "...%.*s", (int)(len-(name-aname)), name);
		snprint(up->env->errstr, ERRMAX, "%#q %s", up->genbuf, tmperrbuf);
		nexterror();
	}

	if(e.mustbedir && !(c->qid.type&QTDIR)){
		npath = e.nelems;
		strcpy(tmperrbuf, "not a directory");
		goto NameError;
	}

	if(amode == Aopen && (omode&3) == OEXEC && (c->qid.type&QTDIR)){
		npath = e.nelems;
		error("cannot exec directory");
	}

	switch(amode){
	case Aaccess:
		if(!nomount)
			domount(&c, nil);
		break;

	case Abind:
		m = nil;
		if(!nomount)
			domount(&c, &m);
		if(c->umh != nil)
			putmhead(c->umh);
		c->umh = m;
		break;

	case Aremove:
	case Aopen:
	Open:
		/* save the name; domount might change c */
		cname = c->name;
		incref(cname);
		m = nil;
		if(!nomount)
			domount(&c, &m);

		/* our own copy to open or remove */
		c = cunique(c);

		/* now it's our copy anyway, we can put the name back */
		cnameclose(c->name);
		c->name = cname;

		switch(amode){
		case Aremove:
			putmhead(m);
			break;

		case Aopen:
		case Acreate:
if(c->umh != nil){
	print("cunique umh\n");
	putmhead(c->umh);
	c->umh = nil;
}

			/* only save the mount head if it's a multiple element union */
			if(m && m->mount && m->mount->next)
				c->umh = m;
			else
				putmhead(m);

			/* save registers else error() in open has wrong value of c saved */
			saveregisters();

			if(omode == OEXEC)
				c->flag &= ~CCACHE;

			c = devtab[c->type]->open(c, omode&~OCEXEC);

			if(omode & OCEXEC)
				c->flag |= CCEXEC;
			if(omode & ORCLOSE)
				c->flag |= CRCLOSE;
			break;
		}
		break;

	case Atodir:
		/*
		 * Directories (e.g. for cd) are left before the mount point,
		 * so one may mount on / or . and see the effect.
		 */
		if(!(c->qid.type & QTDIR))
			error(Enotdir);
		break;

	case Amount:
		/*
		 * When mounting on an already mounted upon directory,
		 * one wants subsequent mounts to be attached to the
		 * original directory, not the replacement.  Don't domount.
		 */
		break;

	case Acreate:
		/*
		 * We've already walked all but the last element.
		 * If the last exists, try to open it OTRUNC.
		 * If omode&OEXCL is set, just give up.
		 */
		e.nelems++;
		if(walk(&c, e.elems+e.nelems-1, 1, nomount, nil) == 0){
			if(omode&OEXCL)
				error(Eexist);
			omode |= OTRUNC;
			goto Open;
		}

		/*
		 * The semantics of the create(2) system call are that if the
		 * file exists and can be written, it is to be opened with truncation.
		 * On the other hand, the create(5) message fails if the file exists.
		 * If we get two create(2) calls happening simultaneously, 
		 * they might both get here and send create(5) messages, but only 
		 * one of the messages will succeed.  To provide the expected create(2)
		 * semantics, the call with the failed message needs to try the above
		 * walk again, opening for truncation.  This correctly solves the 
		 * create/create race, in the sense that any observable outcome can
		 * be explained as one happening before the other.
		 * The create/create race is quite common.  For example, it happens
		 * when two rc subshells simultaneously update the same
		 * environment variable.
		 *
		 * The implementation still admits a create/create/remove race:
		 * (A) walk to file, fails
		 * (B) walk to file, fails
		 * (A) create file, succeeds, returns 
		 * (B) create file, fails
		 * (A) remove file, succeeds, returns
		 * (B) walk to file, return failure.
		 *
		 * This is hardly as common as the create/create race, and is really
		 * not too much worse than what might happen if (B) got a hold of a
		 * file descriptor and then the file was removed -- either way (B) can't do
		 * anything with the result of the create call.  So we don't care about this race.
		 *
		 * Applications that care about more fine-grained decision of the races
		 * can use the OEXCL flag to get at the underlying create(5) semantics;
		 * by default we provide the common case.
		 *
		 * We need to stay behind the mount point in case we
		 * need to do the first walk again (should the create fail).
		 *
		 * We also need to cross the mount point and find the directory
		 * in the union in which we should be creating.
		 *
		 * The channel staying behind is c, the one moving forward is cnew.
		 */
		m = nil;
		cnew = nil;	/* is this assignment necessary? */
		if(!waserror()){	/* try create */
			if(!nomount && findmount(&cnew, &m, c->type, c->dev, c->qid))
				cnew = createdir(cnew, m);
			else{
				cnew = c;
				incref(cnew);
			}

			/*
			 * We need our own copy of the Chan because we're
			 * about to send a create, which will move it.  Once we have
			 * our own copy, we can fix the name, which might be wrong
			 * if findmount gave us a new Chan.
			 */
			cnew = cunique(cnew);
			cnameclose(cnew->name);
			cnew->name = c->name;
			incref(cnew->name);

			devtab[cnew->type]->create(cnew, e.elems[e.nelems-1], omode&~(OEXCL|OCEXEC), perm);
			poperror();
			if(omode & OCEXEC)
				cnew->flag |= CCEXEC;
			if(omode & ORCLOSE)
				cnew->flag |= CRCLOSE;
			if(m)
				putmhead(m);
			cclose(c);
			c = cnew;
			c->name = addelem(c->name, e.elems[e.nelems-1]);
			break;
		}

		/* create failed */
		cclose(cnew);
		if(m)
			putmhead(m);
		if(omode & OEXCL)
			nexterror();
		/* save error */
		createerr = up->env->errstr;
		up->env->errstr = tmperrbuf;
		/* note: we depend that walk does not error */
		if(walk(&c, e.elems+e.nelems-1, 1, nomount, nil) < 0){
			up->env->errstr = createerr;
			error(createerr);	/* report true error */
		}
		up->env->errstr = createerr;
		omode |= OTRUNC;
		goto Open;

	default:
		panic("unknown namec access %d\n", amode);
	}

	poperror();

	/* place final element in genbuf for e.g. exec */
	if(e.nelems > 0)
		kstrcpy(up->genbuf, e.elems[e.nelems-1], sizeof up->genbuf);
	else
		kstrcpy(up->genbuf, ".", sizeof up->genbuf);
	free(e.name);
	free(e.elems);
	free(e.off);

	return c;
}

/*
 * name is valid. skip leading / and ./ as much as possible
 */
char*
skipslash(char *name)
{
	while(name[0]=='/' || (name[0]=='.' && (name[1]==0 || name[1]=='/')))
		name++;
	return name;
}

char isfrog[256]={
	/*NUL*/	1, 1, 1, 1, 1, 1, 1, 1,
	/*BKS*/	1, 1, 1, 1, 1, 1, 1, 1,
	/*DLE*/	1, 1, 1, 1, 1, 1, 1, 1,
	/*CAN*/	1, 1, 1, 1, 1, 1, 1, 1,
	['/']	1,
	[0x7f]	1,
};

/*
 * Check that the name
 *  a) is in valid memory.
 *  b) is shorter than 2^16 bytes, so it can fit in a 9P string field.
 *  c) contains no frogs.
 * The first byte is known to be addressible by the requester, so the
 * routine works for kernel and user memory both.
 * The parameter slashok flags whether a slash character is an error
 * or a valid character.
 */
void
validname(char *aname, int slashok)
{
	char *ename, *name;
	int c;
	Rune r;

	name = aname;
	ename = memchr(name, 0, (1<<16));

	if(ename==nil || ename-name>=(1<<16))
		error("name too long");

	while(*name){
		/* all characters above '~' are ok */
		c = *(uchar*)name;
		if(c >= Runeself)
			name += chartorune(&r, name);
		else{
			if(isfrog[c])
				if(!slashok || c!='/'){
					snprint(up->genbuf, sizeof(up->genbuf), "%s: %q", Ebadchar, aname);
					error(up->genbuf);
			}
			name++;
		}
	}
}

void
isdir(Chan *c)
{
	if(c->qid.type & QTDIR)
		return;
	error(Enotdir);
}

/*
 * This is necessary because there are many
 * pointers to the top of a given mount list:
 *
 *	- the mhead in the namespace hash table
 *	- the mhead in chans returned from findmount:
 *	  used in namec and then by unionread.
 *	- the mhead in chans returned from createdir:
 *	  used in the open/create race protect, which is gone.
 *
 * The RWlock in the Mhead protects the mount list it contains.
 * The mount list is deleted when we cunmount.
 * The RWlock ensures that nothing is using the mount list at that time.
 *
 * It is okay to replace c->mh with whatever you want as 
 * long as you are sure you have a unique reference to it.
 *
 * This comment might belong somewhere else.
 */
void
putmhead(Mhead *m)
{
	if(m && decref(m) == 0){
		m->mount = (Mount*)0xCafeBeef;
		free(m);
	}
}