Import sources from 2011-03-30 iso image

1 files changed, 1775 insertions, 0 deletions

diff --git a/sys/src/9/port/chan.c b/sys/src/9/port/chan.c
new file mode 100755
index 000000000..12af9baa3
--- /dev/null
+++ b/sys/src/9/port/chan.c
@@ -0,0 +1,1775 @@
+#include	"u.h"
+#include	"../port/lib.h"
+#include	"mem.h"
+#include	"dat.h"
+#include	"fns.h"
+#include	"../port/error.h"
+
+int chandebug=0;		/* toggled by sysr1 */
+#define DBG if(chandebug)iprint
+
+enum
+{
+	PATHSLOP	= 20,
+	PATHMSLOP	= 20,
+};
+
+struct
+{
+	Lock;
+	int	fid;
+	Chan	*free;
+	Chan	*list;
+}chanalloc;
+
+typedef struct Elemlist Elemlist;
+
+struct Elemlist
+{
+	char	*aname;	/* original name */
+	char	*name;	/* copy of name, so '/' can be overwritten */
+	int	nelems;
+	char	**elems;
+	int	*off;
+	int	mustbedir;
+	int	nerror;
+	int	prefix;
+};
+
+#define SEP(c) ((c) == 0 || (c) == '/')
+
+static void
+dumpmount(void)		/* DEBUGGING */
+{
+	Pgrp *pg;
+	Mount *t;
+	Mhead **h, **he, *f;
+
+	if(up == nil){
+		print("no process for dumpmount\n");
+		return;
+	}
+	pg = up->pgrp;
+	if(pg == nil){
+		print("no pgrp for dumpmount\n");
+		return;
+	}
+	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){
+			print("head: %#p: %s %#llux.%lud %C %lud -> \n", f,
+				f->from->path->s, f->from->qid.path,
+				f->from->qid.vers, devtab[f->from->type]->dc,
+				f->from->dev);
+			for(t = f->mount; t; t = t->next)
+				print("\t%#p: %s (umh %#p) (path %#.8llux dev %C %lud)\n", t, t->to->path->s, t->to->umh, t->to->qid.path, devtab[t->to->type]->dc, t->to->dev);
+		}
+	}
+	poperror();
+	runlock(&pg->ns);
+}
+
+char*
+chanpath(Chan *c)
+{
+	if(c == nil)
+		return "<nil chan>";
+	if(c->path == nil)
+		return "<nil path>";
+	if(c->path->s == nil)
+		return "<nil path.s>";
+	return c->path->s;
+}
+
+int
+isdotdot(char *p)
+{
+	return p[0]=='.' && p[1]=='.' && p[2]=='\0';
+}
+
+long
+incref(Ref *r)
+{
+	long x;
+
+	lock(r);
+	x = ++r->ref;
+	unlock(r);
+	return x;
+}
+
+long
+decref(Ref *r)
+{
+	long x;
+
+	lock(r);
+	x = --r->ref;
+	unlock(r);
+	if(x < 0)
+		panic("decref pc=%#p", 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;
+
+	todinit();	/* avoid later reentry causing infinite recursion */
+	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->devoffset = 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;
+	memset(&c->mqid, 0, sizeof(c->mqid));
+	c->path = 0;
+	c->ismtpt = 0;
+	
+	return c;
+}
+
+Ref npath;
+
+Path*
+newpath(char *s)
+{
+	int i;
+	Path *p;
+
+	p = smalloc(sizeof(Path));
+	i = strlen(s);
+	p->len = i;
+	p->alen = i+PATHSLOP;
+	p->s = smalloc(p->alen);
+	memmove(p->s, s, i+1);
+	p->ref = 1;
+	incref(&npath);
+
+	/*
+	 * Cannot use newpath for arbitrary names because the mtpt 
+	 * array will not be populated correctly.  The names #/ and / are
+	 * allowed, but other names with / in them draw warnings.
+	 */
+	if(strchr(s, '/') && strcmp(s, "#/") != 0 && strcmp(s, "/") != 0)
+		print("newpath: %s from %#p\n", s, getcallerpc(&s));
+
+	p->mlen = 1;
+	p->malen = PATHMSLOP;
+	p->mtpt = smalloc(p->malen*sizeof p->mtpt[0]);
+	return p;
+}
+
+static Path*
+copypath(Path *p)
+{
+	int i;
+	Path *pp;
+	
+	pp = smalloc(sizeof(Path));
+	pp->ref = 1;
+	incref(&npath);
+	DBG("copypath %s %p => %p\n", p->s, p, pp);
+	
+	pp->len = p->len;
+	pp->alen = p->alen;
+	pp->s = smalloc(p->alen);
+	memmove(pp->s, p->s, p->len+1);
+	
+	pp->mlen = p->mlen;
+	pp->malen = p->malen;
+	pp->mtpt = smalloc(p->malen*sizeof pp->mtpt[0]);
+	for(i=0; i<pp->mlen; i++){
+		pp->mtpt[i] = p->mtpt[i];
+		if(pp->mtpt[i])
+			incref(pp->mtpt[i]);
+	}
+
+	return pp;
+}
+
+void
+pathclose(Path *p)
+{
+	int i;
+	
+	if(p == nil)
+		return;
+//XXX
+	DBG("pathclose %p %s ref=%ld =>", p, p->s, p->ref);
+	for(i=0; i<p->mlen; i++)
+		DBG(" %p", p->mtpt[i]);
+	DBG("\n");
+
+	if(decref(p))
+		return;
+	decref(&npath);
+	free(p->s);
+	for(i=0; i<p->mlen; i++)
+		if(p->mtpt[i])
+			cclose(p->mtpt[i]);
+	free(p->mtpt);
+	free(p);
+}
+
+/*
+ * In place, rewrite name to compress multiple /, eliminate ., and process ..
+ * (Really only called to remove a trailing .. that has been added.
+ * Otherwise would need to update n->mtpt as well.)
+ */
+static void
+fixdotdotname(Path *p)
+{
+	char *r;
+
+	if(p->s[0] == '#'){
+		r = strchr(p->s, '/');
+		if(r == nil)
+			return;
+		cleanname(r);
+
+		/*
+		 * The correct name is #i rather than #i/,
+		 * but the correct name of #/ is #/.
+		 */
+		if(strcmp(r, "/")==0 && p->s[1] != '/')
+			*r = '\0';
+	}else
+		cleanname(p->s);
+	p->len = strlen(p->s);
+}
+
+static Path*
+uniquepath(Path *p)
+{
+	Path *new;
+	
+	if(p->ref > 1){
+		/* copy on write */
+		new = copypath(p);
+		pathclose(p);
+		p = new;
+	}
+	return p;
+}
+
+static Path*
+addelem(Path *p, char *s, Chan *from)
+{
+	char *t;
+	int a, i;
+	Chan *c, **tt;
+
+	if(s[0]=='.' && s[1]=='\0')
+		return p;
+
+	p = uniquepath(p);
+
+	i = strlen(s);
+	if(p->len+1+i+1 > p->alen){
+		a = p->len+1+i+1 + PATHSLOP;
+		t = smalloc(a);
+		memmove(t, p->s, p->len+1);
+		free(p->s);
+		p->s = t;
+		p->alen = a;
+	}
+	/* don't insert extra slash if one is present */
+	if(p->len>0 && p->s[p->len-1]!='/' && s[0]!='/')
+		p->s[p->len++] = '/';
+	memmove(p->s+p->len, s, i+1);
+	p->len += i;
+	if(isdotdot(s)){
+		fixdotdotname(p);
+		DBG("addelem %s .. => rm %p\n", p->s, p->mtpt[p->mlen-1]);
+		if(p->mlen>1 && (c = p->mtpt[--p->mlen])){
+			p->mtpt[p->mlen] = nil;
+			cclose(c);
+		}
+	}else{
+		if(p->mlen >= p->malen){
+			p->malen = p->mlen+1+PATHMSLOP;
+			tt = smalloc(p->malen*sizeof tt[0]);
+			memmove(tt, p->mtpt, p->mlen*sizeof tt[0]);
+			free(p->mtpt);
+			p->mtpt = tt;
+		}
+		DBG("addelem %s %s => add %p\n", p->s, s, from);
+		p->mtpt[p->mlen++] = from;
+		if(from)
+			incref(from);
+	}
+	return p;
+}
+
+void
+chanfree(Chan *c)
+{
+	c->flag = CFREE;
+
+	if(c->dirrock != nil){
+		free(c->dirrock);
+		c->dirrock = 0;
+		c->nrock = 0;
+		c->mrock = 0;
+	}
+	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;
+	}
+
+	pathclose(c->path);
+	c->path = nil;
+
+	lock(&chanalloc);
+	c->next = chanalloc.free;
+	chanalloc.free = c;
+	unlock(&chanalloc);
+}
+
+void
+cclose(Chan *c)
+{
+	if(c->flag&CFREE)
+		panic("cclose %#p", getcallerpc(&c));
+
+	DBG("cclose %p name=%s ref=%ld\n", c, c->path->s, c->ref);
+	if(decref(c))
+		return;
+
+	if(!waserror()){
+		devtab[c->type]->close(c);
+		poperror();
+	}
+	chanfree(c);
+}
+
+/*
+ * Queue a chan to be closed by one of the clunk procs.
+ */
+struct {
+	Chan *head;
+	Chan *tail;
+	int nqueued;
+	int nclosed;
+	Lock l;
+	QLock q;
+	Rendez r;
+} clunkq;
+void closeproc(void*);
+
+void
+ccloseq(Chan *c)
+{
+	if(c->flag&CFREE)
+		panic("cclose %#p", getcallerpc(&c));
+
+	DBG("ccloseq %p name=%s ref=%ld\n", c, c->path->s, c->ref);
+
+	if(decref(c))
+		return;
+
+	lock(&clunkq.l);
+	clunkq.nqueued++;
+	c->next = nil;
+	if(clunkq.head)
+		clunkq.tail->next = c;
+	else
+		clunkq.head = c;
+	clunkq.tail = c;
+	unlock(&clunkq.l);
+
+	if(!wakeup(&clunkq.r))
+		kproc("closeproc", closeproc, nil);	
+}
+
+static int
+clunkwork(void*)
+{
+	return clunkq.head != nil;
+}
+
+void
+closeproc(void*)
+{
+	Chan *c;
+
+	for(;;){
+		qlock(&clunkq.q);
+		if(clunkq.head == nil){
+			if(!waserror()){
+				tsleep(&clunkq.r, clunkwork, nil, 5000);
+				poperror();
+			}
+			if(clunkq.head == nil){
+				qunlock(&clunkq.q);
+				pexit("no work", 1);
+			}
+		}
+		lock(&clunkq.l);
+		c = clunkq.head;
+		clunkq.head = c->next;
+		clunkq.nclosed++;
+		unlock(&clunkq.l);
+		qunlock(&clunkq.q);
+		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 skipvers)
+{
+	if(a->qid.path != b->qid.path)
+		return 0;
+	if(!skipvers && 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 skipvers)
+{
+	if(a->qid.path != qid.path)
+		return 0;
+	if(!skipvers && 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);
+	return mh;
+}
+
+int
+cmount(Chan **newp, Chan *old, int flag, char *spec)
+{
+	int order, flg;
+	Chan *new;
+	Mhead *m, **l, *mh;
+	Mount *nm, *f, *um, **h;
+	Pgrp *pg;
+
+	if(QTDIR & (old->qid.type^(*newp)->qid.type))
+		error(Emount);
+
+	if(old->umh)
+		print("cmount: unexpected umh, caller %#p\n", getcallerpc(&newp));
+
+	order = flag&MORDER;
+
+	if((old->qid.type&QTDIR)==0 && order != MREPL)
+		error(Emount);
+
+	new = *newp;
+	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 allows 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.
+	 */
+	if((flag&MCREATE) && mh && mh->mount
+	&& (mh->mount->next || !(mh->mount->mflag&MCREATE)))
+		error(Emount);
+
+	pg = up->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->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->path = c->path;
+	if(c->path)
+		incref(c->path);
+	return nc;
+}
+
+/* also used by sysfile.c:/^mountfix */
+int
+findmount(Chan **cp, Mhead **mp, int type, int dev, Qid qid)
+{
+	Pgrp *pg;
+	Mhead *m;
+
+	pg = up->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;
+}
+
+/*
+ * Calls findmount but also updates path.
+ */
+static int
+domount(Chan **cp, Mhead **mp, Path **path)
+{
+	Chan **lc;
+	Path *p;
+
+	if(findmount(cp, mp, (*cp)->type, (*cp)->dev, (*cp)->qid) == 0)
+		return 0;
+
+	if(path){
+		p = *path;
+		p = uniquepath(p);
+		if(p->mlen <= 0)
+			print("domount: path %s has mlen==%d\n", p->s, p->mlen);
+		else{
+			lc = &p->mtpt[p->mlen-1];
+DBG("domount %p %s => add %p (was %p)\n", p, p->s, (*mp)->from, p->mtpt[p->mlen-1]);
+			incref((*mp)->from);
+			if(*lc)
+				cclose(*lc);
+			*lc = (*mp)->from;
+		}
+		*path = p;
+	}
+	return 1;
+}
+
+/*
+ * If c is the right-hand-side of a mount point, returns the left hand side.
+ * Changes name to reflect the fact that we've uncrossed the mountpoint,
+ * so name had better be ours to change!
+ */
+static Chan*
+undomount(Chan *c, Path *path)
+{
+	Chan *nc;
+
+	if(path->ref != 1 || path->mlen == 0)
+		print("undomount: path %s ref %ld mlen %d caller %#p\n",
+			path->s, path->ref, path->mlen, getcallerpc(&c));
+
+	if(path->mlen>0 && (nc=path->mtpt[path->mlen-1]) != nil){
+DBG("undomount %p %s => remove %p\n", path, path->s, nc);
+		cclose(c);
+		path->mtpt[path->mlen-1] = nil;
+		c = nc;
+	}
+	return c;
+}
+
+/*
+ * Call dev walk but catch errors.
+ */
+static Walkqid*
+ewalk(Chan *c, Chan *nc, char **name, int nname)
+{
+	Walkqid *wq;
+
+	if(waserror())
+		return nil;
+	wq = devtab[c->type]->walk(c, nc, name, nname);
+	poperror();
+	return wq;
+}
+
+/*
+ * 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, didmount, dotdot, i, n, nhave, ntry, type;
+	Chan *c, *nc, *mtpt;
+	Path *path;
+	Mhead *mh, *nmh;
+	Mount *f;
+	Walkqid *wq;
+
+	c = *cp;
+	incref(c);
+	path = c->path;
+	incref(path);
+	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.
+	 *
+	 * Each time through the loop:
+	 *
+	 *	If didmount==0, c is on the undomount side of the mount point.
+	 *	If didmount==1, c is on the domount side of the mount point.
+	 * 	Either way, c's full path is path.
+	 */
+	didmount = 0;
+	for(nhave=0; nhave<nnames; nhave+=n){
+		if((c->qid.type&QTDIR)==0){
+			if(nerror)
+				*nerror = nhave;
+			pathclose(path);
+			cclose(c);
+			strcpy(up->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 && !didmount)
+			domount(&c, &mh, &path);
+		
+		type = c->type;
+		dev = c->dev;
+
+		if((wq = ewalk(c, nil, names+nhave, ntry)) == nil){
+			/* try a union mount, if any */
+			if(mh && !nomount){
+				/*
+				 * mh->mount->to == c, so start at mh->mount->next
+				 */
+				rlock(&mh->lock);
+				f = mh->mount;
+				for(f = (f? f->next: f); f; f = f->next)
+					if((wq = ewalk(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);
+				pathclose(path);
+				if(nerror)
+					*nerror = nhave+1;
+				if(mh != nil)
+					putmhead(mh);
+				return -1;
+			}
+		}
+
+		didmount = 0;
+		if(dotdot){
+			assert(wq->nqid == 1);
+			assert(wq->clone != nil);
+
+			path = addelem(path, "..", nil);
+			nc = undomount(wq->clone, path);
+			nmh = nil;
+			n = 1;
+		}else{
+			nc = nil;
+			nmh = nil;
+			if(!nomount){
+				for(i=0; i<wq->nqid && i<ntry-1; i++){
+					if(findmount(&nc, &nmh, type, dev, wq->qid[i])){
+						didmount = 1;
+						break;
+					}
+				}
+			}
+			if(nc == nil){	/* no mount points along path */
+				if(wq->clone == nil){
+					cclose(c);
+					pathclose(path);
+					if(wq->nqid==0 || (wq->qid[wq->nqid-1].type&QTDIR)){
+						if(nerror)
+							*nerror = nhave+wq->nqid+1;
+						strcpy(up->errstr, Edoesnotexist);
+					}else{
+						if(nerror)
+							*nerror = nhave+wq->nqid;
+						strcpy(up->errstr, Enotdir);
+					}
+					free(wq);
+					if(mh != nil)
+						putmhead(mh);
+					return -1;
+				}
+				n = wq->nqid;
+				nc = wq->clone;
+			}else{		/* stopped early, at a mount point */
+				didmount = 1;
+				if(wq->clone != nil){
+					cclose(wq->clone);
+					wq->clone = nil;
+				}
+				n = i+1;
+			}
+			for(i=0; i<n; i++){
+				mtpt = nil;
+				if(i==n-1 && nmh)
+					mtpt = nmh->from;
+				path = addelem(path, names[nhave+i], mtpt);
+			}
+		}
+		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;
+	}
+
+	pathclose(c->path);
+	c->path = path;
+
+	cclose(*cp);
+	*cp = c;
+	if(nerror)
+		*nerror = nhave;
+	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)
+{
+}
+
+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+1)*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 *aname, Elemlist *e)
+{
+	char *name, *slash;
+
+	kstrdup(&e->name, aname);
+	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->off[e->nelems] = name+strlen(name) - e->name;
+			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;
+	}
+	
+	if(0 && chandebug){
+		int i;
+		
+		print("parsename %s:", e->name);
+		for(i=0; i<=e->nelems; i++)
+			print(" %d", e->off[i]);
+		print("\n");
+	}
+}
+
+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;
+}
+
+void
+namelenerror(char *aname, int len, char *err)
+{
+	char *ename, *name, *next;
+	int i, errlen;
+
+	/*
+	 * If the name is short enough, just use the whole thing.
+	 */
+	errlen = strlen(err);
+	if(len < ERRMAX/3 || len+errlen < 2*ERRMAX/3)
+		snprint(up->genbuf, sizeof up->genbuf, "%.*s", 
+			utfnlen(aname, len), aname);
+	else{
+		/*
+		 * Print a suffix of the name, but try to get a little info.
+		 */
+		ename = aname+len;
+		next = ename;
+		do{
+			name = next;
+			next = memrchr(aname, '/', name-aname);
+			if(next == nil)
+				next = aname;
+			len = ename-next;
+		}while(len < ERRMAX/3 || len + errlen < 2*ERRMAX/3);
+
+		/*
+		 * If the name is ridiculously long, chop it.
+		 */
+		if(name == ename){
+			name = ename-ERRMAX/4;
+			if(name <= aname)
+				panic("bad math in namelenerror");
+			/* walk out of current UTF sequence */
+			for(i=0; (*name&0xC0)==0x80 && i<UTFmax; i++)
+				name++;
+		}
+		snprint(up->genbuf, sizeof up->genbuf, "...%.*s",
+			utfnlen(name, ename-name), name);
+	}				
+	snprint(up->errstr, ERRMAX, "%#q %s", up->genbuf, err);
+	nexterror();
+}
+
+void
+nameerror(char *name, char *err)
+{
+	namelenerror(name, strlen(name), err);
+}
+
+/*
+ * 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, Aremove, or Aaccess 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 or Amount does not guarantee this.
+ *
+ * Under certain circumstances, opening Aaccess will cause
+ * an unnecessary clone in order to get a cunique Chan so it
+ * can attach the correct name.  Sysstat and sys_stat need the
+ * correct name so they can rewrite the stat info.
+ */
+Chan*
+namec(char *aname, int amode, int omode, ulong perm)
+{
+	int len, n, t, nomount;
+	Chan *c, *cnew;
+	Path *path;
+	Elemlist e;
+	Rune r;
+	Mhead *m;
+	char *createerr, tmperrbuf[ERRMAX];
+	char *name;
+
+	if(aname[0] == '\0')
+		error("empty file name");
+	aname = validnamedup(aname, 1);
+	if(waserror()){
+		free(aname);
+		nexterror();
+	}
+	DBG("namec %s %d %d\n", aname, amode, omode);
+	name = aname;
+
+	/*
+	 * 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->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';
+		/*
+		 *  noattach is sandboxing.
+		 *
+		 *  the OK exceptions are:
+		 *	|  it only gives access to pipes you create
+		 *	d  this process's file descriptors
+		 *	e  this process's environment
+		 *  the iffy exceptions are:
+		 *	c  time and pid, but also cons and consctl
+		 *	p  control of your own processes (and unfortunately
+		 *	   any others left unprotected)
+		 */
+		n = chartorune(&r, up->genbuf+1)+1;
+		/* actually / is caught by parsing earlier */
+		if(utfrune("M", r))
+			error(Enoattach);
+		if(up->pgrp->noattach && utfrune("|decp", r)==nil)
+			error(Enoattach);
+		t = devno(r, 1);
+		if(t == -1)
+			error(Ebadsharp);
+		c = devtab[t]->attach(up->genbuf+n);
+		break;
+
+	default:
+		c = up->dot;
+		incref(c);
+		break;
+	}
+
+	e.aname = aname;
+	e.prefix = name - aname;
+	e.name = nil;
+	e.elems = nil;
+	e.off = nil;
+	e.nelems = 0;
+	e.nerror = 0;
+	if(waserror()){
+		cclose(c);
+		free(e.name);
+		free(e.elems);
+		/*
+		 * Prepare nice error, showing first e.nerror elements of name.
+		 */
+		if(e.nerror == 0)
+			nexterror();
+		strcpy(tmperrbuf, up->errstr);
+		if(e.off[e.nerror]==0)
+			print("nerror=%d but off=%d\n",
+				e.nerror, e.off[e.nerror]);
+		if(0 && chandebug)
+			print("showing %d+%d/%d (of %d) of %s (%d %d)\n", e.prefix, e.off[e.nerror], e.nerror, e.nelems, aname, e.off[0], e.off[1]);
+		len = e.prefix+e.off[e.nerror];
+		free(e.off);
+		namelenerror(aname, len, tmperrbuf);
+	}
+
+	/*
+	 * Build a list of elements in the name.
+	 */
+	parsename(name, &e);
+
+	/*
+	 * On create, ....
+	 */
+	if(amode == Acreate){
+		/* perm must have DMDIR if last element is / or /. */
+		if(e.mustbedir && !(perm&DMDIR)){
+			e.nerror = e.nelems;
+			error("create without DMDIR");
+		}
+
+		/* 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, &e.nerror) < 0){
+		if(e.nerror < 0 || e.nerror > e.nelems){
+			print("namec %s walk error nerror=%d\n", aname, e.nerror);
+			e.nerror = 0;
+		}
+		nexterror();
+	}
+
+	if(e.mustbedir && !(c->qid.type&QTDIR))
+		error("not a directory");
+
+	if(amode == Aopen && (omode&3) == OEXEC && (c->qid.type&QTDIR))
+		error("cannot exec directory");
+
+	switch(amode){
+	case Abind:
+		/* no need to maintain path - cannot dotdot an Abind */
+		m = nil;
+		if(!nomount)
+			domount(&c, &m, nil);
+		if(c->umh != nil)
+			putmhead(c->umh);
+		c->umh = m;
+		break;
+
+	case Aaccess:
+	case Aremove:
+	case Aopen:
+	Open:
+		/* save&update the name; domount might change c */
+		path = c->path;
+		incref(path);
+		m = nil;
+		if(!nomount)
+			domount(&c, &m, &path);
+
+		/* our own copy to open or remove */
+		c = cunique(c);
+
+		/* now it's our copy anyway, we can put the name back */
+		pathclose(c->path);
+		c->path = path;
+
+		/* record whether c is on a mount point */
+		c->ismtpt = m!=nil;
+
+		switch(amode){
+		case Aaccess:
+		case Aremove:
+			putmhead(m);
+			break;
+
+		case Aopen:
+		case Acreate:
+if(c->umh != nil){
+	print("cunique umh Open\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++;
+		e.nerror++;
+		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);
+			pathclose(cnew->path);
+			cnew->path = c->path;
+			incref(cnew->path);
+
+			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->path = addelem(c->path, e.elems[e.nelems-1], nil);
+			break;
+		}
+
+		/* create failed */
+		cclose(cnew);
+		if(m)
+			putmhead(m);
+		if(omode & OEXCL)
+			nexterror();
+		/* save error */
+		createerr = up->errstr;
+		up->errstr = tmperrbuf;
+		/* note: we depend that walk does not error */
+		if(walk(&c, e.elems+e.nelems-1, 1, nomount, nil) < 0){
+			up->errstr = createerr;
+			error(createerr);	/* report true error */
+		}
+		up->errstr = createerr;
+		omode |= OTRUNC;
+		goto Open;
+
+	default:
+		panic("unknown namec access %d\n", amode);
+	}
+
+	/* 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);
+	poperror();	/* e c */
+	free(aname);
+	poperror();	/* aname */
+
+	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.
+ *
+ * The parameter dup flags whether the string should be copied
+ * out of user space before being scanned the second time.
+ * (Otherwise a malicious thread could remove the NUL, causing us
+ * to access unchecked addresses.) 
+ */
+static char*
+validname0(char *aname, int slashok, int dup, ulong pc)
+{
+	char *ename, *name, *s;
+	int c, n;
+	Rune r;
+
+	name = aname;
+	if((ulong)name < KZERO){
+		if(!dup)
+			print("warning: validname called from %#p with user pointer", pc);
+		ename = vmemchr(name, 0, (1<<16));
+	}else
+		ename = memchr(name, 0, (1<<16));
+
+	if(ename==nil || ename-name>=(1<<16))
+		error("name too long");
+
+	s = nil;
+	if(dup){
+		n = ename-name;
+		s = smalloc(n+1);
+		memmove(s, name, n);
+		s[n] = 0;
+		aname = s;
+		name = s;
+		setmalloctag(s, pc);
+	}
+	
+	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);
+					free(s);
+					error(up->genbuf);
+			}
+			name++;
+		}
+	}
+	return s;
+}
+
+void
+validname(char *aname, int slashok)
+{
+	validname0(aname, slashok, 0, getcallerpc(&aname));
+}
+
+char*
+validnamedup(char *aname, int slashok)
+{
+	return validname0(aname, slashok, 1, getcallerpc(&aname));
+}
+
+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);
+	}
+}
+