chan.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

#include	"u.h"
#include	"lib.h"
#include	"dat.h"
#include	"fns.h"
#include	"error.h"

int chandebug=0;		/* toggled by sysr1 */
QLock chanprint;		/* probably asking for trouble (deadlocks) -rsc */

int domount(Chan**, Mhead**);

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 0x%llux.%lud %C %lud -> \n", f,
				f->from->name->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->name->s, t->to->umh, t->to->qid.path, devtab[t->to->type]->dc, t->to->dev);
		}
	}
	poperror();
	runlock(&pg->ns);
}


char*
c2name(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 lk;
	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->lk);
	x = ++r->ref;
	unlock(&r->lk);
	return x;
}

int
decref(Ref *r)
{
	int x;

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

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

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

	/* if you get an error before associating with a dev,
	   close calls rootclose, a nop */
	c->type = 0;
	c->flag = 0;
	c->ref.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;
	memset(&c->mqid, 0, sizeof(c->mqid));
	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.ref = 1;
	incref(&ncname);
	return n;
}

void
cnameclose(Cname *n)
{
	if(n == nil)
		return;
	if(decref(&n->ref))
		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.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.lk);
	c->next = chanalloc.free;
	chanalloc.free = c;
	unlock(&chanalloc.lk);
}

void
cclose(Chan *c)
{
	if(c->flag&CFREE)
		panic("cclose %p", getcallerpc(&c));

	if(decref(&c->ref))
		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.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.ref = 1;
	mh->from = from;
	incref(&from->ref);

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

	if(QTDIR & (old->qid.type^(*newp)->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);

	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 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->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->name = c->name;
	if(c->name)
		incref(&c->name->ref);
	return nc;
}

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->ref);
				if(*mp != nil)
					putmhead(*mp);
				*mp = m;
			}
			if(*cp != nil)
				cclose(*cp);
			incref(&m->mount->to->ref);
			*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->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->ref);
					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;