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kernel: make the mntcache robust against fileserver like fossil that do not change the qid.vers on wstat

introducing new ctrunc() function that invalidates any caches
for the passed in chan, invoked when handling wstat with a
specified file length or on file creation/truncation.

test program to reproduce the problem:

#include <u.h>
#include <libc.h>
#include <libsec.h>

void
main(int argc, char *argv[])
{
	int fd;
	Dir *d, nd;

	fd = create("xxx", ORDWR, 0666);
	write(fd, "1234", 4);
	d = dirstat("xxx");
	assert(d->length == 4);
	nulldir(&nd);
	nd.length = 0;
	dirwstat("xxx", &nd);
	d = dirstat("xxx");
	assert(d->length == 0);
	fd = open("xxx", OREAD);
	assert(read(fd, (void*)&d, 4) == 0);
}

--- a/sys/src/9/port/cache.c

+++ b/sys/src/9/port/cache.c

@@ -187,7 +187,7 @@

 	return nil;

 }

-void

+int

 copen(Chan *c)

 {

 	Mntcache *m, *f, **l;

@@ -195,19 +195,20 @@

 	/* directories aren't cacheable */

 	if(c->qid.type&QTDIR){

 		c->mcp = nil;

-		return;

+		return 0;

 	}

 	lock(&cache);

-	m = clookup(c, 1);

-	if(m == nil)

-		m = cache.head;

-	else if(m->qid.vers == c->qid.vers) {

+	m = clookup(c, 0);

+	if(m != nil){

 		ctail(m);

 		unlock(&cache);

 		c->mcp = m;

-		return;

+		return 1;

 	}

+	m = clookup(c, 1);

+	if(m == nil)

+		m = cache.head;

 	ctail(m);

 	l = &cache.hash[m->qid.path%NHASH];

@@ -234,7 +235,7 @@

 			unlock(&cache);

 			cacheunlock(m);

 			c->mcp = f;

-			return;

+			return 1;

 		}

 	}

@@ -251,10 +252,9 @@

 	m->rah.vers = m->qid.vers;

 	mntrahinit(&m->rah);

 	cnodata(m);

-

 	cacheunlock(m);

-

 	c->mcp = m;

+	return 0;

 }

 enum {

@@ -480,6 +480,31 @@

 		return;

 	}

 	cachedata(m, buf, len, off);

+}

+

+void

+ctrunc(Chan *c)

+{

+	Mntcache *m;

+

+	if(c->qid.type&QTDIR)

+		return;

+

+	if((c->flag&COPEN) == 0){

+		lock(&cache);

+		c->mcp = clookup(c, 0);

+		unlock(&cache);

+	}

+

+	m = ccache(c);

+	if(m == nil)

+		return;

+	mntrahinit(&m->rah);

+	cnodata(m);

+	cacheunlock(m);

+

+	if((c->flag&COPEN) == 0)

+		c->mcp = nil;

 }

 void

--- a/sys/src/9/port/devmnt.c

+++ b/sys/src/9/port/devmnt.c

@@ -521,8 +521,11 @@

 	poperror();

 	mntfree(r);

-	if(c->flag & CCACHE)

-		copen(c);

+	if(c->flag & CCACHE){

+		if(copen(c))

+		if(type == Tcreate || (omode&OTRUNC) != 0)

+			ctrunc(c);

+	}

 	return c;

 }

@@ -620,6 +623,11 @@

 	mountrpc(m, r);

 	poperror();

 	mntfree(r);

+

+	if(c->flag & CCACHE)

+	if(GBIT64(&dp[STATFIXLEN-4*BIT16SZ-BIT64SZ]) != ~0ULL)

+		ctrunc(c);

+

 	return n;

 }

--- a/sys/src/9/port/portfns.h

+++ b/sys/src/9/port/portfns.h

@@ -41,7 +41,7 @@

 int		consactive(void);

 void		(*consdebug)(void);

 void		cpushutdown(void);

-void		copen(Chan*);

+int		copen(Chan*);

 void		cclunk(Chan*);

 Block*		concatblock(Block*);

 Block*		copyblock(Block*, int);

@@ -48,6 +48,7 @@

 void		copypage(Page*, Page*);

 void		countpagerefs(ulong*, int);

 int		cread(Chan*, uchar*, int, vlong);

+void		ctrunc(Chan*);

 void		cunmount(Chan*, Chan*);

 void		cupdate(Chan*, uchar*, int, vlong);

 void		cwrite(Chan*, uchar*, int, vlong);