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upas/fs: write index out to temp file

pridx() can be extremely slow when reading from
imap, because it ensures each message is cached.
Ensuring each message is cached may take a round
trip to the IMAP server.

Instead, we should write the index to a temp file,
and rename it, to reduce the window wher the index
can be broken.

--- a/sys/src/cmd/upas/fs/idx.c

+++ b/sys/src/cmd/upas/fs/idx.c

@@ -240,12 +240,12 @@

 int

 wridxfile(Mailbox *mb)

 {

-	char buf[Pathlen + 4];

+	char *p, buf[Pathlen + 32];

 	int r, fd;

 	Biobuf b;

-	Dir *d;

+	Dir *d, n;

-	snprint(buf, sizeof buf, "%s.idx", mb->path);

+	snprint(buf, sizeof buf, "%s.idx.tmp", mb->path);

 	iprint("wridxfile %s\n", buf);

 	if((fd = exopen(buf)) == -1){

 		rerrstr(buf, sizeof buf);

@@ -258,8 +258,21 @@

 	Binit(&b, fd, OWRITE);

 	r = pridx(&b, mb);

 	Bterm(&b);

+

+	/* remove and rename over the old index */

+	snprint(buf, sizeof buf, "%s.idx", mb->path);

+	remove(buf);

+

+	if((p = strrchr(mb->path, '/')) == nil)

+		sysfatal("index path is directory");

+	snprint(buf, sizeof buf, "%s.idx", p+1);

+	nulldir(&n);

+	n.name = buf;

+	if(dirfwstat(fd, &n) == -1)

+		sysfatal("dirfwstat: %r");

+

 	d = dirfstat(fd);

-	if(d == 0)

+	if(d == nil)

 		sysfatal("dirfstat: %r");

 	mb->qid = d->qid;

 	free(d);