Wikipost expects it can mount a wikifs from /srv.  But consider the
case where we are using https plus passwords to export a "private"
wiki via httpd, and to modify pages with wikipost.  Then it is not
so desirable for a publicly readable /srv entry to show the private
bits to everybody who can log in to the web server.

This patch leverages private namespaces to fix the problem.  Before
trying to mount /srv/wiki.%s onto /mnt/wiki, wikipost will first see
if a /mnt/wiki.%s exists in its (httpd's) namespace.  If so, it will
bind that /mnt/wiki.%s onto /mnt/wiki and use it.  In other words,
the wikifs can be "stored" in httpd's namespace instead of being
"stored" in /srv.

The code falls back to the pre-existing behavior if there is no
/mnt/wiki.%s.

Reference: /n/sources/patch/applied/wikipost-trybind
Date: Thu Jun  5 00:36:28 CES 2008
Signed-off-by: davide+p9@cs.cmu.edu

--- /sys/src/cmd/ip/httpd/wikipost.c	Thu Jun  5 00:28:42 2008
+++ /sys/src/cmd/ip/httpd/wikipost.c	Thu Jun  5 20:59:47 2008
@@ -120,6 +120,18 @@
 	char buf[64];
 	int fd;
 
+	/* already in (possibly private) namespace? */
+	snprint(buf, sizeof buf, "/mnt/wiki.%s/new", service);
+	if (access(buf, AREAD) == 0){
+		if (bind(buf, "/mnt/wiki", MREPL) < 0){
+			syslog(0, LOG, "%s bind /mnt/wiki failed: %r", hp->remotesys);
+			hfail(c, HNotFound);
+			exits("bind /mnt/wiki failed");
+		}
+		return;
+	}
+
+	/* old way: public wikifs from /srv */
 	snprint(buf, sizeof buf, "/srv/wiki.%s", service);
 	if((fd = open(buf, ORDWR)) < 0){
 		syslog(0, LOG, "%s open %s failed: %r", buf, hp->remotesys);