Change rootfs pinning mechnism

Chane pinning mechanism: Use $rootfs/lxc.hold instead of $rootfs.hold
(in case $rootfs is a mountpoint itself), but delete the file
immediately after creating it (but keep it open). This will keep the
root filesystem busy but does not leave any unnecessary files lying
around.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>

src/lxc/conf.c

@@ -671,9 +671,10 @@ static int mount_rootfs_block(const char *rootfs, const char *target)
 
 /*
  * pin_rootfs
- * if rootfs is a directory, then open ${rootfs}.hold for writing for the
- * duration of the container run, to prevent the container from marking the
- * underlying fs readonly on shutdown.
+ * if rootfs is a directory, then open ${rootfs}/lxc.hold for writing for
+ * the duration of the container run, to prevent the container from marking
+ * the underlying fs readonly on shutdown. unlink the file immediately so
+ * no name pollution is happens
  * return -1 on error.
  * return -2 if nothing needed to be pinned.
  * return an open fd (>=0) if we pinned it.
@@ -700,11 +701,14 @@ int pin_rootfs(const char *rootfs)
 	if (!S_ISDIR(s.st_mode))
 		return -2;
 
-	ret = snprintf(absrootfspin, MAXPATHLEN, "%s%s", absrootfs, ".hold");
+	ret = snprintf(absrootfspin, MAXPATHLEN, "%s/lxc.hold", absrootfs);
 	if (ret >= MAXPATHLEN)
 		return -1;
 
 	fd = open(absrootfspin, O_CREAT | O_RDWR, S_IWUSR|S_IRUSR);
+	if (fd < 0)
+		return fd;
+	(void)unlink(absrootfspin);
 	return fd;
 }