cache_lock: do xstrdup/free on lockfile

Since fmt() uses 8 alternating static buffers, and cache_lock might call
cache_create_dirs() multiple times, which in turn might call fmt() twice,
after four iterations lockfile would be overwritten by a cachedirectory
path.

In worst case, this could cause the cachedirectory to be unlinked and replaced
by a cachefile.

Fix: use xstrdup() on the result from fmt() before assigning to lockfile, and
call free(lockfile) before exit.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cache.c

@@ -74,7 +74,7 @@ int cache_refill_overdue(const char *lockfile)
 int cache_lock(struct cacheitem *item)
 {
 	int i = 0;
-	char *lockfile = fmt("%s.lock", item->name);
+	char *lockfile = xstrdup(fmt("%s.lock", item->name));
 
  top:
 	if (++i > cgit_max_lock_attempts)
@@ -90,6 +90,7 @@ int cache_lock(struct cacheitem *item)
 	    cache_refill_overdue(lockfile) && !unlink(lockfile))
 			goto top;
 
+	free(lockfile);
 	return (item->fd > 0);
 }