Avoid infinite loops in caching layer

Add a global variable, cgit_max_lock_attemps, to avoid the possibility of
infinite loops when failing to acquire a lockfile. This could happen on
broken setups or under crazy server load.

Incidentally, this also fixes a lurking bug in cache_lock() where an
uninitialized returnvalue was used.

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

1 files changed, 20 insertions, 11 deletions

diff --git a/cache.c b/cache.c
index 0ffa502..2e1ef8c 100644
--- a/cache.c
+++ b/cache.c
@@ -40,59 +40,68 @@ int cache_exist(struct cacheitem *item)
 	}
 	return 1;
 }
 
 int cache_create_dirs()
 {
 	char *path;
 
 	if (!cgit_query_repo)
 		return 0;
 
 	path = fmt("%s/%s", cgit_cache_root, cgit_query_repo);
 	if (mkdir(path, S_IRWXU) && errno!=EEXIST)
 		return 0;
 
 	if (cgit_query_page) {
 		path = fmt("%s/%s/%s", cgit_cache_root, cgit_query_repo, 
 			   cgit_query_page);
 		if (mkdir(path, S_IRWXU) && errno!=EEXIST)
 			return 0;
 	}
 	return 1;
 }
 
+int cache_refill_overdue(const char *lockfile)
+{
+	struct stat st;
+
+	if (stat(lockfile, &st))
+		return 0;
+	else
+		return (time(NULL) - st.st_mtime > cgit_cache_max_create_time);
+}
+
 int cache_lock(struct cacheitem *item)
 {
-	int ret;
+	int i = 0;
 	char *lockfile = fmt("%s.lock", item->name);
 
  top:  
+	if (++i > cgit_max_lock_attempts)
+		die("cache_lock: unable to lock %s: %s",
+		    item->name, strerror(errno));
+
        	item->fd = open(lockfile, O_WRONLY | O_CREAT | O_EXCL, S_IRUSR|S_IWUSR);
+
 	if (item->fd == NOLOCK && errno == ENOENT && cache_create_dirs())
 		goto top;
-	if (item->fd == NOLOCK && errno == EEXIST) {
-		struct stat st;
-		time_t t;
-		if (stat(lockfile, &st))
-			return ret;
-		t = time(NULL);
-		if (t-st.st_mtime > cgit_cache_max_create_time && 
-		    !unlink(lockfile))
+
+	if (item->fd == NOLOCK && errno == EEXIST &&
+	    cache_refill_overdue(lockfile) && !unlink(lockfile))
 			goto top;
-		return 0;
-	}
+
 	return (item->fd > 0);
 }
 
 int cache_unlock(struct cacheitem *item)
 {
 	close(item->fd);
 	return (rename(fmt("%s.lock", item->name), item->name) == 0);
 }
 
 int cache_expired(struct cacheitem *item)
 {
 	if (item->ttl < 0)
 		return 0;
 	return item->st.st_mtime + item->ttl * 60 < time(NULL);
 }