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, 22 insertions, 13 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:  
-               item->fd = open(lockfile, O_WRONLY | O_CREAT | O_EXCL, S_IRUSR|S_IWUSR);
+ 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);
 }