Don't truncate valid cachefiles

An embarrassing thinko in cgit_check_cache() would truncate valid cachefiles
in the following situation:
  1) process A notices a missing/expired cachefile
  2) process B gets scheduled, locks, fills and unlocks the cachefile
  3) process A gets scheduled, locks the cachefile, notices that the cachefile
     now exist/is not expired anymore, and continues to overwrite it with an
     empty lockfile.

Thanks to Linus for noticing (again).

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

3 files changed, 16 insertions, 4 deletions

@@ -99,6 +99,11 @@ int cache_unlock(struct cacheitem *item)
         return (rename(fmt("%s.lock", item->name), item->name) == 0);
 }
 
+int cache_cancel_lock(struct cacheitem *item)
+{
+        return (unlink(fmt("%s.lock", item->name)) == 0);
+}
+
 int cache_expired(struct cacheitem *item)
 {
         if (item->ttl < 0)
@@ -61,13 +61,19 @@ static void cgit_check_cache(struct cacheitem *item)
                         sleep(1);
                         goto top;
                 }
-                if (!cache_exist(item))
+                if (!cache_exist(item)) {
                         cgit_fill_cache(item);
-                cache_unlock(item);
+                        cache_unlock(item);
+                } else {
+                        cache_cancel_lock(item);
+                }
         } else if (cache_expired(item) && cache_lock(item)) {
-                if (cache_expired(item))
+                if (cache_expired(item)) {
                         cgit_fill_cache(item);
-                cache_unlock(item);
+                        cache_unlock(item);
+                } else {
+                        cache_cancel_lock(item);
+                }
         }
 }
 
@@ -66,6 +66,7 @@ extern int cgit_parse_query(char *txt, configfn fn);
 extern void cache_prepare(struct cacheitem *item);
 extern int cache_lock(struct cacheitem *item);
 extern int cache_unlock(struct cacheitem *item);
+extern int cache_cancel_lock(struct cacheitem *item);
 extern int cache_exist(struct cacheitem *item);
 extern int cache_expired(struct cacheitem *item);