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>

cache.c

@@ -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)

cgit.c

@@ -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);
+		}
 	}
 }
 

cgit.h

@@ -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);