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, 14 insertions, 2 deletions

diff --git a/cache.c b/cache.c
index 2e1ef8c..b947a34 100644
--- a/cache.c
+++ b/cache.c
@@ -100,4 +100,9 @@ int cache_unlock(struct cacheitem *item)
 }
 
+int cache_cancel_lock(struct cacheitem *item)
+{
+	return (unlink(fmt("%s.lock", item->name)) == 0);
+}
+
 int cache_expired(struct cacheitem *item)
 {
diff --git a/cgit.c b/cgit.c
index d1abaa0..0f72f2d 100644
--- a/cgit.c
+++ b/cgit.c
@@ -62,11 +62,17 @@ static void cgit_check_cache(struct cacheitem *item)
 			goto top;
 		}
-		if (!cache_exist(item))
+		if (!cache_exist(item)) {
 			cgit_fill_cache(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);
+		} else {
+			cache_cancel_lock(item);
+		}
 	}
 }
diff --git a/cgit.h b/cgit.h
index c1dcc97..c9554a7 100644
--- a/cgit.h
+++ b/cgit.h
@@ -67,4 +67,5 @@ 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);