packed_ref_cache: increment refcount when locked

Increment the packed_ref_cache reference count while it is locked to
prevent its being freed.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -820,7 +820,9 @@ struct packed_ref_cache {
 	/*
 	 * Iff the packed-refs file associated with this instance is
 	 * currently locked for writing, this points at the associated
-	 * lock (which is owned by somebody else).
+	 * lock (which is owned by somebody else).  The referrer count
+	 * is also incremented when the file is locked and decremented
+	 * when it is unlocked.
 	 */
 	struct lock_file *lock;
 };
@@ -2099,6 +2101,8 @@ int lock_packed_refs(int flags)
 	/* Read the current packed-refs while holding the lock: */
 	packed_ref_cache = get_packed_ref_cache(&ref_cache);
 	packed_ref_cache->lock = &packlock;
+	/* Increment the reference count to prevent it from being freed: */
+	acquire_packed_ref_cache(packed_ref_cache);
 	return 0;
 }
 
@@ -2119,6 +2123,7 @@ int commit_packed_refs(void)
 	if (commit_lock_file(packed_ref_cache->lock))
 		error = -1;
 	packed_ref_cache->lock = NULL;
+	release_packed_ref_cache(packed_ref_cache);
 	return error;
 }
 
@@ -2131,6 +2136,7 @@ void rollback_packed_refs(void)
 		die("internal error: packed-refs not locked");
 	rollback_lock_file(packed_ref_cache->lock);
 	packed_ref_cache->lock = NULL;
+	release_packed_ref_cache(packed_ref_cache);
 	clear_packed_ref_cache(&ref_cache);
 }