[PATCH 12/12] refs: do not invalidate the packed-refs cache unnecessarily

[Michael Haggerty <mhagger@alum.mit.edu>]

Now that we keep track of the packed-refs file metadata, we can detect
when the packed-refs file has been modified since we last read it, and
we do so automatically every time that get_packed_ref_cache() is
called.  So there is no need to invalidate the cache automatically
when lock_packed_refs() is called; usually the old copy will still be
valid.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
This patch is optional.  It makes the assumption that the metadata
stored in stat_validity are adequate to reliably detect when the
packed-refs file has changed.  Given that we are about to rewrite the
file, it is perhaps even more crucial not to make a mistake in this
codepath than in others.  So if the stat_validity check is not
considered safe enough, it might be prudent to omit this patch and
continue to reload the packed-refs data here unconditionally.

 refs.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/refs.c b/refs.c
index aa4641b..d9cbfc4 100644
--- a/refs.c
+++ b/refs.c
@@ -2134,12 +2134,15 @@ int lock_packed_refs(struct lock_file *lock, int flags)
 	int fd;
 	struct packed_ref_cache *packed_ref_cache;
 
-	/* Discard the old cache because it might be invalid: */
-	clear_packed_ref_cache(&ref_cache);
 	fd = hold_lock_file_for_update(lock, git_path("packed-refs"), flags);
 	if (fd < 0)
 		return -1;
-	/* Read the current packed-refs while holding the lock: */
+	/*
+	 * Get the current packed-refs while holding the lock.  If the
+	 * packed-refs file has been modified since we last read it,
+	 * this will automatically invalidate the cache and re-read
+	 * the packed-refs file.
+	 */
 	packed_ref_cache = get_packed_ref_cache(&ref_cache);
 	packed_ref_cache->lock = lock;
 	packed_ref_cache->fd = fd;
-- 
1.8.3