[PATCH v2 01/11] packed-backend: don't adjust the reference count on lock/unlock

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

The old code incremented the packed ref cache reference count when
acquiring the packed-refs lock, and decremented the count when
releasing the lock. This is unnecessary because:

* Another process cannot change the packed-refs file because it is
  locked.

* When we ourselves change the packed-refs file, we do so by first
  modifying the packed ref-cache, and then writing the data from the
  ref-cache to disk. So the packed ref-cache remains fresh because any
  changes that we plan to make to the file are made in the cache first
  anyway.

So there is no reason for the cache to become stale.

Moreover, the extra reference count causes a problem if we
intentionally clear the packed refs cache, as we sometimes need to do
if we change the cache in anticipation of writing a change to disk,
but then the write to disk fails. In that case, `packed_refs_unlock()`
would have no easy way to find the cache whose reference count it
needs to decrement.

This whole issue will soon become moot due to upcoming changes that
avoid changing the in-memory cache as part of updating the packed-refs
on disk, but this change makes that transition easier.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 refs/packed-backend.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 412c85034f..b76f14e5b3 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -525,7 +525,6 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 				"packed_refs_lock");
 	static int timeout_configured = 0;
 	static int timeout_value = 1000;
-	struct packed_ref_cache *packed_ref_cache;
 
 	if (!timeout_configured) {
 		git_config_get_int("core.packedrefstimeout", &timeout_value);
@@ -560,9 +559,11 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 	 */
 	validate_packed_ref_cache(refs);
 
-	packed_ref_cache = get_packed_ref_cache(refs);
-	/* Increment the reference count to prevent it from being freed: */
-	acquire_packed_ref_cache(packed_ref_cache);
+	/*
+	 * Now make sure that the packed-refs file as it exists in the
+	 * locked state is loaded into the cache:
+	 */
+	get_packed_ref_cache(refs);
 	return 0;
 }
 
@@ -576,7 +577,6 @@ void packed_refs_unlock(struct ref_store *ref_store)
 	if (!is_lock_file_locked(&refs->lock))
 		die("BUG: packed_refs_unlock() called when not locked");
 	rollback_lock_file(&refs->lock);
-	release_packed_ref_cache(refs->cache);
 }
 
 int packed_refs_is_locked(struct ref_store *ref_store)
-- 
2.14.1