[PATCH 3/4] refs.c: change update_refs to run the commit loops once all work is finished

[Ronnie Sahlberg <sahlberg@google.com>]

During update_refs we will both be deleting refs as well as updating/changing
the refs.
Since both delete and update now use the same lock/update/commit pattern

    lock = lock_ref_sha1_basic() (or varient of)
    write_ref_sha1(lock)/delete_ref_loose(lock)
    unlock_ref(lock) | commit_ref_lock(lock)

we can now simplify update_update refs to have one loop that locks all
involved refs.
A second loop that writes or flags for deletion, but does not commit, all
the refs.
And a final third loop that commits all the refs once all the work and
preparations are complete.

This makes updating/deleting multiple refs 'more atomic' since we will not start
the commit phase until all the preparations have completed successfully.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
---
 refs.c | 38 +++++++++++++++++++++-----------------
 1 file changed, 21 insertions(+), 17 deletions(-)

diff --git a/refs.c b/refs.c
index 038614d..1678e12 100644
--- a/refs.c
+++ b/refs.c
@@ -3368,34 +3368,38 @@ int update_refs(const char *action, const struct ref_update **updates_orig,
 		}
 	}
 
-	/* Perform updates first so live commits remain referenced */
-	for (i = 0; i < n; i++)
-		if (!is_null_sha1(updates[i]->new_sha1)) {
+	/* Go through and prepare all updates and deletes */
+	for (i = 0; i < n; i++) {
+		if (!is_null_sha1(updates[i]->new_sha1))
 			ret = update_ref_write(action,
 					       updates[i]->ref_name,
 					       updates[i]->new_sha1,
 					       locks[i], onerr);
-			if (ret)
-				unlock_ref(locks[i]);
-			else
-				commit_ref_lock(locks[i]);
-			locks[i] = NULL;
-			if (ret)
-				goto cleanup;
+		else {
+			delnames[delnum++] = locks[i]->ref_name;
+			ret = delete_ref_loose(locks[i], types[i]);
 		}
+		if (ret)
+			goto cleanup;
+	}
 
-	/* Perform deletes now that updates are safely completed */
+	ret = repack_without_refs(delnames, delnum);
+	for (i = 0; i < delnum; i++)
+		unlink_or_warn(git_path("logs/%s", delnames[i]));
+	clear_loose_ref_cache(&ref_cache);
+
+	/* Perform updates first so live commits remain referenced */
+	for (i = 0; i < n; i++)
+		if (locks[i] && !locks[i]->delete_ref) {
+			ret |= commit_ref_lock(locks[i]);
+			locks[i] = NULL;
+		}
+	/* And finally perform all deletes */
 	for (i = 0; i < n; i++)
 		if (locks[i]) {
-			delnames[delnum++] = locks[i]->ref_name;
-			ret |= delete_ref_loose(locks[i], types[i]);
 			ret |= commit_ref_lock(locks[i]);
 			locks[i] = NULL;
 		}
-	ret |= repack_without_refs(delnames, delnum);
-	for (i = 0; i < delnum; i++)
-		unlink_or_warn(git_path("logs/%s", delnames[i]));
-	clear_loose_ref_cache(&ref_cache);
 
 cleanup:
 	for (i = 0; i < n; i++)
-- 
1.9.1.478.ga5a8238.dirty