receive-pack.c: use a reference transaction for updating the refs

Wrap all the ref updates inside a transaction.

In the new API there is no distinction between failure to lock and
failure to write a ref.  Both can be permanent (e.g., a ref
"refs/heads/topic" is blocking creation of the lock file
"refs/heads/topic/1.lock") or transient (e.g., file system full) and
there's no clear difference in how the client should respond, so
replace the two statuses "failed to lock" and "failed to write" with
a single status "failed to update ref".  In both cases a more
detailed message is sent by sideband to diagnose the problem.

Example, before:

 error: there are still refs under 'refs/heads/topic'
 remote: error: failed to lock refs/heads/topic
 To foo
  ! [remote rejected] HEAD -> topic (failed to lock)

After:

 error: there are still refs under 'refs/heads/topic'
 remote: error: Cannot lock the ref 'refs/heads/topic'.
 To foo
  ! [remote rejected] HEAD -> topic (failed to update ref)

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -475,7 +475,6 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 	const char *namespaced_name;
 	unsigned char *old_sha1 = cmd->old_sha1;
 	unsigned char *new_sha1 = cmd->new_sha1;
-	struct ref_lock *lock;
 
 	/* only refs/... are allowed */
 	if (!starts_with(name, "refs/") || check_refname_format(name + 5, 0)) {
@@ -576,19 +575,27 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 		return NULL; /* good */
 	}
 	else {
+		struct strbuf err = STRBUF_INIT;
+		struct ref_transaction *transaction;
+
 		if (shallow_update && si->shallow_ref[cmd->index] &&
 		    update_shallow_ref(cmd, si))
 			return "shallow error";
 
-		lock = lock_any_ref_for_update(namespaced_name, old_sha1,
-					       0, NULL);
-		if (!lock) {
-			rp_error("failed to lock %s", name);
-			return "failed to lock";
-		}
-		if (write_ref_sha1(lock, new_sha1, "push")) {
-			return "failed to write"; /* error() already called */
+		transaction = ref_transaction_begin(&err);
+		if (!transaction ||
+		    ref_transaction_update(transaction, namespaced_name,
+					   new_sha1, old_sha1, 0, 1, &err) ||
+		    ref_transaction_commit(transaction, "push", &err)) {
+			ref_transaction_free(transaction);
+
+			rp_error("%s", err.buf);
+			strbuf_release(&err);
+			return "failed to update ref";
 		}
+
+		ref_transaction_free(transaction);
+		strbuf_release(&err);
 		return NULL; /* good */
 	}
 }