branch.c: use ref transaction for all ref updates

Change create_branch to use a ref transaction when creating the new branch.

This also fixes a race condition in the old code where two concurrent
create_branch could race since the lock_any_ref_for_update/write_ref_sha1
did not protect against the ref already existing. I.e. one thread could end up
overwriting a branch even if the forcing flag is false.

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>

branch.c

@@ -226,7 +226,6 @@ void create_branch(const char *head,
 		   int force, int reflog, int clobber_head,
 		   int quiet, enum branch_track track)
 {
-	struct ref_lock *lock = NULL;
 	struct commit *commit;
 	unsigned char sha1[20];
 	char *real_ref, msg[PATH_MAX + 20];
@@ -285,29 +284,33 @@ void create_branch(const char *head,
 		die(_("Not a valid branch point: '%s'."), start_name);
 	hashcpy(sha1, commit->object.sha1);
 
-	if (!dont_change_ref) {
-		lock = lock_any_ref_for_update(ref.buf, NULL, 0, NULL);
-		if (!lock)
-			die_errno(_("Failed to lock ref for update"));
-	}
-
-	if (reflog)
-		log_all_ref_updates = 1;
-
 	if (forcing)
 		snprintf(msg, sizeof msg, "branch: Reset to %s",
 			 start_name);
 	else if (!dont_change_ref)
 		snprintf(msg, sizeof msg, "branch: Created from %s",
 			 start_name);
 
+	if (reflog)
+		log_all_ref_updates = 1;
+
+	if (!dont_change_ref) {
+		struct ref_transaction *transaction;
+		struct strbuf err = STRBUF_INIT;
+
+		transaction = ref_transaction_begin(&err);
+		if (!transaction ||
+		    ref_transaction_update(transaction, ref.buf, sha1,
+					   null_sha1, 0, !forcing, &err) ||
+		    ref_transaction_commit(transaction, msg, &err))
+			die("%s", err.buf);
+		ref_transaction_free(transaction);
+		strbuf_release(&err);
+	}
+
 	if (real_ref && track)
 		setup_tracking(ref.buf + 11, real_ref, track, quiet);
 
-	if (!dont_change_ref)
-		if (write_ref_sha1(lock, sha1, msg) < 0)
-			die_errno(_("Failed to write ref"));
-
 	strbuf_release(&ref);
 	free(real_ref);
 }