walker.c: use ref transaction for ref updates

Switch to using ref transactions in walker_fetch(). As part of the refactoring
to use ref transactions we also fix a potential memory leak where in the
original code if write_ref_sha1() would fail we would end up returning from
the function without free()ing the msg string.

Note that this function is only called when fetching from a remote HTTP
repository onto the local (most of the time single-user) repository which
likely means that the type of collisions that the previous locking would
protect against and cause the fetch to fail for are even more rare.

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>

walker.c

@@ -251,40 +251,40 @@ void walker_targets_free(int targets, char **target, const char **write_ref)
 int walker_fetch(struct walker *walker, int targets, char **target,
 		 const char **write_ref, const char *write_ref_log_details)
 {
-	struct ref_lock **lock = xcalloc(targets, sizeof(struct ref_lock *));
+	struct strbuf refname = STRBUF_INIT;
+	struct strbuf err = STRBUF_INIT;
+	struct ref_transaction *transaction = NULL;
 	unsigned char *sha1 = xmalloc(targets * 20);
-	char *msg;
-	int ret;
-	int i;
+	char *msg = NULL;
+	int i, ret = -1;
 
 	save_commit_buffer = 0;
 
-	for (i = 0; i < targets; i++) {
-		if (!write_ref || !write_ref[i])
-			continue;
-
-		lock[i] = lock_ref_sha1(write_ref[i], NULL);
-		if (!lock[i]) {
-			error("Can't lock ref %s", write_ref[i]);
-			goto unlock_and_fail;
+	if (write_ref) {
+		transaction = ref_transaction_begin(&err);
+		if (!transaction) {
+			error("%s", err.buf);
+			goto done;
 		}
 	}
-
 	if (!walker->get_recover)
 		for_each_ref(mark_complete, NULL);
 
 	for (i = 0; i < targets; i++) {
 		if (interpret_target(walker, target[i], &sha1[20 * i])) {
 			error("Could not interpret response from server '%s' as something to pull", target[i]);
-			goto unlock_and_fail;
+			goto done;
 		}
 		if (process(walker, lookup_unknown_object(&sha1[20 * i])))
-			goto unlock_and_fail;
+			goto done;
 	}
 
 	if (loop(walker))
-		goto unlock_and_fail;
-
+		goto done;
+	if (!write_ref) {
+		ret = 0;
+		goto done;
+	}
 	if (write_ref_log_details) {
 		msg = xmalloc(strlen(write_ref_log_details) + 12);
 		sprintf(msg, "fetch from %s", write_ref_log_details);
@@ -292,23 +292,33 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 		msg = NULL;
 	}
 	for (i = 0; i < targets; i++) {
-		if (!write_ref || !write_ref[i])
+		if (!write_ref[i])
 			continue;
-		ret = write_ref_sha1(lock[i], &sha1[20 * i], msg ? msg : "fetch (unknown)");
-		lock[i] = NULL;
-		if (ret)
-			goto unlock_and_fail;
+		strbuf_reset(&refname);
+		strbuf_addf(&refname, "refs/%s", write_ref[i]);
+		if (ref_transaction_update(transaction, refname.buf,
+					   &sha1[20 * i], NULL, 0, 0,
+					   &err)) {
+			error("%s", err.buf);
+			goto done;
+		}
 	}
+	if (ref_transaction_commit(transaction,
+				   msg ? msg : "fetch (unknown)",
+				   &err)) {
+		error("%s", err.buf);
+		goto done;
+	}
+
+	ret = 0;
+
+done:
+	ref_transaction_free(transaction);
 	free(msg);
-
-	return 0;
-
-unlock_and_fail:
-	for (i = 0; i < targets; i++)
-		if (lock[i])
-			unlock_ref(lock[i]);
-
-	return -1;
+	free(sha1);
+	strbuf_release(&err);
+	strbuf_release(&refname);
+	return ret;
 }
 
 void walker_free(struct walker *walker)