builtin/fetch: avoid aborting closed reference transaction

As part of the reference transaction commit phase, the transaction is
set to a closed state regardless of whether it was successful of not.
Attempting to abort a closed transaction via `ref_transaction_abort()`
results in a `BUG()`.

In c92abe71df (builtin/fetch: fix leaking transaction with `--atomic`,
2024-08-22), logic to free a transaction after the commit phase is moved
to the centralized exit path. In cases where the transaction commit
failed, this results in a closed transaction being aborted and signaling
a bug.

Free the transaction and set it to NULL when the commit fails. This
allows the exit path to correctly handle the error without attempting to
abort the transaction.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -1732,8 +1732,15 @@ static int do_fetch(struct transport *transport,
 			goto cleanup;
 
 		retcode = ref_transaction_commit(transaction, &err);
-		if (retcode)
+		if (retcode) {
+			/*
+			 * Explicitly handle transaction cleanup to avoid
+			 * aborting an already closed transaction.
+			 */
+			ref_transaction_free(transaction);
+			transaction = NULL;
 			goto cleanup;
+		}
 	}
 
 	commit_fetch_head(&fetch_head);

t/t5510-fetch.sh

@@ -345,6 +345,19 @@ test_expect_success 'fetch --atomic --append appends to FETCH_HEAD' '
 	test_cmp expected atomic/.git/FETCH_HEAD
 '
 
+test_expect_success REFFILES 'fetch --atomic fails transaction if reference locked' '
+	test_when_finished "rm -rf upstream repo" &&
+
+	git init upstream &&
+	git -C upstream commit --allow-empty -m 1 &&
+	git -C upstream switch -c foobar &&
+	git clone --mirror upstream repo &&
+	git -C upstream commit --allow-empty -m 2 &&
+	touch repo/refs/heads/foobar.lock &&
+
+	test_must_fail git -C repo fetch --atomic origin
+'
+
 test_expect_success '--refmap="" ignores configured refspec' '
 	cd "$TRASH_DIRECTORY" &&
 	git clone "$D" remote-refs &&