fetch: fix failed batched updates skipping operations

Fix a regression introduced with batched updates in 0e358de64a (fetch:
use batched reference updates, 2025-05-19) when fetching references. In
the `do_fetch()` function, we jump to cleanup if committing the
transaction fails, regardless of whether using batched or atomic
updates. This skips three subsequent operations:

  - Update 'FETCH_HEAD' as part of `commit_fetch_head()`.

  - Add upstream tracking information via `set_upstream()`.

  - Setting remote 'HEAD' values when `do_set_head` is true.

For atomic updates, this is expected behavior. For batched updates,
we want to continue with these operations even if some refs fail to
update.

Skipping `commit_fetch_head()` isn't actually a regression because
'FETCH_HEAD' is already updated via `append_fetch_head()` when not
using '--atomic'. However, we add a test to validate this behavior.

Skipping the other two operations (upstream tracking and remote HEAD)
is a regression. Fix this by only jumping to cleanup when using
'--atomic', allowing batched updates to continue with post-fetch
operations. Add tests to prevent future regressions.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -1890,7 +1890,11 @@ static int do_fetch(struct transport *transport,
 
 	retcode = commit_ref_transaction(&transaction, atomic_fetch,
 					 transport->remote->name, &err);
-	if (retcode)
+	/*
+	 * With '--atomic', bail out if the transaction fails. Without '--atomic',
+	 * continue to fetch head and perform other post-fetch operations.
+	 */
+	if (retcode && atomic_fetch)
 		goto cleanup;
 
 	commit_fetch_head(&fetch_head);

t/t5510-fetch.sh

@@ -1639,6 +1639,94 @@ test_expect_success "backfill tags when providing a refspec" '
 	test_cmp expect actual
 '
 
+test_expect_success REFFILES "FETCH_HEAD is updated even if ref updates fail" '
+	test_when_finished rm -rf base repo &&
+
+	git init base &&
+	(
+		cd base &&
+		test_commit "updated" &&
+
+		git update-ref refs/heads/foo @ &&
+		git update-ref refs/heads/branch @
+	) &&
+
+	git init --bare repo &&
+	(
+		cd repo &&
+		rm -f FETCH_HEAD &&
+		git remote add origin ../base &&
+		>refs/heads/foo.lock &&
+		test_must_fail git fetch -f origin "refs/heads/*:refs/heads/*" 2>err &&
+		test_grep "error: fetching ref refs/heads/foo failed: reference already exists" err &&
+		test_grep "branch ${SQ}branch${SQ} of ../base" FETCH_HEAD &&
+		test_grep "branch ${SQ}foo${SQ} of ../base" FETCH_HEAD
+	)
+'
+
+test_expect_success "upstream tracking info is added with --set-upstream" '
+	test_when_finished rm -rf base repo &&
+
+	git init --initial-branch=main base &&
+	test_commit -C base "updated" &&
+
+	git init --bare --initial-branch=main repo &&
+	(
+		cd repo &&
+		git remote add origin ../base &&
+		git fetch origin --set-upstream main &&
+		git config get branch.main.remote >actual &&
+		echo "origin" >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success REFFILES "upstream tracking info is added even with conflicts" '
+	test_when_finished rm -rf base repo &&
+
+	git init --initial-branch=main base &&
+	test_commit -C base "updated" &&
+
+	git init --bare --initial-branch=main repo &&
+	(
+		cd repo &&
+		git remote add origin ../base &&
+		test_must_fail git config get branch.main.remote &&
+
+		mkdir -p refs/remotes/origin &&
+		>refs/remotes/origin/main.lock &&
+		test_must_fail git fetch origin --set-upstream main &&
+		git config get branch.main.remote >actual &&
+		echo "origin" >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success REFFILES "HEAD is updated even with conflicts" '
+	test_when_finished rm -rf base repo &&
+
+	git init base &&
+	(
+		cd base &&
+		test_commit "updated" &&
+
+		git update-ref refs/heads/foo @ &&
+		git update-ref refs/heads/branch @
+	) &&
+
+	git init --bare repo &&
+	(
+		cd repo &&
+		git remote add origin ../base &&
+
+		test_path_is_missing refs/remotes/origin/HEAD &&
+		mkdir -p refs/remotes/origin &&
+		>refs/remotes/origin/branch.lock &&
+		test_must_fail git fetch origin &&
+		test -f refs/remotes/origin/HEAD
+	)
+'
+
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd