[PATCH v2] refs/files: avoid packed-refs lock for root ref deletion

[Ariel Keselman <skariel@gmail.com>]

Deleting a root ref queues a packed-ref transaction in the files
backend, even though root refs cannot be packed. For example, holding
.git/packed-refs.lock makes "git update-ref --no-deref -d AUTO_MERGE"
fail, whether or not AUTO_MERGE exists.

This also affects post-commit cleanup, which deletes AUTO_MERGE after
updating HEAD. In a linked worktree with read-only shared metadata,
commit succeeds but cleanup reports a packed-refs.lock error. Deleting
CHERRY_PICK_HEAD and REVERT_HEAD is affected as well.

Skip the packed transaction for root-ref deletions. Keep loose-ref
locking and packed-ref deletion for other refs unchanged.

Test deleting a root ref with packed-refs.lock held, and check that a
transaction deleting both a root ref and a packed branch still fails
without changing either ref.

Signed-off-by: Ariel Keselman <skariel@gmail.com>
---
Thanks for the review, Patrick.

Changes since v1:
- Keep the packed-ref deletion comment focused on its original purpose.
- Use one existing root ref and include a packed-refs file in the test.
- Drop the timeout overrides and redundant individual-ref cases.
- Add a transaction deleting a root ref and a packed branch together;
  check that a held packed-ref lock causes failure and preserves both refs.

AI assistance was used to generate the patch, tests, and commit message,
including this revision.

The root-ref deletion regression fails without the fix. With the fix,
123 test scripts / 4078 tests pass, along with 249 unit tests and t0600
with SHA-256 (one platform skip in t0600).

 refs/files-backend.c        |  8 ++++---
 t/t0600-reffiles-backend.sh | 43 +++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+), 3 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index a4c7858787..0333d5d355 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -2981,10 +2981,12 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 
 		if (update->flags & REF_DELETING &&
 		    !(update->flags & REF_LOG_ONLY) &&
-		    !(update->flags & REF_IS_PRUNING)) {
+		    !(update->flags & REF_IS_PRUNING) &&
+		    !is_root_ref(update->refname)) {
 			/*
-			 * This reference has to be deleted from
-			 * packed-refs if it exists there.
+			 * This reference has to be deleted from packed-refs if it
+			 * exists there. Root refs are never packed, so we do not
+			 * have to delete them from packed-refs.
 			 */
 			if (!packed_transaction) {
 				packed_transaction = ref_store_transaction_begin(
diff --git a/t/t0600-reffiles-backend.sh b/t/t0600-reffiles-backend.sh
index 74bfa2e9ba..65ca19e84b 100755
--- a/t/t0600-reffiles-backend.sh
+++ b/t/t0600-reffiles-backend.sh
@@ -519,4 +519,47 @@ test_expect_success 'symref transaction supports false symlink config' '
 	test_cmp expect actual
 '
 
+test_expect_success 'deleting a root ref does not lock packed-refs' '
+	test_when_finished "rm -rf root-ref" &&
+	git init root-ref &&
+	(
+		cd root-ref &&
+		test_commit initial &&
+		git pack-refs --all &&
+		cp .git/packed-refs expect &&
+		git update-ref AUTO_MERGE HEAD &&
+		: >.git/packed-refs.lock &&
+		git update-ref --no-deref -d AUTO_MERGE &&
+		test_path_is_missing .git/AUTO_MERGE &&
+		test_path_is_file .git/packed-refs.lock &&
+		test_cmp expect .git/packed-refs
+	)
+'
+
+test_expect_success 'deleting root and packed refs in one transaction requires packed-refs lock' '
+	test_when_finished "rm -rf root-ref" &&
+	git init root-ref &&
+	(
+		cd root-ref &&
+		test_commit initial &&
+		git update-ref refs/heads/packed-branch HEAD &&
+		git pack-refs --all &&
+		test_path_is_missing .git/refs/heads/packed-branch &&
+		git update-ref AUTO_MERGE HEAD &&
+		git rev-parse AUTO_MERGE refs/heads/packed-branch >expect &&
+		cat >stdin <<-EOF &&
+		start
+		delete AUTO_MERGE
+		delete refs/heads/packed-branch
+		prepare
+		commit
+		EOF
+		: >.git/packed-refs.lock &&
+		test_must_fail git update-ref --no-deref --stdin <stdin 2>err &&
+		test_grep "Unable to create .*packed-refs.lock" err &&
+		git rev-parse AUTO_MERGE refs/heads/packed-branch >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done
-- 
2.55.0