refs: fix creation of corrupted reflogs for symrefs

The commit 297c09e (refs: allow multiple reflog entries for the same
refname, 2024-12-16) added logic for reflogs to exit early in
`lock_ref_for_update()` after obtaining the required lock. This was
added as a performance optimization as it was assumed that no further
processing was required for reflog only updates. However this was
incorrect since for a symref's reflog entry, the update needs to be
populated with the old_oid value. This is done right after the early
exit.

This caused a bug in Git 2.48 where target references of symrefs being
updated would create a corrupted reflog entry for the symref since the
old_oid is not populated. Undo the skip in logic to fix this issue and
also add a test to ensure that such an issue doesn't arise in the
future.

Reported-by: Nika Layzell <[email protected]>
Co-authored-by: Jeff King <[email protected]>
Signed-off-by: Karthik Nayak <[email protected]>
Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: 2 people

refs/files-backend.c

@@ -2611,9 +2611,6 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 
 	update->backend_data = lock;
 
-	if (update->flags & REF_LOG_ONLY)
-		goto out;
-
 	if (update->type & REF_ISSYMREF) {
 		if (update->flags & REF_NO_DEREF) {
 			/*

t/t1400-update-ref.sh

@@ -2068,4 +2068,13 @@ do
 
 done
 
+test_expect_success 'update-ref should also create reflog for HEAD' '
+	test_commit to-rewind &&
+	git rev-parse HEAD >expect &&
+	head=$(git symbolic-ref HEAD) &&
+	git update-ref --create-reflog "$head" HEAD~ &&
+	git rev-parse HEAD@{1} >actual &&
+	test_cmp expect actual
+'
+
 test_done