[PATCH v2] push: fix --force-if-includes when remote-tracking ref has no reflog

[Aleksei Sviridkin <f@lex.la>]

Since 99a1f9ae10 (push: add reflog check for "--force-if-includes",
2020-10-03), is_reachable_in_reflog() stops walking the reflog of the
local branch at entries older than the newest reflog entry of the
remote-tracking ref. That timestamp is read by a callback of
refs_for_each_reflog_ent_reverse() into a variable that is never
initialized, so when the remote-tracking ref has no reflog the walk
is cut off at whatever happens to be on the stack.

With the files backend a remote-tracking ref created by "git clone"
has no reflog and does not get one until it moves. On my machine the
leftover value exceeds any real timestamp: the walk stops at the very
first entry, never reaches the "Created from" entry that "checkout
--track" wrote, and the push is rejected with "remote ref updated
since checkout" although nothing on the remote has changed.

Initialize the timestamp to zero, so that a remote-tracking ref
without reflog makes the walk cover the whole reflog of the local
branch, as documented.

Assisted-by: LLM
Signed-off-by: Aleksei Sviridkin <f@lex.la>
---
Changes since v1:
  - sign-off is now the last trailer
  - test_when_finished moved ahead of the setup so a failed init, push
    or clone still cleans up

 remote.c            |  2 +-
 t/t5533-push-cas.sh | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/remote.c b/remote.c
index 00723b385e..6d301698ca 100644
--- a/remote.c
+++ b/remote.c
@@ -2751,7 +2751,7 @@ static int check_and_collect_until(const char *refname UNUSED,
  */
 static int is_reachable_in_reflog(const char *local, const struct ref *remote)
 {
-	timestamp_t date;
+	timestamp_t date = 0;
 	struct commit *commit;
 	struct commit **chunk;
 	struct check_and_collect_until_cb_data cb;
diff --git a/t/t5533-push-cas.sh b/t/t5533-push-cas.sh
index cba26a872d..bb8878c593 100755
--- a/t/t5533-push-cas.sh
+++ b/t/t5533-push-cas.sh
@@ -396,4 +396,22 @@ test_expect_success '"--force-if-includes" should allow deletes' '
 	)
 '
 
+test_expect_success '"--force-if-includes" should allow forced update when remote-tracking ref has no reflog' '
+	rm -fr dst src &&
+	test_when_finished "rm -fr dst src" &&
+	git init --bare dst &&
+	git push dst main main:branch &&
+	git clone --no-local dst src &&
+	(
+		cd src &&
+		# a clone leaves the remote-tracking refs without reflog
+		# entries with the files backend, but not with reftable
+		git reflog expire --all --expire=all &&
+		git switch -c branch --track origin/branch &&
+		git reset --hard HEAD^ &&
+		test_commit D &&
+		git push --force-if-includes --force-with-lease="branch"
+	)
+'
+
 test_done

base-commit: e9019fcafe0040228b8631c30f97ae1adb61bcdc
-- 
2.55.0