reftable: write correct max_update_index to header

In 297c09eabb (refs: allow multiple reflog entries for the same refname,
2024-12-16), the reftable backend learned to handle multiple reflog
entries within the same transaction. This was done modifying the
`update_index` for reflogs with multiple indices. During writing the
logs, the `max_update_index` of the writer was modified to ensure the
limits were raised to the modified `update_index`s.

However, since ref entries are written before the modification to the
`max_update_index`, if there are multiple blocks to be written, the
reftable backend writes the header with the old `max_update_index`. When
all logs are finally written, the footer will be written with the new
`min_update_index`. This causes a mismatch between the header and the
footer and causes the reftable file to be corrupted. The existing tests
only spawn a single block and since headers are lazily written with the
first block, the tests didn't capture this bug.

To fix the issue, the appropriate `max_update_index` limit must be set
even before the first block is written. Add a `max_index` field to the
transaction which holds the `max_index` within all its updates, then
propagate this value to the reftable backend, wherein this is used to
the set the `max_update_index` correctly.

Add a test which creates a few thousand reference updates with multiple
reflog entries, which should trigger the bug.

Reported-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/t/t1460-refs-migrate.sh b/t/t1460-refs-migrate.sh
index f59bc4860f..307b2998ef 100755
--- a/t/t1460-refs-migrate.sh
+++ b/t/t1460-refs-migrate.sh
@@ -227,6 +227,18 @@ do
 	done
 done
 
+test_expect_success 'multiple reftable blocks with multiple entries' '
+	test_when_finished "rm -rf repo" &&
+	git init --ref-format=files repo &&
+	test_commit -C repo first &&
+	printf "create refs/heads/ref-%d HEAD\n" $(test_seq 5000) >stdin &&
+	git -C repo update-ref --stdin <stdin &&
+	test_commit -C repo second &&
+	printf "update refs/heads/ref-%d HEAD\n" $(test_seq 3000) >stdin &&
+	git -C repo update-ref --stdin <stdin &&
+	test_migration repo reftable
+'
+
 test_expect_success 'migrating from files format deletes backend files' '
 	test_when_finished "rm -rf repo" &&
 	git init --ref-format=files repo &&