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>

refs.c

@@ -1297,6 +1297,13 @@ int ref_transaction_update_reflog(struct ref_transaction *transaction,
 	update->flags &= ~REF_HAVE_OLD;
 	update->index = index;
 
+	/*
+	 * Reference backends may need to know the max index to optimize
+	 * their writes. So we store the max_index on the transaction level.
+	 */
+	if (index > transaction->max_index)
+		transaction->max_index = index;
+
 	return 0;
 }
 

refs/refs-internal.h

@@ -203,6 +203,7 @@ struct ref_transaction {
 	enum ref_transaction_state state;
 	void *backend_data;
 	unsigned int flags;
+	unsigned int max_index;
 };
 
 /*

refs/reftable-backend.c

@@ -852,6 +852,7 @@ struct write_transaction_table_arg {
 	size_t updates_nr;
 	size_t updates_alloc;
 	size_t updates_expected;
+	unsigned int max_index;
 };
 
 struct reftable_transaction_data {
@@ -1302,7 +1303,6 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 	struct reftable_log_record *logs = NULL;
 	struct ident_split committer_ident = {0};
 	size_t logs_nr = 0, logs_alloc = 0, i;
-	uint64_t max_update_index = ts;
 	const char *committer_info;
 	int ret = 0;
 
@@ -1312,7 +1312,12 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 
 	QSORT(arg->updates, arg->updates_nr, transaction_update_cmp);
 
-	reftable_writer_set_limits(writer, ts, ts);
+	/*
+	 * During reflog migration, we add indexes for a single reflog with
+	 * multiple entries. Each entry will contain a different update_index,
+	 * so set the limits accordingly.
+	 */
+	reftable_writer_set_limits(writer, ts, ts + arg->max_index);
 
 	for (i = 0; i < arg->updates_nr; i++) {
 		struct reftable_transaction_update *tx_update = &arg->updates[i];
@@ -1414,12 +1419,6 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 				 */
 				log->update_index = ts + u->index;
 
-				/*
-				 * Note the max update_index so the limit can be set later on.
-				 */
-				if (log->update_index > max_update_index)
-					max_update_index = log->update_index;
-
 				log->refname = xstrdup(u->refname);
 				memcpy(log->value.update.new_hash,
 				       u->new_oid.hash, GIT_MAX_RAWSZ);
@@ -1483,8 +1482,6 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 	 * and log blocks.
 	 */
 	if (logs) {
-		reftable_writer_set_limits(writer, ts, max_update_index);
-
 		ret = reftable_writer_add_logs(writer, logs, logs_nr);
 		if (ret < 0)
 			goto done;
@@ -1505,6 +1502,9 @@ static int reftable_be_transaction_finish(struct ref_store *ref_store UNUSED,
 	struct reftable_transaction_data *tx_data = transaction->backend_data;
 	int ret = 0;
 
+	if (tx_data->args)
+		tx_data->args->max_index = transaction->max_index;
+
 	for (size_t i = 0; i < tx_data->args_nr; i++) {
 		ret = reftable_addition_add(tx_data->args[i].addition,
 					    write_transaction_table, &tx_data->args[i]);

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 &&