reftable/stack: perform auto-compaction with transactional interface

Whenever updating references or reflog entries in the reftable stack, we
need to add a new table to the stack, thus growing the stack's length by
one. The stack can grow to become quite long rather quickly, leading to
performance issues when trying to read records. But besides performance
issues, this can also lead to exhaustion of file descriptors very
rapidly as every single table requires a separate descriptor when
opening the stack.

While git-pack-refs(1) fixes this issue for us by merging the tables, it
runs too irregularly to keep the length of the stack within reasonable
limits. This is why the reftable stack has an auto-compaction mechanism:
`reftable_stack_add()` will call `reftable_stack_auto_compact()` after
its added the new table, which will auto-compact the stack as required.

But while this logic works alright for `reftable_stack_add()`, we do not
do the same in `reftable_addition_commit()`, which is the transactional
equivalent to the former function that allows us to write multiple
updates to the stack atomically. Consequentially, we will easily run
into file descriptor exhaustion in code paths that use many separate
transactions like e.g. non-atomic fetches.

Fix this issue by calling `reftable_stack_auto_compact()`.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

reftable/stack.c

@@ -584,6 +584,12 @@ int reftable_addition_commit(struct reftable_addition *add)
 	add->new_tables_len = 0;
 
 	err = reftable_stack_reload(add->stack);
+	if (err)
+		goto done;
+
+	if (!add->stack->disable_auto_compact)
+		err = reftable_stack_auto_compact(add->stack);
+
 done:
 	reftable_addition_close(add);
 	return err;

reftable/stack_test.c

@@ -289,6 +289,61 @@ static void test_reftable_stack_transaction_api(void)
 	clear_dir(dir);
 }
 
+static void test_reftable_stack_transaction_api_performs_auto_compaction(void)
+{
+	char *dir = get_tmp_dir(__LINE__);
+	struct reftable_write_options cfg = {0};
+	struct reftable_addition *add = NULL;
+	struct reftable_stack *st = NULL;
+	int i, n = 20, err;
+
+	err = reftable_new_stack(&st, dir, cfg);
+	EXPECT_ERR(err);
+
+	for (i = 0; i <= n; i++) {
+		struct reftable_ref_record ref = {
+			.update_index = reftable_stack_next_update_index(st),
+			.value_type = REFTABLE_REF_SYMREF,
+			.value.symref = "master",
+		};
+		char name[100];
+
+		snprintf(name, sizeof(name), "branch%04d", i);
+		ref.refname = name;
+
+		/*
+		 * Disable auto-compaction for all but the last runs. Like this
+		 * we can ensure that we indeed honor this setting and have
+		 * better control over when exactly auto compaction runs.
+		 */
+		st->disable_auto_compact = i != n;
+
+		err = reftable_stack_new_addition(&add, st);
+		EXPECT_ERR(err);
+
+		err = reftable_addition_add(add, &write_test_ref, &ref);
+		EXPECT_ERR(err);
+
+		err = reftable_addition_commit(add);
+		EXPECT_ERR(err);
+
+		reftable_addition_destroy(add);
+
+		/*
+		 * The stack length should grow continuously for all runs where
+		 * auto compaction is disabled. When enabled, we should merge
+		 * all tables in the stack.
+		 */
+		if (i != n)
+			EXPECT(st->merged->stack_len == i + 1);
+		else
+			EXPECT(st->merged->stack_len == 1);
+	}
+
+	reftable_stack_destroy(st);
+	clear_dir(dir);
+}
+
 static void test_reftable_stack_validate_refname(void)
 {
 	struct reftable_write_options cfg = { 0 };
@@ -1016,6 +1071,7 @@ int stack_test_main(int argc, const char *argv[])
 	RUN_TEST(test_reftable_stack_log_normalize);
 	RUN_TEST(test_reftable_stack_tombstone);
 	RUN_TEST(test_reftable_stack_transaction_api);
+	RUN_TEST(test_reftable_stack_transaction_api_performs_auto_compaction);
 	RUN_TEST(test_reftable_stack_update_index_check);
 	RUN_TEST(test_reftable_stack_uptodate);
 	RUN_TEST(test_reftable_stack_validate_refname);