Patrick Steinhardt [off-list ref] writes:

We're lacking test coverage for compacting tables when some of the
tables that we are about to compact are locked. Add two tests that
exercise this, one for auto-compaction and one for full compaction.

So this patch prepares for the upcoming fixes by adding tests which fail
compaction. Makes sense.

[snip]

+static void test_reftable_stack_compaction_with_locked_tables(void)
+{
+	struct reftable_write_options opts = {
+		.disable_auto_compact = 1,
+	};
+	struct reftable_stack *st = NULL;
+	struct strbuf buf = STRBUF_INIT;
+	char *dir = get_tmp_dir(__LINE__);
+	int err;
+
+	err = reftable_new_stack(&st, dir, &opts);
+	EXPECT_ERR(err);
+
+	write_n_ref_tables(st, &opts, 3);
+	EXPECT(st->merged->stack_len == 3);
+
+	/* Lock one of the tables that we're about to compact. */
+	strbuf_reset(&buf);
+	strbuf_addf(&buf, "%s/%s.lock", dir, st->readers[1]->name);
+	write_file_buf(buf.buf, "", 0);
+
+	/*
+	 * Compaction is expected to fail given that we were not able to
+	 * compact all tables.
+	 */
+	err = reftable_stack_compact_all(st, NULL);
+	EXPECT(err == REFTABLE_LOCK_ERROR);
+	/* TODO: this is wrong, we should get notified about the failure. */
+	EXPECT(st->stats.failures == 0);

This is a good catch. The autocompaction code has a wrapper
`stack_compact_range_stats` which handles this exact scenario.

[snip]