reftable/writer: fix writing multi-level indices

When finishing a section we will potentially write an index that makes
it more efficient to look up relevant blocks. The index records written
will encode, for each block of the indexed section, what the offset of
that block is as well as the last key of that block. Thus, the reader
would iterate through the index records to find the first key larger or
equal to the wanted key and then use the encoded offset to look up the
desired block.

When there are a lot of blocks to index though we may end up writing
multiple index blocks, too. To not require a linear search across all
index blocks we instead end up writing a multi-level index. Instead of
referring to the block we are after, an index record may point to
another index block. The reader will then access the highest-level index
and follow down the chain of index blocks until it hits the sought-after
block.

It has been observed though that it is impossible to seek ref records of
the last ref block when using a multi-level index. While the multi-level
index exists and looks fine for most of the part, the highest-level
index was missing an index record pointing to the last block of the next
index. Thus, every additional level made more refs become unseekable at
the end of the ref section.

The root cause is that we are not flushing the last block of the current
level once done writing the level. Consequently, it wasn't recorded in
the blocks that need to be indexed by the next-higher level and thus we
forgot about it.

Fix this bug by flushing blocks after we have written all index records.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 60 insertions, 4 deletions

diff --git a/reftable/readwrite_test.c b/reftable/readwrite_test.c
index 6b99daeaf2..853923397e 100644
--- a/reftable/readwrite_test.c
+++ b/reftable/readwrite_test.c
@@ -866,6 +866,61 @@ static void test_write_multiple_indices(void)
 	strbuf_release(&buf);
 }
 
+static void test_write_multi_level_index(void)
+{
+	struct reftable_write_options opts = {
+		.block_size = 100,
+	};
+	struct strbuf writer_buf = STRBUF_INIT, buf = STRBUF_INIT;
+	struct reftable_block_source source = { 0 };
+	struct reftable_iterator it = { 0 };
+	const struct reftable_stats *stats;
+	struct reftable_writer *writer;
+	struct reftable_reader *reader;
+	int err;
+
+	writer = reftable_new_writer(&strbuf_add_void, &noop_flush, &writer_buf, &opts);
+	reftable_writer_set_limits(writer, 1, 1);
+	for (size_t i = 0; i < 200; i++) {
+		struct reftable_ref_record ref = {
+			.update_index = 1,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = {i},
+		};
+
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "refs/heads/%03" PRIuMAX, (uintmax_t)i);
+		ref.refname = buf.buf,
+
+		err = reftable_writer_add_ref(writer, &ref);
+		EXPECT_ERR(err);
+	}
+	reftable_writer_close(writer);
+
+	/*
+	 * The written refs should be sufficiently large to result in a
+	 * multi-level index.
+	 */
+	stats = reftable_writer_stats(writer);
+	EXPECT(stats->ref_stats.max_index_level == 2);
+
+	block_source_from_strbuf(&source, &writer_buf);
+	err = reftable_new_reader(&reader, &source, "filename");
+	EXPECT_ERR(err);
+
+	/*
+	 * Seeking the last ref should work as expected.
+	 */
+	err = reftable_reader_seek_ref(reader, &it, "refs/heads/199");
+	EXPECT_ERR(err);
+
+	reftable_iterator_destroy(&it);
+	reftable_writer_free(writer);
+	reftable_reader_free(reader);
+	strbuf_release(&writer_buf);
+	strbuf_release(&buf);
+}
+
 static void test_corrupt_table_empty(void)
 {
 	struct strbuf buf = STRBUF_INIT;
@@ -916,5 +971,6 @@ int readwrite_test_main(int argc, const char *argv[])
 	RUN_TEST(test_write_object_id_length);
 	RUN_TEST(test_write_object_id_min_length);
 	RUN_TEST(test_write_multiple_indices);
+	RUN_TEST(test_write_multi_level_index);
 	return 0;
 }
diff --git a/reftable/writer.c b/reftable/writer.c
index b5bcce0292..0349094d29 100644
--- a/reftable/writer.c
+++ b/reftable/writer.c
@@ -418,15 +418,15 @@ static int writer_finish_section(struct reftable_writer *w)
 				return err;
 		}
 
+		err = writer_flush_block(w);
+		if (err < 0)
+			return err;
+
 		for (i = 0; i < idx_len; i++)
 			strbuf_release(&idx[i].last_key);
 		reftable_free(idx);
 	}
 
-	err = writer_flush_block(w);
-	if (err < 0)
-		return err;
-
 	writer_clear_index(w);
 
 	bstats = writer_reftable_block_stats(w, typ);