reftable/reader: iterate to next block in place

The table iterator has to iterate towards the next block once it has
yielded all records of the current block. This is done by creating a new
table iterator, initializing it to the next block, releasing the old
iterator and then copying over the data.

Refactor the code to instead advance the table iterator in place. This
is simpler and unlocks some optimizations in subsequent patches. Also,
it allows us to avoid some allocations.

The following measurements show a single matching ref out of 1 million
refs. Before this change:

  HEAP SUMMARY:
      in use at exit: 13,603 bytes in 125 blocks
    total heap usage: 7,235 allocs, 7,110 frees, 301,481 bytes allocated

After:

  HEAP SUMMARY:
      in use at exit: 13,603 bytes in 125 blocks
    total heap usage: 315 allocs, 190 frees, 107,027 bytes allocated

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

2 files changed, 28 insertions, 21 deletions

diff --git a/reftable/block.c b/reftable/block.c
index 2d8d0668b3..0c4e71eae3 100644
--- a/reftable/block.c
+++ b/reftable/block.c
@@ -188,6 +188,8 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
 	uint8_t *restart_bytes = NULL;
 	uint8_t *uncompressed = NULL;
 
+	reftable_block_done(&br->block);
+
 	if (!reftable_is_block_type(typ)) {
 		err =  REFTABLE_FORMAT_ERROR;
 		goto done;
diff --git a/reftable/reader.c b/reftable/reader.c
index b77b639751..dd4de294a1 100644
--- a/reftable/reader.c
+++ b/reftable/reader.c
@@ -312,26 +312,20 @@ static void table_iter_close(struct table_iter *ti)
 	block_iter_close(&ti->bi);
 }
 
-static int table_iter_next_block(struct table_iter *dest,
-				 struct table_iter *src)
+static int table_iter_next_block(struct table_iter *ti)
 {
-	uint64_t next_block_off = src->block_off + src->br.full_block_size;
+	uint64_t next_block_off = ti->block_off + ti->br.full_block_size;
 	int err;
 
-	dest->r = src->r;
-	dest->typ = src->typ;
-	dest->block_off = next_block_off;
-
-	err = reader_init_block_reader(src->r, &dest->br, next_block_off, src->typ);
+	err = reader_init_block_reader(ti->r, &ti->br, next_block_off, ti->typ);
 	if (err > 0)
-		dest->is_finished = 1;
-	if (err) {
-		table_iter_block_done(dest);
+		ti->is_finished = 1;
+	if (err)
 		return err;
-	}
 
-	dest->is_finished = 0;
-	block_iter_seek_start(&dest->bi, &dest->br);
+	ti->block_off = next_block_off;
+	ti->is_finished = 0;
+	block_iter_seek_start(&ti->bi, &ti->br);
 
 	return 0;
 }
@@ -342,7 +336,6 @@ static int table_iter_next(struct table_iter *ti, struct reftable_record *rec)
 		return REFTABLE_API_ERROR;
 
 	while (1) {
-		struct table_iter next = TABLE_ITER_INIT;
 		int err;
 
 		if (ti->is_finished)
@@ -362,14 +355,11 @@ static int table_iter_next(struct table_iter *ti, struct reftable_record *rec)
 		 * table and retry. If there are no more blocks then the
 		 * iterator is drained.
 		 */
-		err = table_iter_next_block(&next, ti);
+		err = table_iter_next_block(ti);
 		if (err) {
 			ti->is_finished = 1;
 			return err;
 		}
-
-		table_iter_close(ti);
-		*ti = next;
 	}
 }
 
@@ -453,9 +443,24 @@ static int reader_seek_linear(struct table_iter *ti,
 	 * have no other way to do this.
 	 */
 	while (1) {
-		struct table_iter next = TABLE_ITER_INIT;
+		struct table_iter next = *ti;
+
+		/*
+		 * We must be careful to not modify underlying data of `ti`
+		 * because we may find that `next` does not contain our desired
+		 * block, but that `ti` does. In that case, we would discard
+		 * `next` and continue with `ti`.
+		 *
+		 * This also means that we cannot reuse allocated memory for
+		 * `next` here. While it would be great if we could, it should
+		 * in practice not be too bad given that we should only ever
+		 * end up doing linear seeks with at most three blocks. As soon
+		 * as we have more than three blocks we would have an index, so
+		 * we would not do a linear search there anymore.
+		 */
+		memset(&next.br.block, 0, sizeof(next.br.block));
 
-		err = table_iter_next_block(&next, ti);
+		err = table_iter_next_block(&next);
 		if (err < 0)
 			goto done;
 		if (err > 0)