reftable/reader: avoid copying index iterator

When doing an indexed seek we need to walk down the multi-level index
until we finally hit a record of the desired indexed type. This loop
performs a copy of the index iterator on every iteration, which is both
hard to understand and completely unnecessary.

Refactor the code so that we use a single iterator to walk down the
indices, only.

Note that while this should improve performance, the improvement is
negligible in all but the most unreasonable repositories. This is
because the effect is only really noticeable when we have to walk down
many levels of indices, which is not something that a repository would
typically have. So the motivation for this change is really only about
readability.

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

Author: pks-t

reftable/reader.c

@@ -510,13 +510,11 @@ static int reader_seek_indexed(struct reftable_reader *r,
 		.type = BLOCK_TYPE_INDEX,
 		.u.idx = { .last_key = STRBUF_INIT },
 	};
-	struct table_iter index_iter = TABLE_ITER_INIT;
-	struct table_iter empty = TABLE_ITER_INIT;
-	struct table_iter next = TABLE_ITER_INIT;
+	struct table_iter ti = TABLE_ITER_INIT, *malloced;
 	int err = 0;
 
 	reftable_record_key(rec, &want_index.u.idx.last_key);
-	err = reader_start(r, &index_iter, reftable_record_type(rec), 1);
+	err = reader_start(r, &ti, reftable_record_type(rec), 1);
 	if (err < 0)
 		goto done;
 
@@ -526,7 +524,7 @@ static int reader_seek_indexed(struct reftable_reader *r,
 	 * highest layer that identifies the relevant index block as well as
 	 * the record inside that block that corresponds to our wanted key.
 	 */
-	err = reader_seek_linear(&index_iter, &want_index);
+	err = reader_seek_linear(&ti, &want_index);
 	if (err < 0)
 		goto done;
 
@@ -552,44 +550,36 @@ static int reader_seek_indexed(struct reftable_reader *r,
 		 * all levels of the index only to find out that the key does
 		 * not exist.
 		 */
-		err = table_iter_next(&index_iter, &index_result);
+		err = table_iter_next(&ti, &index_result);
 		if (err != 0)
 			goto done;
 
-		err = reader_table_iter_at(r, &next, index_result.u.idx.offset,
-					   0);
+		err = reader_table_iter_at(r, &ti, index_result.u.idx.offset, 0);
 		if (err != 0)
 			goto done;
 
-		err = block_iter_seek_key(&next.bi, &next.br, &want_index.u.idx.last_key);
+		err = block_iter_seek_key(&ti.bi, &ti.br, &want_index.u.idx.last_key);
 		if (err < 0)
 			goto done;
 
-		if (next.typ == reftable_record_type(rec)) {
+		if (ti.typ == reftable_record_type(rec)) {
 			err = 0;
 			break;
 		}
 
-		if (next.typ != BLOCK_TYPE_INDEX) {
+		if (ti.typ != BLOCK_TYPE_INDEX) {
 			err = REFTABLE_FORMAT_ERROR;
-			break;
+			goto done;
 		}
-
-		table_iter_close(&index_iter);
-		index_iter = next;
-		next = empty;
 	}
 
-	if (err == 0) {
-		struct table_iter *malloced = reftable_calloc(1, sizeof(*malloced));
-		*malloced = next;
-		next = empty;
-		iterator_from_table_iter(it, malloced);
-	}
+	REFTABLE_ALLOC_ARRAY(malloced, 1);
+	*malloced = ti;
+	iterator_from_table_iter(it, malloced);
 
 done:
-	table_iter_close(&next);
-	table_iter_close(&index_iter);
+	if (err)
+		table_iter_close(&ti);
 	reftable_record_release(&want_index);
 	reftable_record_release(&index_result);
 	return err;