reftable/merged: reuse buffer to compute record keys

When iterating over entries in the merged iterator's queue, we compute
the key of each of the entries and write it into a buffer. We do not
reuse the buffer though and thus re-allocate it on every iteration,
which is wasteful given that we never transfer ownership of the
allocated bytes outside of the loop.

Refactor the code to reuse the buffer. This also fixes a potential
memory leak when `merged_iter_advance_subiter()` returns an error.

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

reftable/merged.c

@@ -52,6 +52,8 @@ static void merged_iter_close(void *p)
 		reftable_iterator_destroy(&mi->stack[i]);
 	}
 	reftable_free(mi->stack);
+	strbuf_release(&mi->key);
+	strbuf_release(&mi->entry_key);
 }
 
 static int merged_iter_advance_nonnull_subiter(struct merged_iter *mi,
@@ -85,7 +87,6 @@ static int merged_iter_advance_subiter(struct merged_iter *mi, size_t idx)
 static int merged_iter_next_entry(struct merged_iter *mi,
 				  struct reftable_record *rec)
 {
-	struct strbuf entry_key = STRBUF_INIT;
 	struct pq_entry entry = { 0 };
 	int err = 0;
 
@@ -105,33 +106,31 @@ static int merged_iter_next_entry(struct merged_iter *mi,
 	  such a deployment, the loop below must be changed to collect all
 	  entries for the same key, and return new the newest one.
 	*/
-	reftable_record_key(&entry.rec, &entry_key);
+	reftable_record_key(&entry.rec, &mi->entry_key);
 	while (!merged_iter_pqueue_is_empty(mi->pq)) {
 		struct pq_entry top = merged_iter_pqueue_top(mi->pq);
-		struct strbuf k = STRBUF_INIT;
-		int err = 0, cmp = 0;
+		int cmp = 0;
 
-		reftable_record_key(&top.rec, &k);
+		reftable_record_key(&top.rec, &mi->key);
 
-		cmp = strbuf_cmp(&k, &entry_key);
-		strbuf_release(&k);
-
-		if (cmp > 0) {
+		cmp = strbuf_cmp(&mi->key, &mi->entry_key);
+		if (cmp > 0)
 			break;
-		}
 
 		merged_iter_pqueue_remove(&mi->pq);
 		err = merged_iter_advance_subiter(mi, top.index);
-		if (err < 0) {
-			return err;
-		}
+		if (err < 0)
+			goto done;
 		reftable_record_release(&top.rec);
 	}
 
 	reftable_record_copy_from(rec, &entry.rec, hash_size(mi->hash_id));
+
+done:
 	reftable_record_release(&entry.rec);
-	strbuf_release(&entry_key);
-	return 0;
+	strbuf_release(&mi->entry_key);
+	strbuf_release(&mi->key);
+	return err;
 }
 
 static int merged_iter_next(struct merged_iter *mi, struct reftable_record *rec)
@@ -248,6 +247,8 @@ static int merged_table_seek_record(struct reftable_merged_table *mt,
 		.typ = reftable_record_type(rec),
 		.hash_id = mt->hash_id,
 		.suppress_deletions = mt->suppress_deletions,
+		.key = STRBUF_INIT,
+		.entry_key = STRBUF_INIT,
 	};
 	int n = 0;
 	int err = 0;

reftable/merged.h

@@ -31,6 +31,8 @@ struct merged_iter {
 	uint8_t typ;
 	int suppress_deletions;
 	struct merged_iter_pqueue pq;
+	struct strbuf key;
+	struct strbuf entry_key;
 };
 
 void merged_table_release(struct reftable_merged_table *mt);