[PATCH 7/8] reftable/merged: reuse buffer to compute record keys

[Patrick Steinhardt <ps@xxxxxx>]

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@xxxxxx>
---
 reftable/merged.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/reftable/merged.c b/reftable/merged.c
index 5ded470c08..cd03f6da13 100644
--- a/reftable/merged.c
+++ b/reftable/merged.c
@@ -85,7 +85,7 @@ 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 strbuf entry_key = STRBUF_INIT, k = STRBUF_INIT;
 	struct pq_entry entry = { 0 };
 	int err = 0;
 
@@ -108,30 +108,28 @@ static int merged_iter_next_entry(struct merged_iter *mi,
 	reftable_record_key(&entry.rec, &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);
 
 		cmp = strbuf_cmp(&k, &entry_key);
-		strbuf_release(&k);
-
-		if (cmp > 0) {
+		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(&k);
+	return err;
 }
 
 static int merged_iter_next(struct merged_iter *mi, struct reftable_record *rec)
-- 
2.42.0

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