reftable/block: reuse buffer to compute record keys

When iterating over entries in the block iterator 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.

Refactor the code to reuse the buffer.

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

reftable/block.c

@@ -323,30 +323,28 @@ int block_iter_next(struct block_iter *it, struct reftable_record *rec)
  .len = it->br->block_len - it->next_off,
  };
  struct string_view start = in;
- struct strbuf key = STRBUF_INIT;
  uint8_t extra = 0;
  int n = 0;
 
  if (it->next_off >= it->br->block_len)
  return 1;
 
- n = reftable_decode_key(&key, &extra, it->last_key, in);
+ n = reftable_decode_key(&it->key, &extra, it->last_key, in);
  if (n < 0)
  return -1;
 
- if (!key.len)
+ if (!it->key.len)
  return REFTABLE_FORMAT_ERROR;
 
  string_view_consume(&in, n);
- n = reftable_record_decode(rec, key, extra, in, it->br->hash_size);
+ n = reftable_record_decode(rec, it->key, extra, in, it->br->hash_size);
  if (n < 0)
  return -1;
  string_view_consume(&in, n);
 
  strbuf_reset(&it->last_key);
- strbuf_addbuf(&it->last_key, &key);
+ strbuf_addbuf(&it->last_key, &it->key);
  it->next_off += start.len - in.len;
- strbuf_release(&key);
  return 0;
 }
 
@@ -377,6 +375,7 @@ int block_iter_seek(struct block_iter *it, struct strbuf *want)
 void block_iter_close(struct block_iter *it)
 {
  strbuf_release(&it->last_key);
+ strbuf_release(&it->key);
 }
 
 int block_reader_seek(struct block_reader *br, struct block_iter *it,
@@ -387,7 +386,6 @@ int block_reader_seek(struct block_reader *br, struct block_iter *it,
  .r = br,
  };
  struct reftable_record rec = reftable_new_record(block_reader_type(br));
- struct strbuf key = STRBUF_INIT;
  int err = 0;
  struct block_iter next = BLOCK_ITER_INIT;
 
@@ -414,8 +412,8 @@ int block_reader_seek(struct block_reader *br, struct block_iter *it,
  if (err < 0)
  goto done;
 
- reftable_record_key(&rec, &key);
- if (err > 0 || strbuf_cmp(&key, want) >= 0) {
+ reftable_record_key(&rec, &it->key);
+ if (err > 0 || strbuf_cmp(&it->key, want) >= 0) {
  err = 0;
  goto done;
  }
@@ -424,8 +422,7 @@ int block_reader_seek(struct block_reader *br, struct block_iter *it,
  }
 
 done:
- strbuf_release(&key);
- strbuf_release(&next.last_key);
+ block_iter_close(&next);
  reftable_record_release(&rec);
 
  return err;

reftable/block.h

@@ -84,10 +84,12 @@ struct block_iter {
 
  /* key for last entry we read. */
  struct strbuf last_key;
+ struct strbuf key;
 };
 
 #define BLOCK_ITER_INIT { \
  .last_key = STRBUF_INIT, \
+ .key = STRBUF_INIT, \
 }
 
 /* initializes a block reader. */