reftable/block: move ownership of block reader into struct table_iter

The table iterator allows the caller to iterate through all records in a
reftable table. To do so it iterates through all blocks of the desired
type one by one, where for each block it creates a new block iterator
and yields all its entries.

One of the things that is somewhat confusing in this context is who owns
the block reader that is being used to read the blocks and pass them to
the block iterator. Intuitively, as the table iterator is responsible
for iterating through the blocks, one would assume that this iterator is
also responsible for managing the lifecycle of the reader. And while it
somewhat is, the block reader is ultimately stored inside of the block
iterator.

Refactor the code such that the block reader is instead fully managed by
the table iterator. Instead of passing the reader to the block iterator,
we now only end up passing the block data to it. Despite clearing up the
lifecycle of the reader, it will also allow for better reuse of the
reader in subsequent patches.

The following benchmark prints a single matching ref out of 1 million
refs. Before:

  HEAP SUMMARY:
      in use at exit: 13,603 bytes in 125 blocks
    total heap usage: 6,607 allocs, 6,482 frees, 509,635 bytes allocated

After:

  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

Note that while there are more allocation and free calls now, the
overall number of bytes allocated is significantly lower. The number of
allocations will be reduced significantly by the next patch though.

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

reftable/block.c

@@ -261,12 +261,12 @@ void block_reader_release(struct block_reader *br)
 	reftable_block_done(&br->block);
 }
 
-uint8_t block_reader_type(struct block_reader *r)
+uint8_t block_reader_type(const struct block_reader *r)
 {
 	return r->block.data[r->header_off];
 }
 
-int block_reader_first_key(struct block_reader *br, struct strbuf *key)
+int block_reader_first_key(const struct block_reader *br, struct strbuf *key)
 {
 	int off = br->header_off + 4, n;
 	struct string_view in = {
@@ -286,14 +286,16 @@ int block_reader_first_key(struct block_reader *br, struct strbuf *key)
 	return 0;
 }
 
-static uint32_t block_reader_restart_offset(struct block_reader *br, int i)
+static uint32_t block_reader_restart_offset(const struct block_reader *br, int i)
 {
 	return get_be24(br->restart_bytes + 3 * i);
 }
 
-void block_iter_seek_start(struct block_iter *it, struct block_reader *br)
+void block_iter_seek_start(struct block_iter *it, const struct block_reader *br)
 {
-	it->br = br;
+	it->block = br->block.data;
+	it->block_len = br->block_len;
+	it->hash_size = br->hash_size;
 	strbuf_reset(&it->last_key);
 	it->next_off = br->header_off + 4;
 }
@@ -301,7 +303,7 @@ void block_iter_seek_start(struct block_iter *it, struct block_reader *br)
 struct restart_needle_less_args {
 	int error;
 	struct strbuf needle;
-	struct block_reader *reader;
+	const struct block_reader *reader;
 };
 
 static int restart_needle_less(size_t idx, void *_args)
@@ -340,9 +342,11 @@ static int restart_needle_less(size_t idx, void *_args)
 	return args->needle.len < suffix_len;
 }
 
-void block_iter_copy_from(struct block_iter *dest, struct block_iter *src)
+void block_iter_copy_from(struct block_iter *dest, const struct block_iter *src)
 {
-	dest->br = src->br;
+	dest->block = src->block;
+	dest->block_len = src->block_len;
+	dest->hash_size = src->hash_size;
 	dest->next_off = src->next_off;
 	strbuf_reset(&dest->last_key);
 	strbuf_addbuf(&dest->last_key, &src->last_key);
@@ -351,14 +355,14 @@ void block_iter_copy_from(struct block_iter *dest, struct block_iter *src)
 int block_iter_next(struct block_iter *it, struct reftable_record *rec)
 {
 	struct string_view in = {
-		.buf = it->br->block.data + it->next_off,
-		.len = it->br->block_len - it->next_off,
+		.buf = (unsigned char *) it->block + it->next_off,
+		.len = it->block_len - it->next_off,
 	};
 	struct string_view start = in;
 	uint8_t extra = 0;
 	int n = 0;
 
-	if (it->next_off >= it->br->block_len)
+	if (it->next_off >= it->block_len)
 		return 1;
 
 	n = reftable_decode_key(&it->last_key, &extra, in);
@@ -368,7 +372,7 @@ int block_iter_next(struct block_iter *it, struct reftable_record *rec)
 		return REFTABLE_FORMAT_ERROR;
 
 	string_view_consume(&in, n);
-	n = reftable_record_decode(rec, it->last_key, extra, in, it->br->hash_size,
+	n = reftable_record_decode(rec, it->last_key, extra, in, it->hash_size,
 				   &it->scratch);
 	if (n < 0)
 		return -1;
@@ -378,13 +382,22 @@ int block_iter_next(struct block_iter *it, struct reftable_record *rec)
 	return 0;
 }
 
+void block_iter_reset(struct block_iter *it)
+{
+	strbuf_reset(&it->last_key);
+	it->next_off = 0;
+	it->block = NULL;
+	it->block_len = 0;
+	it->hash_size = 0;
+}
+
 void block_iter_close(struct block_iter *it)
 {
 	strbuf_release(&it->last_key);
 	strbuf_release(&it->scratch);
 }
 
-int block_iter_seek_key(struct block_iter *it, struct block_reader *br,
+int block_iter_seek_key(struct block_iter *it, const struct block_reader *br,
 			struct strbuf *want)
 {
 	struct restart_needle_less_args args = {
@@ -436,7 +449,9 @@ int block_iter_seek_key(struct block_iter *it, struct block_reader *br,
 		it->next_off = block_reader_restart_offset(br, i - 1);
 	else
 		it->next_off = br->header_off + 4;
-	it->br = br;
+	it->block = br->block.data;
+	it->block_len = br->block_len;
+	it->hash_size = br->hash_size;
 
 	reftable_record_init(&rec, block_reader_type(br));