Patrick Steinhardt <ps@xxxxxx> writes:
> Restart points record the location of reftable records that do not use
> prefix compression and are used to perform a binary search inside of a
> block. These restart points are encoded at the end of a block, between
> the record data and the footer of a table.
>
> The block structure contains three different variables related to these
> restart points:
>
> - The block length contains the length of the reftable block up to the
> restart points.
>
> - The restart count contains the number of restart points contained in
> the block.
>
> - The restart bytes variable tracks where the restart point data
> begins.
>
> Tracking all three of these variables is unnecessary though as the data
> can be derived from one another: the block length without restart points
> is the exact same as the offset of the restart count data, which we
> already track via the `restart_bytes` data.
>
Nit: This para makes it seem as if we'd eliminate 'block length' in
support of having/keeping `restart_bytes`, but we remove both.
> Refactor the code so that we track the location of restart bytes not as
> a pointer, but instead as an offset. This allows us to trivially get rid
> of the `block_len` variable as described above. This avoids having the
> confusing `block_len` variable and allows us to do less bookkeeping
> overall.
>
> Signed-off-by: Patrick Steinhardt <ps@xxxxxx>
> ---
> reftable/block.c | 25 ++++++++++++-------------
> reftable/block.h | 8 +++++---
> reftable/table.c | 2 +-
> 3 files changed, 18 insertions(+), 17 deletions(-)
>
> diff --git a/reftable/block.c b/reftable/block.c
> index 97740187259..f2567a8f0fd 100644
> --- a/reftable/block.c
> +++ b/reftable/block.c
> @@ -216,10 +216,9 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
> uint32_t full_block_size = table_block_size;
> uint8_t typ = block->data[header_off];
> uint32_t sz = reftable_get_be24(block->data + header_off + 1);
> - int err = 0;
> - uint16_t restart_count = 0;
> - uint32_t restart_start = 0;
> - uint8_t *restart_bytes = NULL;
> + uint16_t restart_count;
> + uint32_t restart_off;
Nit: I guess this is to be consistent with `header_off`, but I would
think spelling it out as `header_offset` is much easier to understand.
> + int err;
>
> block_source_return_block(&br->block);
>
> @@ -300,8 +299,7 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
> }
>
> restart_count = reftable_get_be16(block->data + sz - 2);
> - restart_start = sz - 2 - 3 * restart_count;
> - restart_bytes = block->data + restart_start;
> + restart_off = sz - 2 - 3 * restart_count;
>
So each block looks like:
'r'
uint24( block_len )
ref_record+
uint24( restart_offset )+
uint16( restart_count )
So this does make sense.
> /* transfer ownership. */
> br->block = *block;
> @@ -309,11 +307,12 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
> block->len = 0;
>
> br->hash_size = hash_size;
> - br->block_len = restart_start;
> + br->restart_off = restart_off;
> br->full_block_size = full_block_size;
> br->header_off = header_off;
> br->restart_count = restart_count;
> - br->restart_bytes = restart_bytes;
> +
> + err = 0;
>
> done:
> return err;
> @@ -337,7 +336,7 @@ int block_reader_first_key(const struct block_reader *br, struct reftable_buf *k
> int off = br->header_off + 4, n;
> struct string_view in = {
> .buf = br->block.data + off,
> - .len = br->block_len - off,
> + .len = br->restart_off - off,
> };
> uint8_t extra = 0;
>
> @@ -354,13 +353,13 @@ int block_reader_first_key(const struct block_reader *br, struct reftable_buf *k
>
> static uint32_t block_reader_restart_offset(const struct block_reader *br, size_t idx)
> {
> - return reftable_get_be24(br->restart_bytes + 3 * idx);
> + return reftable_get_be24(br->block.data + br->restart_off + 3 * idx);
> }
>
> void block_iter_seek_start(struct block_iter *it, const struct block_reader *br)
> {
> it->block = br->block.data;
> - it->block_len = br->block_len;
> + it->block_len = br->restart_off;
> it->hash_size = br->hash_size;
> reftable_buf_reset(&it->last_key);
> it->next_off = br->header_off + 4;
> @@ -378,7 +377,7 @@ static int restart_needle_less(size_t idx, void *_args)
> uint32_t off = block_reader_restart_offset(args->reader, idx);
> struct string_view in = {
> .buf = args->reader->block.data + off,
> - .len = args->reader->block_len - off,
> + .len = args->reader->restart_off - off,
> };
> uint64_t prefix_len, suffix_len;
> uint8_t extra;
> @@ -505,7 +504,7 @@ int block_iter_seek_key(struct block_iter *it, const struct block_reader *br,
> else
> it->next_off = br->header_off + 4;
> it->block = br->block.data;
> - it->block_len = br->block_len;
> + it->block_len = br->restart_off;
> it->hash_size = br->hash_size;
>
> err = reftable_record_init(&rec, block_reader_type(br));
> diff --git a/reftable/block.h b/reftable/block.h
> index 203b07d9a44..b78f322e646 100644
> --- a/reftable/block.h
> +++ b/reftable/block.h
> @@ -79,10 +79,12 @@ struct block_reader {
> unsigned char *uncompressed_data;
> size_t uncompressed_cap;
>
> - /* size of the data, excluding restart data. */
> - uint32_t block_len;
> - uint8_t *restart_bytes;
> + /*
> + * Restart point data. Restart points are located after the block's
> + * record data.
> + */
> uint16_t restart_count;
> + uint32_t restart_off;
>
> /* size of the data in the file. For log blocks, this is the compressed
> * size. */
> diff --git a/reftable/table.c b/reftable/table.c
> index d18e17b0d44..ec84545707c 100644
> --- a/reftable/table.c
> +++ b/reftable/table.c
> @@ -838,7 +838,7 @@ int reftable_table_print_blocks(const char *tablename)
> printf("%s:\n", sections[i].name);
>
> while (1) {
> - printf(" - length: %u\n", ti.br.block_len);
> + printf(" - length: %u\n", ti.br.restart_off);
> printf(" restarts: %u\n", ti.br.restart_count);
>
> err = table_iter_next_block(&ti);
>
> --
> 2.49.0.604.gff1f9ca942.dirty
The patch looks good.
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