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reftable/block: open-code call to uncompress2()

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The reftable format stores log blocks in a compressed format. Thus,
whenever we want to read such a block we first need to decompress it.
This is done by calling the convenience function `uncompress2()` of the
zlib library, which is a simple wrapper that manages the lifecycle of
the `zstream` structure for us.

While nice for one-off inflation of data, when iterating through reflogs
we will likely end up inflating many such log blocks. This requires us
to reallocate the state of the `zstream` every single time, which adds
up over time. It would thus be great to reuse the `zstream` instead of
discarding it after every inflation.

Open-code the call to `uncompress2()` such that we can start reusing the
`zstream` in the subsequent commit. Note that our open-coded variant is
different from `uncompress2()` in two ways:

  - We do not loop around `inflate()` until we have processed all input.
    As our input is limited by the maximum block size, which is 16MB, we
    should not hit limits of `inflate()`.

  - We use `Z_FINISH` instead of `Z_NO_FLUSH`. Quoting the `inflate()`
    documentation: "inflate() should normally be called until it returns
    Z_STREAM_END or an error. However if all decompression is to be
    performed in a single step (a single call of inflate), the parameter
    flush should be set to Z_FINISH."

    Furthermore, "Z_FINISH also informs inflate to not maintain a
    sliding window if the stream completes, which reduces inflate's
    memory footprint."

Other than that this commit is expected to be functionally equivalent
and does not yet reuse the `zstream`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Patrick Steinhardt 2024-04-08 14:16:59 +02:00 committed by Junio C Hamano
parent dd347bbce6
commit 15a60b747e
1 changed files with 28 additions and 10 deletions
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38
reftable/block.c
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@ -195,10 +195,10 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
}
if (typ == BLOCK_TYPE_LOG) {
int block_header_skip = 4 + header_off;
uLongf dst_len = sz - block_header_skip; /* total size of dest
buffer. */
uLongf src_len = block->len - block_header_skip;
uint32_t block_header_skip = 4 + header_off;
uLong dst_len = sz - block_header_skip;
uLong src_len = block->len - block_header_skip;
z_stream stream = {0};
/* Log blocks specify the *uncompressed* size in their header. */
REFTABLE_ALLOC_GROW(br->uncompressed_data, sz,
@ -207,15 +207,33 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
/* Copy over the block header verbatim. It's not compressed. */
memcpy(br->uncompressed_data, block->data, block_header_skip);
/* Uncompress */
if (Z_OK !=
uncompress2(br->uncompressed_data + block_header_skip, &dst_len,
block->data + block_header_skip, &src_len)) {
err = inflateInit(&stream);
if (err != Z_OK) {
err = REFTABLE_ZLIB_ERROR;
goto done;
}
if (dst_len + block_header_skip != sz) {
stream.next_in = block->data + block_header_skip;
stream.avail_in = src_len;
stream.next_out = br->uncompressed_data + block_header_skip;
stream.avail_out = dst_len;
/*
* We know both input as well as output size, and we know that
* the sizes should never be bigger than `uInt_MAX` because
* blocks can at most be 16MB large. We can thus use `Z_FINISH`
* here to instruct zlib to inflate the data in one go, which
* is more efficient than using `Z_NO_FLUSH`.
*/
err = inflate(&stream, Z_FINISH);
inflateEnd(&stream);
if (err != Z_STREAM_END) {
err = REFTABLE_ZLIB_ERROR;
goto done;
}
err = 0;
if (stream.total_out + block_header_skip != sz) {
err = REFTABLE_FORMAT_ERROR;
goto done;
}
@ -224,7 +242,7 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
reftable_block_done(block);
block->data = br->uncompressed_data;
block->len = sz;
full_block_size = src_len + block_header_skip;
full_block_size = src_len + block_header_skip - stream.avail_in;
} else if (full_block_size == 0) {
full_block_size = sz;
} else if (sz < full_block_size && sz < block->len &&