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[zstd] Fix unneededly allocate large decompression buffer #120

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Merged
Viq111 merged 2 commits into from
Jun 6, 2022
Merged

[zstd] Fix unneededly allocate large decompression buffer #120

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43c36f4
[zstd] Fix unneededly allocate large decompression buffer
Viq111 Jun 2, 2022
8eebfba
[zstd] Always give a decompression hint of > 0
Viq111 Jun 2, 2022
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7 changes: 5 additions & 2 deletions zstd.go
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Original file line number Diff line number Diff line change
Expand Up @@ -36,7 +36,7 @@ const (
// decompressed, err := zstd.Decompress(dst, src)
decompressSizeBufferLimit = 1000 * 1000

zstdFrameHeaderSizeMax = 18 // From zstd.h. Since it's experimental API, hardcoding it
zstdFrameHeaderSizeMin = 2 // From zstd.h. Since it's experimental API, hardcoding it
)

// CompressBound returns the worst case size needed for a destination buffer,
Expand Down Expand Up @@ -67,11 +67,14 @@ func decompressSizeHint(src []byte) int {
}

hint := upperBound
if len(src) >= zstdFrameHeaderSizeMax {
if len(src) >= zstdFrameHeaderSizeMin {
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@david-ds david-ds Jun 6, 2022

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why not check the upper bound as well?

len(src) >= zstdFrameHeaderSizeMin && len(src) <= zstdFrameHeaderSizeMax

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@Viq111 Viq111 Jun 6, 2022

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I'm not sure what you mean ? That check if just to make sure the header is in the payload but src will 99% of the time be bigger than zstdFrameHeaderSizeMax (and that's fine)

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@david-ds david-ds Jun 6, 2022

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👍 sorry I thought that src would only be the header, I didn't realize it was the full payload.

hint = int(C.ZSTD_getFrameContentSize(unsafe.Pointer(&src[0]), C.size_t(len(src))))
if hint < 0 { // On error, just use upperBound
hint = upperBound
}
if hint == 0 { // When compressing the empty slice, we need an output of at least 1 to pass down to the C lib
hint = 1
}
}

// Take the minimum of both
Expand Down
19 changes: 19 additions & 0 deletions zstd_test.go
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Expand Up @@ -293,6 +293,25 @@ func TestBadPayloadZipBomb(t *testing.T) {
}
}

func TestSmallPayload(t *testing.T) {
// Test that we can compress really small payloads and this doesn't generate a huge output buffer
compressed, err := Compress(nil, []byte("a"))
if err != nil {
t.Fatalf("failed to compress: %s", err)
}

preAllocated := make([]byte, 1, 64) // Don't use more than that
decompressed, err := Decompress(preAllocated, compressed)
if err != nil {
t.Fatalf("failed to compress: %s", err)
}

if &(preAllocated[0]) != &(decompressed[0]) { // They should point to the same spot (no realloc)
t.Fatal("Compression buffer was changed")
}

}

func BenchmarkCompression(b *testing.B) {
if raw == nil {
b.Fatal(ErrNoPayloadEnv)
Expand Down