compress/flate: do not emit the preset dictionary into the output by joechenrh · Pull Request #80539 · golang/go

joechenrh · 2026-07-23T04:03:25Z

When using a dictionary, fillWindow was not setting blockStart, which
caused a non-compressed block in first position to write the dictionary.
This mixup can happen because window stores both the look back window
and the block we are about to write. Set blockStart to skip over the
dictionary if we have to write the window.

Fixes #80538

google-cla · 2026-07-23T04:03:48Z

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joechenrh · 2026-07-23T04:23:49Z

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gopherbot · 2026-07-23T04:44:56Z

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gopherbot · 2026-07-23T04:44:56Z

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NewWriterDict primes the compressor with fillWindow. That function copies the dictionary into d.window and advances d.windowEnd and the match index, but it leaves d.blockStart at 0. When the first block is emitted, writeBlock passes d.window[blockStart:index] to the huffman bit writer as the raw-bytes fallback. Since blockStart is 0, this slice starts with the dictionary. If the input is incompressible, storing the raw bytes is smaller than the encoded tokens, so the writer emits them as-is, and the dictionary ends up in the output. Decompressing then yields the dictionary followed by the data. Start the first block at the end of the dictionary instead, so the fallback only sees real input. The dictionary still primes matches through the hash chains, which do not use blockStart, so compression of compressible data is unchanged. Fixes golang#80538

gopherbot · 2026-07-23T05:37:03Z

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gopherbot · 2026-07-23T05:37:03Z

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gopherbot · 2026-07-23T05:37:12Z

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gopherbot · 2026-07-23T05:54:27Z

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gopherbot · 2026-07-23T05:54:28Z

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gopherbot · 2026-07-23T05:54:28Z

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gopherbot · 2026-07-23T05:54:29Z

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gopherbot · 2026-07-23T06:11:47Z

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gopherbot · 2026-07-27T15:50:30Z

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joechenrh force-pushed the flate-dict-blockstart branch from 7d27d7d to 1049404 Compare July 23, 2026 04:06

joechenrh force-pushed the flate-dict-blockstart branch from 65ae4b0 to 9fedf56 Compare July 23, 2026 04:26

joechenrh force-pushed the flate-dict-blockstart branch from 9fedf56 to 478d270 Compare July 23, 2026 05:20

address review: rename test, check buffer fully consumed

469f26b

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joechenrh commented Jul 23, 2026 •

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