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[3.15] gh-160310: Fix quadratic one-shot decompress() with many members (GH-160311) - #161037
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…ny members (pythonGH-160311) gzip.decompress(), bz2.decompress(), lzma.decompress() and compression.zstd.decompress() handed each new decompressor all of the remaining input and copied the rest back through unused_data, so input made of many concatenated members took quadratic time. Each member is now fed in chunks that start at 1 KiB and double, and the loop steps back over unused_data by offset. The first chunk is the whole input, so single-member data still takes one decompressor call. bz2, lzma and zstd share the loop through a private helper in compression._common._streams; gzip keeps its own because it parses the member header, trailer and padding itself. ANT-2026-9R8SO5S2 (cherry picked from commit e80962a)
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gzip.decompress(), bz2.decompress(), lzma.decompress() and
compression.zstd.decompress() handed each new decompressor all of the
remaining input and copied the rest back through unused_data, so input
made of many concatenated members took quadratic time. Each member is
now fed in chunks that start at 1 KiB and double, and the loop steps
back over unused_data by offset. The first chunk is the whole input,
so single-member data still takes one decompressor call.
bz2, lzma and zstd share the loop through a private helper in
compression._common._streams; gzip keeps its own because it parses the
member header, trailer and padding itself.
ANT-2026-9R8SO5S2
(cherry picked from commit e80962a)
Manual backport notes (3.15): the cherry-pick conflicted in Lib/bz2.py and Lib/compression/zstd/init.py, only on surrounding context that exists on main but not on 3.15: the module-level getattr at the end of bz2.py and the extra names main imports from _zstd. Each conflict was resolved by keeping 3.15's own code and adding only what the original commit adds or changes. The rest applied cleanly, and test_gzip, test_bz2, test_lzma, test_zstd, test_tarfile, test_shutil, test_zipfile and test_zipimport pass.
decompress()ingzip,bz2,lzmaandcompression.zstdis quadratic in the number of members #160310