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GeezerVison is showing github.com — gh-116738: Make lzma module thread-safe by yoney · Pull Request #142947 · python/cpython

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yoney commented Dec 18, 2025 •

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Make the attributes in lzma module thread-safe on the free-threading build. Attributes (check, eof, needs_input, unused_data) are now stored atomically or accessed via mutex-protected getters.

Even though the operations are protected by locks, the attributes exposed via PyMemberDef (check, eof, needs_input, unused_data) should still be updated atomically while holding the lock, or accessed via mutex‑protected getters, because they can be read without acquiring the lock.

This issue was not addressed in #140711. A small addition that repeatedly reads these attributes shows the issue under tsan build.

WARNING: ThreadSanitizer: data race (pid=2367860)
  Atomic read of size 1 at 0x7fdbc9506ee0 by thread T19:
    #0 _Py_atomic_load_char_relaxed /workspace/ft_lzma_tsan/./Include/cpython/pyatomic_gcc.h:311 (python+0x99e392) (BuildId: ce943d12d16f92ec125f1db1856a1617d3d75854)
    #1 PyMember_GetOne /workspace/ft_lzma_tsan/Python/structmember.c:37 (python+0x99e392)
    #2 member_get /workspace/ft_lzma_tsan/Objects/descrobject.c:180 (python+0x60f4d5) (BuildId: ce943d12d16f92ec125f1db1856a1617d3d75854)
...
...
...
Previous write of size 1 at 0x7fdbc9506ee0 by thread T11:
    #0 decompress /workspace/ft_lzma_tsan/./Modules/_lzmamodule.c:1067 (_lzma.cpython-315td-x86_64-linux-gnu.so+0x7256) (BuildId: 978a710f12e938f2fbc3233c01c40cf693105ef1)
    #1 _lzma_LZMADecompressor_decompress_impl /workspace/ft_lzma_tsan/./Modules/_lzmamodule.c:1139 (_lzma.cpython-315td-x86_64-linux-gnu.so+0x7256)
    #2 _lzma_LZMADecompressor_decompress /workspace/ft_lzma_tsan/./Modules/clinic/_lzmamodule.c.h:157 (_lzma.cpython-315td-x86_64-linux-gnu.so+0x7256)
...
...
...
SUMMARY: ThreadSanitizer: data race /workspace/ft_lzma_tsan/./Modules/_lzmamodule.c:1043 in decompress

cc: @mpage @colesbury

Issue: Audit all built-in modules for thread safety #116738

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          pythongh-116738: Make lzma module thread-safe

b422600

Make the attributes in lzma module thread-safe on the free-threading build.
Attributes (check, eof, needs_input, unused_data) are now stored atomically
or accessed via mutex-protected getters.

bedevere-app Bot mentioned this pull request

Dec 18, 2025

Audit all built-in modules for thread safety #116738

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yoney marked this pull request as ready for review

December 18, 2025 17:42

bedevere-app Bot added the awaiting review label

Dec 18, 2025

colesbury requested review from colesbury and emmatyping

December 18, 2025 21:19

colesbury added the topic-free-threading label

Dec 18, 2025

ZeroIntensity reviewed

Dec 19, 2025

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Misc/NEWS.d/next/Library/2025-12-19-12-38-01.gh-issue-116738.iMt3Ol.rst Resolved

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emmatyping reviewed

Dec 19, 2025

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Modules/_lzmamodule.c Resolved

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yoney added 3 commits

December 19, 2025 12:44


          Correct the NEWS section

2640d35


          Improve unused_data getter

637cc78


          Merge branch 'main' into ft_lzma_2

f26846f

emmatyping commented Dec 23, 2025

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Looks like jobs failed due to a spurious GitHub outage. I've re-run the failed jobs.

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picnixz reviewed

Dec 28, 2025

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Modules/_lzmamodule.c Resolved

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          Update Modules/_lzmamodule.c

bdc32fe

Co-authored-by: Bénédikt Tran <[email protected]>

colesbury approved these changes

Jan 5, 2026

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LGTM

@emmatyping @picnixz - do you have any further feedback on the PR?

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bedevere-app Bot added awaiting merge and removed awaiting review labels

Jan 5, 2026

colesbury requested review from emmatyping and picnixz

January 5, 2026 18:09

picnixz approved these changes

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Misc/NEWS.d/next/Library/2025-12-19-12-38-01.gh-issue-116738.iMt3Ol.rst Resolved

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Modules/_lzmamodule.c Resolved

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kumaraditya303 approved these changes

Jan 13, 2026

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          Merge branch 'main' into ft_lzma_2

ff7bf31

kumaraditya303 enabled auto-merge (squash)

January 13, 2026 13:36

kumaraditya303 merged commit fca7fec into python:main

Jan 13, 2026

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bedevere-app Bot removed the awaiting merge label

Jan 13, 2026

yoney deleted the ft_lzma_2 branch

January 27, 2026 17:07

yoney mentioned this pull request

Jan 30, 2026

gh-116738: Align bz2 unused_data getter to lzma implementation #144351

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thunder-coding pushed a commit to thunder-coding/cpython that referenced this pull request

Feb 15, 2026


          pythongh-116738: Make lzma module thread-safe (python#142947)

313c23b

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colesbury

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yoney commented Dec 18, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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yoney commented Dec 18, 2025 •

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