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GeezerVison is showing github.com — gh-159458: Fix free-threading data race in datetime strftime century normalization by AnnaAr321 · Pull Request #159520 · python/cpython

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@AnnaAr321

@AnnaAr321 AnnaAr321 commented Oct 16, 2026 •
edited by bedevere-app Bot
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In Modules/_datetimemodule.c, normalize_century() lazily initializes a function-local static int cache = -1; on the first strftime() call using %Y, %G, %F, or %C. Under free-threaded builds (Py_GIL_DISABLED), concurrent initial calls to strftime() race on reading and writing cache, triggering ThreadSanitizer data race warnings.
Use FT_ATOMIC_LOAD_INT_RELAXED and FT_ATOMIC_STORE_INT_RELAXED to atomically load and store cache in free-threaded builds with zero overhead in default GIL-enabled builds.

  • Issue: Free-threading: data race on static cache in _datetime.normalize_century() #159458

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@AnnaAr321
Fix free-threading data race in datetime strftime century normalization
bb256a0
@AnnaAr321
AnnaAr321 requested review from StanFromIreland and pganssle as code owners October 16, 2026 10:23
@bedevere-app bedevere-app Bot mentioned this pull request Oct 16, 2026
Free-threading: data race on static cache in _datetime.normalize_century() #159458
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@bedevere-app bedevere-app Bot added the awaiting review label Oct 16, 2026
@blurb-it
📜🤖 Added by blurb_it.
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@encukou

encukou commented Oct 16, 2026

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Thank you.
AFAIK, this doesn't affect users in practice, so the NEWS blurb isn't really necessary. But since you already wrote it, we can keep it.
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@encukou
encukou merged commit ddc647a into python:main Oct 16, 2026
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@bedevere-app bedevere-app Bot removed the awaiting review label Oct 16, 2026
@encukou encukou added awaiting review needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes labels Oct 16, 2026
@miss-islington-app

miss-islington-app Bot commented Oct 16, 2026

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Thanks @AnnaAr321 for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖
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@miss-islington-app

miss-islington-app Bot commented Oct 16, 2026

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Thanks @AnnaAr321 for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖
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@bedevere-app

bedevere-app Bot commented Oct 16, 2026

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GH-159536 is a backport of this pull request to the 3.15 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label Oct 16, 2026
@bedevere-app

bedevere-app Bot commented Oct 16, 2026

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GH-159537 is a backport of this pull request to the 3.14 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Oct 16, 2026
StanFromIreland
StanFromIreland reviewed Oct 16, 2026
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This was merged before I could review it, so a couple of notes for future PRs.

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Comment thread Lib/test/test_free_threading/test_datetime.py

@threading_helper.requires_working_threading()
class TestDatetime(unittest.TestCase):
def test_concurrent_strftime_normalize_century(self):

@StanFromIreland StanFromIreland Oct 16, 2026

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I think this test does not work in our current testing configuration, as it relies on the cache being empty, correct? In the CI job the whole suite runs in parallel threads, so by the time this test runs the cache is almost certainly already initialised...

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Comment thread Misc/NEWS.d/next/Library/2026-10-16-10-25-22.gh-issue-159458.q02EnY.rst
@@ -0,0 +1 @@
Fix a ThreadSanitizer data race on the static cache in ``normalize_century()`` of the datetime module under the free-threaded build.

@StanFromIreland StanFromIreland Oct 16, 2026

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Suggested change
Fix a ThreadSanitizer data race on the static cache in ``normalize_century()`` of the datetime module under the free-threaded build.
Fix a data race in :meth:`datetime.date.strftime` and :meth:`datetime.datetime.strftime` on the free-threaded build.

Please only include user-facing details in blurbs.

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corona10 pushed a commit that referenced this pull request Oct 20, 2026
@miss-islington @AnnaAr321
[3.15] gh-159458: Fix free-threading data race in datetime strftime c…
d89f40b 
…entury normalization (GH-159520) (gh-159536)

gh-159458: Fix free-threading data race in datetime strftime century normalization (GH-159520)
(cherry picked from commit ddc647a)

Co-authored-by: Anna <[email protected]>
corona10 pushed a commit that referenced this pull request Oct 20, 2026
@miss-islington @AnnaAr321
[3.14] gh-159458: Fix free-threading data race in datetime strftime c…
9d6f05e 
…entury normalization (GH-159520) (gh-159537)

gh-159458: Fix free-threading data race in datetime strftime century normalization (GH-159520)
(cherry picked from commit ddc647a)

Co-authored-by: Anna <[email protected]>
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@StanFromIreland StanFromIreland StanFromIreland left review comments
@pganssle pganssle Awaiting requested review from pganssle pganssle is a code owner

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@AnnaAr321 @encukou @StanFromIreland
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