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gh-159458: Fix free-threading data race in datetime strftime century normalization #159520

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encukou merged 2 commits into from
Oct 16, 2026
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gh-159458: Fix free-threading data race in datetime strftime century normalization #159520

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Fix free-threading data race in datetime strftime century normalization
AnnaAr321 Oct 16, 2026
5b57447
📜🤖 Added by blurb_it.
blurb-it[bot] Oct 16, 2026
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25 changes: 25 additions & 0 deletions Lib/test/test_free_threading/test_datetime.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,25 @@
import datetime
import unittest

from test.support import threading_helper
from test.support.threading_helper import run_concurrently


NTHREADS = 10


@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...

d = datetime.date(2026, 4, 12)
results = []

def worker():
results.append(d.strftime("%Y-%m-%d"))

run_concurrently(worker_func=worker, nthreads=NTHREADS)
self.assertEqual(results, ["2026-04-12"] * NTHREADS)


if __name__ == "__main__":
unittest.main()
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2026-10-16-10-25-22.gh-issue-159458.q02EnY.rst
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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.

10 changes: 6 additions & 4 deletions Modules/_datetimemodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1783,17 +1783,19 @@ inline static int
normalize_century(void)
{
static int cache = -1;
if (cache < 0) {
int res = FT_ATOMIC_LOAD_INT_RELAXED(cache);
if (res < 0) {
char year[5];
struct tm date = {
.tm_year = -1801,
.tm_mon = 0,
.tm_mday = 1
};
cache = (strftime(year, sizeof(year), "%Y", &date) &&
strcmp(year, "0099") != 0);
res = (strftime(year, sizeof(year), "%Y", &date) &&
strcmp(year, "0099") != 0);
FT_ATOMIC_STORE_INT_RELAXED(cache, res);
}
return cache;
return res;
}

static PyObject *
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