From 4e0750d272cb7fbe457605b5b0c18f2b7cd578fb Mon Sep 17 00:00:00 2001
From: Anna <araslanova.anna.a@gmail.com>
Date: Fri, 16 Oct 2026 16:16:09 +0200
Subject: [PATCH] gh-159458: Fix free-threading data race in datetime strftime
 century normalization (GH-159520) (cherry picked from commit
 ddc647a3410f05e99b2c65d40283e4ad6d903bef)

Co-authored-by: Anna <araslanova.anna.a@gmail.com>
---
 Lib/test/test_free_threading/test_datetime.py | 25 +++++++++++++++++++
 ...-10-16-10-25-22.gh-issue-159458.q02EnY.rst |  1 +
 Modules/_datetimemodule.c                     | 10 +++++---
 3 files changed, 32 insertions(+), 4 deletions(-)
 create mode 100644 Lib/test/test_free_threading/test_datetime.py
 create mode 100644 Misc/NEWS.d/next/Library/2026-10-16-10-25-22.gh-issue-159458.q02EnY.rst

diff --git a/Lib/test/test_free_threading/test_datetime.py b/Lib/test/test_free_threading/test_datetime.py
new file mode 100644
index 00000000000000..6cea76b32d9ffc
--- /dev/null
+++ b/Lib/test/test_free_threading/test_datetime.py
@@ -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):
+        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()
diff --git a/Misc/NEWS.d/next/Library/2026-10-16-10-25-22.gh-issue-159458.q02EnY.rst b/Misc/NEWS.d/next/Library/2026-10-16-10-25-22.gh-issue-159458.q02EnY.rst
new file mode 100644
index 00000000000000..6565f4265b5877
--- /dev/null
+++ b/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.
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index d0f2e8c7dcabe4..fc07b4b7bc01fb 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -1780,17 +1780,19 @@ 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 *