[3.15] gh-159458: Fix free-threading data race in datetime strftime century normalization (GH-159520)

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()

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.

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 *