gh-142495: make defaultdict keep existed value when racing with __missing__

Lib/test/test_free_threading/test_defaultdict.py

@@ -0,0 +1,43 @@
+import unittest
+from collections import defaultdict
+
+from threading import Barrier, Thread
+from unittest import TestCase
+
+try:
+    import _testcapi
+except ImportError:
+    _testcapi = None
+
+from test.support import threading_helper
+
+@threading_helper.requires_working_threading()
+class TestDefaultDict(TestCase):
+    def test_default_factory_race(self):
+        wait_barrier = Barrier(2)
+        write_barrier = Barrier(2)
+        key = "default_race_key"
+
+        def default_factory():
+            wait_barrier.wait()
+            write_barrier.wait()
+            return "default_value"
+
+        test_dict = defaultdict(default_factory)
+
+        def writer():
+            wait_barrier.wait()
+            test_dict[key] = "writer_value"
+            write_barrier.wait()
+
+        default_factory_thread = Thread(target=lambda: test_dict[key])
+        writer_thread = Thread(target=writer)
+        default_factory_thread.start()
+        writer_thread.start()
+        default_factory_thread.join()
+        writer_thread.join()
+        self.assertEqual(test_dict[key], "writer_value")
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Library/2025-12-13-23-26-42.gh-issue-142495.I88Uv_.rst

@@ -0,0 +1,3 @@
+:class:`collections.defaultdict` now prioritizes :meth:`~object.__setitem__`
+when inserting default values from ``default_factory``. This prevents race
+conditions where a default value would overwrite a value set by another thread.

Modules/_collectionsmodule.c

@@ -2231,11 +2231,11 @@ defdict_missing(PyObject *op, PyObject *key)
     value = _PyObject_CallNoArgs(factory);
     if (value == NULL)
         return value;
-    if (PyObject_SetItem(op, key, value) < 0) {
-        Py_DECREF(value);
-        return NULL;
-    }
-    return value;
+    PyObject *result = NULL;
+    (void)PyDict_SetDefaultRef(op, key, value, &result);
+    // 'result' is NULL, or a strong reference to 'value' or 'op[key]'
+    Py_DECREF(value);
+    return result;
 }
 
 static inline PyObject*