gh-142495 fix __missing__ data race condition

Lib/test/test_defaultdict.py

@@ -2,9 +2,12 @@
 
 import copy
 import pickle
+import threading
+import time
 import unittest
 
 from collections import defaultdict
+from test.support import threading_helper
 
 def foobar():
     return list
@@ -186,5 +189,62 @@ def test_union(self):
         with self.assertRaises(TypeError):
             i |= None
 
+    @threading_helper.requires_working_threading()
+    def test_no_value_overwrite_race_condition(self):
+        call_count = 0
+
+        def unique_factory():
+            nonlocal call_count
+            call_count += 1
+            return f"value_{call_count}_{threading.get_ident()}"
+
+        d = defaultdict(unique_factory)
+        results = {}
+
+        def worker(thread_id):
+            for _ in range(5):
+                value = d['shared_key']
+                if 'shared_key' not in results:
+                    results['shared_key'] = value
+                time.sleep(0.001)
+
+        threads = []
+        for i in range(3):
+            t = threading.Thread(target=worker, args=(i,))
+            threads.append(t)
+            t.start()
+
+        for t in threads:
+            t.join()
+
+        self.assertIn('shared_key', d)
+
+        final_value = d['shared_key']
+        self.assertEqual(results['shared_key'], final_value)
+
+        self.assertTrue(final_value.startswith('value_'))
+
+        self.assertEqual(call_count, 1)
+
+    def test_factory_called_only_when_key_missing(self):
+        factory_calls = []
+
+        def tracked_factory():
+            factory_calls.append(threading.get_ident())
+            return [1, 2, 3]
+
+        d = defaultdict(tracked_factory)
+
+        value1 = d['key']
+        self.assertEqual(value1, [1, 2, 3])
+        initial_call_count = len(factory_calls)
+
+        for _ in range(10):
+            value = d['key']
+            self.assertEqual(value, [1, 2, 3])
+
+        self.assertEqual(len(factory_calls), initial_call_count)
+
+
 if __name__ == "__main__":
     unittest.main()

Modules/_collectionsmodule.c

@@ -2228,14 +2228,23 @@ defdict_missing(PyObject *op, PyObject *key)
         Py_DECREF(tup);
         return NULL;
     }
+
     value = _PyObject_CallNoArgs(factory);
     if (value == NULL)
-        return value;
-    if (PyObject_SetItem(op, key, value) < 0) {
-        Py_DECREF(value);
         return NULL;
+
+    /* Use PyDict_SetDefaultRef to atomically insert the value only if the key is absent.
+     * This ensures we don't overwrite a value that another thread inserted
+     * between the factory call and this insertion.
+     */
+    PyObject *result_value = NULL;
+    int res = PyDict_SetDefaultRef(op, key, value, &result_value);
+
+    if (res != 0) {
+        Py_DECREF(value);
     }
-    return value;
+
+    return result_value;
 }
 
 static inline PyObject*