add unit test.

Author: LindaSummer

Lib/test/test_defaultdict.py

@@ -5,6 +5,7 @@
 import unittest
 
 from collections import defaultdict
+from threading import Condition, Thread
 
 def foobar():
     return list
@@ -185,6 +186,32 @@ def test_union(self):
 
         with self.assertRaises(TypeError):
             i |= None
+            
+    def test_default_race(self):
+        cv = Condition()
+        key = "default_race_key"
+        
+        def default_factory(cv: Condition, cv_flag: list[bool]):
+            with cv:
+                while not cv_flag[0]:
+                    cv.wait()
+                return "default_value"
+        ready_flag = [False]
+        test_dict = defaultdict(lambda: default_factory(cv, ready_flag))
+        
+        def writer(cv: Condition, cv_flag: list[bool], race_dict: dict):
+            with cv:
+                race_dict[key] = "writer_value"
+                cv_flag[0] = True
+                cv.notify()
+        
+        default_factory_thread = Thread(target=lambda: test_dict[key])
+        writer_thread = Thread(target=lambda: writer(cv, ready_flag, test_dict))
+        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()

Modules/_collectionsmodule.c

@@ -2231,18 +2231,16 @@ 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;
-    // }
     PyObject* result;
     int res = PyDict_SetDefaultRef(op, key, value, &result);
     if (res < 0) {
+        // set default ref with error
         PyObject *tup;
         tup = PyTuple_Pack(1, key);
         if (!tup) return NULL;
         PyErr_SetObject(PyExc_KeyError, tup);
         Py_DECREF(tup);
+        Py_DECREF(value);
     } else if (res > 0) {
         // Key was already in the dict
         Py_DECREF(value);