fix reference count leak

Author: LindaSummer

Lib/test/test_defaultdict.py

@@ -5,7 +5,6 @@
 import unittest
 
 from collections import defaultdict
-from threading import Condition, Thread
 
 def foobar():
     return list
@@ -187,31 +186,5 @@ 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()

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()

Modules/_collectionsmodule.c

@@ -2232,12 +2232,12 @@ defdict_missing(PyObject *op, PyObject *key)
     if (value == NULL)
         return value;
     PyObject* result;
-    int res = PyDict_SetDefaultRef(op, key, value, &result);
-    if (res != 0) {
-        // when res < 0, result will be NULL
-        // when res > 0, result is a new reference to the existing value
-        Py_DECREF(value);
-    }
+    PyDict_SetDefaultRef(op, key, value, &result);
+    // when PyDict_SetDefaultRef() < 0, result will be NULL
+    // when PyDict_SetDefaultRef() == 0, result is a new reference of default value
+    // when PyDict_SetDefaultRef() > 0, result is a new reference to the existing value
+    // so the value reference must be decref'ed in all cases
+    Py_DECREF(value);
     return result;
 }