add code and simple test case.

Author: LindaSummer

Modules/_collectionsmodule.c

@@ -2231,11 +2231,23 @@ defdict_missing(PyObject *op, PyObject *key)
     value = _PyObject_CallNoArgs(factory);
     if (value == NULL)
         return value;
-    if (PyObject_SetItem(op, key, value) < 0) {
+    // 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) {
+        PyObject *tup;
+        tup = PyTuple_Pack(1, key);
+        if (!tup) return NULL;
+        PyErr_SetObject(PyExc_KeyError, tup);
+        Py_DECREF(tup);
+    } else if (res > 0) {
+        // Key was already in the dict
         Py_DECREF(value);
-        return NULL;
     }
-    return value;
+    return result;
 }
 
 static inline PyObject*

edward_defaultdict_test.py

@@ -0,0 +1,39 @@
+from collections import defaultdict
+from threading import Barrier, Condition, Thread
+
+
+def test():
+    cv = Condition()
+
+    key = 'my-key'
+    setting_ready_flag = False
+
+    def default_factory():
+        with cv:
+            while not setting_ready_flag:
+                print("Waiting for value to be set...")
+                cv.wait()
+            print("missing value is set, returning it.")
+            return "default"
+
+    mydict = defaultdict(default_factory)
+
+    def writer():
+        with cv:
+            nonlocal setting_ready_flag
+            mydict[key] = "setted value"
+            setting_ready_flag = True
+            print("Value set, notifying all waiting threads.")
+            cv.notify()
+        
+    default_fact_thread = Thread(target=lambda: mydict[key])
+    writer_thread = Thread(target=writer)
+    default_fact_thread.start()
+    writer_thread.start()
+
+    default_fact_thread.join()
+    writer_thread.join()
+
+    print(mydict.items())  # Should print "setted value" but actually "default"
+    
+test()