gh-130327: Always traverse managed dictionaries, even when inline values are available (#130469)

Author: ZeroIntensity

Lib/test/test_dict.py

@@ -1569,6 +1569,26 @@ def make_pairs():
                 self.assertEqual(d.get(key3_3), 44)
                 self.assertGreaterEqual(eq_count, 1)
 
+    def test_overwrite_managed_dict(self):
+        # GH-130327: Overwriting an object's managed dictionary with another object's
+        # skipped traversal in favor of inline values, causing the GC to believe that
+        # the __dict__ wasn't reachable.
+        import gc
+
+        class Shenanigans:
+            pass
+
+        to_be_deleted = Shenanigans()
+        to_be_deleted.attr = "whatever"
+        holds_reference = Shenanigans()
+        holds_reference.__dict__ = to_be_deleted.__dict__
+        holds_reference.ref = {"circular": to_be_deleted, "data": 42}
+
+        del to_be_deleted
+        gc.collect()
+        self.assertEqual(holds_reference.ref['data'], 42)
+        self.assertEqual(holds_reference.attr, "whatever")
+
     def test_unhashable_key(self):
         d = {'a': 1}
         key = [1, 2, 3]

Misc/NEWS.d/next/Core_and_Builtins/2025-02-19-21-06-30.gh-issue-130327.z3TaR8.rst

@@ -0,0 +1,2 @@
+Fix erroneous clearing of an object's :attr:`~object.__dict__` if
+overwritten at runtime.

Objects/dictobject.c

@@ -4825,10 +4825,8 @@ dict_traverse(PyObject *op, visitproc visit, void *arg)
 
     if (DK_IS_UNICODE(keys)) {
         if (_PyDict_HasSplitTable(mp)) {
-            if (!mp->ma_values->embedded) {
-                for (i = 0; i < n; i++) {
-                    Py_VISIT(mp->ma_values->values[i]);
-                }
+            for (i = 0; i < n; i++) {
+                Py_VISIT(mp->ma_values->values[i]);
             }
         }
         else {
@@ -7413,16 +7411,21 @@ PyObject_VisitManagedDict(PyObject *obj, visitproc visit, void *arg)
     if((tp->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0) {
         return 0;
     }
-    if (tp->tp_flags & Py_TPFLAGS_INLINE_VALUES) {
+    PyDictObject *dict = _PyObject_ManagedDictPointer(obj)->dict;
+    if (dict != NULL) {
+        // GH-130327: If there's a managed dictionary available, we should
+        // *always* traverse it. The dict is responsible for traversing the
+        // inline values if it points to them.
+        Py_VISIT(dict);
+    }
+    else if (tp->tp_flags & Py_TPFLAGS_INLINE_VALUES) {
         PyDictValues *values = _PyObject_InlineValues(obj);
         if (values->valid) {
             for (Py_ssize_t i = 0; i < values->capacity; i++) {
                 Py_VISIT(values->values[i]);
             }
-            return 0;
         }
     }
-    Py_VISIT(_PyObject_ManagedDictPointer(obj)->dict);
     return 0;
 }