[3.13] pythonGH-133543: Maintain tracking for materialized instance dictionaries (pythonGH-133617)

Author: brandtbucher

Lib/test/test_dict.py

@@ -1006,6 +1006,18 @@ class MyDict(dict):
             pass
         self._tracked(MyDict())
 
+    @support.cpython_only
+    def test_track_lazy_instance_dicts(self):
+        class C:
+            pass
+        o = C()
+        d = o.__dict__
+        self._not_tracked(d)
+        o.untracked = 42
+        self._not_tracked(d)
+        o.tracked = []
+        self._tracked(d)
+
     def make_shared_key_dict(self, n):
         class C:
             pass

Misc/NEWS.d/next/Core_and_Builtins/2025-05-07-10-48-31.gh-issue-133543.4jcszP.rst

@@ -0,0 +1,2 @@
+Fix a possible memory leak that could occur when directly accessing instance
+dictionaries (``__dict__``) that later become part of a reference cycle.

Objects/dictobject.c

@@ -6839,6 +6839,9 @@ store_instance_attr_lock_held(PyObject *obj, PyDictValues *values,
                                    value == NULL ? PyDict_EVENT_DELETED :
                                    PyDict_EVENT_MODIFIED);
         _PyDict_NotifyEvent(interp, event, dict, name, value);
+        if (value) {
+            MAINTAIN_TRACKING(dict, name, value);
+        }
     }
 
     FT_ATOMIC_STORE_PTR_RELEASE(values->values[ix], Py_XNewRef(value));