GH-95589: Dont crash when subclassing extension classes with multiple inheritance (GH-96028)

* Treat tp_weakref and tp_dictoffset like other opaque slots for multiple inheritance.

* Document Py_TPFLAGS_MANAGED_DICT and Py_TPFLAGS_MANAGED_WEAKREF in what's new.

Doc/whatsnew/3.12.rst

@@ -450,6 +450,11 @@ New Features
   inherit the ``Py_TPFLAGS_HAVE_VECTORCALL`` flag.
   (Contributed by Petr Viktorin in :gh:`93274`.)
 
+  The :const:`Py_TPFLAGS_MANAGED_DICT` and :const:`Py_TPFLAGS_MANAGED_WEAKREF`
+  flags have been added. This allows extensions classes to support object
+  ``__dict__`` and weakrefs with less bookkeeping,
+  using less memory and with faster access.
+
 Porting to Python 3.12
 ----------------------
 
@@ -486,6 +491,18 @@ Porting to Python 3.12
   :c:func:`PyUnicode_FromFormatV`.
   (Contributed by Philip Georgi in :gh:`95504`.)
 
+* Extension classes wanting to add a ``__dict__`` or weak reference slot
+  should use :const:`Py_TPFLAGS_MANAGED_DICT` and
+  :const:`Py_TPFLAGS_MANAGED_WEAKREF` instead of ``tp_dictoffset`` and
+  ``tp_weaklistoffset``, respectively.
+  The use of ``tp_dictoffset`` and ``tp_weaklistoffset`` is still
+  supported, but does not fully support multiple inheritance
+  (:gh: `95589`), and performance may be worse.
+  Classes declaring :const:`Py_TPFLAGS_MANAGED_DICT` should call
+  :c:func:`_PyObject_VisitManagedDict` and :c:func:`_PyObject_ClearManagedDict`
+  to traverse and clear their instance's dictionaries.
+  To clear weakrefs, call :c:func:`PyObject_ClearWeakRefs`, as before.
+
 Deprecated
 ----------
 

Lib/test/test_capi.py

@@ -677,21 +677,43 @@ class CAPITest(unittest.TestCase):
 
     def test_multiple_inheritance_ctypes_with_weakref_or_dict(self):
 
+        with self.assertRaises(TypeError):
             class Both1(_testcapi.HeapCTypeWithWeakref, _testcapi.HeapCTypeWithDict):
                 pass
+        with self.assertRaises(TypeError):
             class Both2(_testcapi.HeapCTypeWithDict, _testcapi.HeapCTypeWithWeakref):
                 pass
 
-        for cls in (_testcapi.HeapCTypeWithDict, _testcapi.HeapCTypeWithDict2,
-            _testcapi.HeapCTypeWithWeakref, _testcapi.HeapCTypeWithWeakref2):
-            for cls2 in (_testcapi.HeapCTypeWithDict, _testcapi.HeapCTypeWithDict2,
-                _testcapi.HeapCTypeWithWeakref, _testcapi.HeapCTypeWithWeakref2):
-                if cls is not cls2:
+    def test_multiple_inheritance_ctypes_with_weakref_or_dict_and_other_builtin(self):
+
+        with self.assertRaises(TypeError):
+            class C1(_testcapi.HeapCTypeWithDict, list):
+                pass
+
+        with self.assertRaises(TypeError):
+            class C2(_testcapi.HeapCTypeWithWeakref, list):
+                pass
+
+        class C3(_testcapi.HeapCTypeWithManagedDict, list):
+            pass
+        class C4(_testcapi.HeapCTypeWithManagedWeakref, list):
+            pass
+
+        inst = C3()
+        inst.append(0)
+        str(inst.__dict__)
+
+        inst = C4()
+        inst.append(0)
+        str(inst.__weakref__)
+
+        for cls in (_testcapi.HeapCTypeWithManagedDict, _testcapi.HeapCTypeWithManagedWeakref):
+            for cls2 in (_testcapi.HeapCTypeWithDict, _testcapi.HeapCTypeWithWeakref):
                 class S(cls, cls2):
                     pass
-            class B1(Both1, cls):
+            class B1(C3, cls):
                 pass
-            class B2(Both1, cls):
+            class B2(C4, cls):
                 pass
 
     def test_pytype_fromspec_with_repeated_slots(self):

Misc/NEWS.d/next/C API/2022-08-16-16-54-42.gh-issue-95589.6xE1ar.rst

@@ -0,0 +1,4 @@
+Extensions classes that set ``tp_dictoffset`` and ``tp_weaklistoffset``
+lose the support for multiple inheritance, but are now safe. Extension
+classes should use :const:`Py_TPFLAGS_MANAGED_DICT` and
+:const:`Py_TPFLAGS_MANAGED_WEAKREF` instead.

Objects/typeobject.c

@@ -2330,36 +2330,13 @@ best_base(PyObject *bases)
     return base;
 }
 
-#define ADDED_FIELD_AT_OFFSET(name, offset) \
-    (type->tp_ ## name  && (base->tp_ ##name == 0) && \
-    type->tp_ ## name + sizeof(PyObject *) == (offset) && \
-    type->tp_flags & Py_TPFLAGS_HEAPTYPE)
-
 static int
-extra_ivars(PyTypeObject *type, PyTypeObject *base)
+shape_differs(PyTypeObject *t1, PyTypeObject *t2)
 {
-    size_t t_size = type->tp_basicsize;
-    size_t b_size = base->tp_basicsize;
-
-    assert(t_size >= b_size); /* Else type smaller than base! */
-    if (type->tp_itemsize || base->tp_itemsize) {
-        /* If itemsize is involved, stricter rules */
-        return t_size != b_size ||
-            type->tp_itemsize != base->tp_itemsize;
-    }
-    /* Check for __dict__ and __weakrefs__ slots in either order */
-    if (ADDED_FIELD_AT_OFFSET(weaklistoffset, t_size)) {
-        t_size -= sizeof(PyObject *);
-    }
-    if ((type->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0 &&
-        ADDED_FIELD_AT_OFFSET(dictoffset, t_size)) {
-        t_size -= sizeof(PyObject *);
-    }
-    /* Check __weakrefs__ again, in case it precedes __dict__ */
-    if (ADDED_FIELD_AT_OFFSET(weaklistoffset, t_size)) {
-        t_size -= sizeof(PyObject *);
-    }
-    return t_size != b_size;
+    return (
+        t1->tp_basicsize != t2->tp_basicsize ||
+        t1->tp_itemsize != t2->tp_itemsize
+    );
 }
 
 static PyTypeObject *
@@ -2367,14 +2344,18 @@ solid_base(PyTypeObject *type)
 {
     PyTypeObject *base;
 
-    if (type->tp_base)
+    if (type->tp_base) {
         base = solid_base(type->tp_base);
-    else
+    }
+    else {
         base = &PyBaseObject_Type;
-    if (extra_ivars(type, base))
+    }
+    if (shape_differs(type, base)) {
         return type;
-    else
+    }
+    else {
         return base;
+    }
 }
 
 static void object_dealloc(PyObject *);