bpo-41295: Reimplement the Carlo Verre "hackcheck" (GH-21528)

Walk down the MRO backwards to find the type that originally defined the final `tp_setattro`, then make sure we are not jumping over intermediate C-level bases with the Python-level call.

Automerge-Triggered-By: @gvanrossum
(cherry picked from commit c53b310)

Co-authored-by: scoder <stefan_ml@behnel.de>

Author: miss-islington

Lib/test/test_descr.py

@@ -4315,6 +4315,42 @@ def test_carloverre(self):
         else:
             self.fail("Carlo Verre __delattr__ succeeded!")
 
+    def test_carloverre_multi_inherit_valid(self):
+        class A(type):
+            def __setattr__(cls, key, value):
+                type.__setattr__(cls, key, value)
+
+        class B:
+            pass
+
+        class C(B, A):
+            pass
+
+        obj = C('D', (object,), {})
+        try:
+            obj.test = True
+        except TypeError:
+            self.fail("setattr through direct base types should be legal")
+
+    def test_carloverre_multi_inherit_invalid(self):
+        class A(type):
+            def __setattr__(cls, key, value):
+                object.__setattr__(cls, key, value)  # this should fail!
+
+        class B:
+            pass
+
+        class C(B, A):
+            pass
+
+        obj = C('D', (object,), {})
+        try:
+            obj.test = True
+        except TypeError:
+            pass
+        else:
+            self.fail("setattr through indirect base types should be rejected")
+
     def test_weakref_segfault(self):
         # Testing weakref segfault...
         # SF 742911

Misc/NEWS.d/next/Core and Builtins/2020-07-18-08-15-32.bpo-41295.pu8Ezo.rst

@@ -0,0 +1,3 @@
+Resolve a regression in CPython 3.8.4 where defining "__setattr__" in a
+multi-inheritance setup and calling up the hierarchy chain could fail
+if builtins/extension types were involved in the base types.

Objects/typeobject.c

@@ -5955,14 +5955,29 @@ hackcheck(PyObject *self, setattrofunc func, const char *what)
         return 1;
     }
     assert(PyTuple_Check(mro));
-    Py_ssize_t i, n;
-    n = PyTuple_GET_SIZE(mro);
-    for (i = 0; i < n; i++) {
+
+    /* Find the (base) type that defined the type's slot function. */
+    PyTypeObject *defining_type = type;
+    Py_ssize_t i;
+    for (i = PyTuple_GET_SIZE(mro) - 1; i >= 0; i--) {
         PyTypeObject *base = (PyTypeObject*) PyTuple_GET_ITEM(mro, i);
+        if (base->tp_setattro == slot_tp_setattro) {
+            /* Ignore Python classes:
+               they never define their own C-level setattro. */
+        }
+        else if (base->tp_setattro == type->tp_setattro) {
+            defining_type = base;
+            break;
+        }
+    }
+
+    /* Reject calls that jump over intermediate C-level overrides. */
+    for (PyTypeObject *base = defining_type; base; base = base->tp_base) {
         if (base->tp_setattro == func) {
-            /* 'func' is the earliest non-Python implementation in the MRO. */
+            /* 'func' is the right slot function to call. */
             break;
-        } else if (base->tp_setattro != slot_tp_setattro) {
+        }
+        else if (base->tp_setattro != slot_tp_setattro) {
             /* 'base' is not a Python class and overrides 'func'.
                Its tp_setattro should be called instead. */
             PyErr_Format(PyExc_TypeError,
@@ -5972,8 +5987,6 @@ hackcheck(PyObject *self, setattrofunc func, const char *what)
             return 0;
         }
     }
-    /* Either 'func' is not in the mro (which should fail when checking 'self'),
-       or it's the right slot function to call. */
     return 1;
 }