GH-92678: Fix tp_dictoffset inheritance. (GH-95596)

* Add test for inheriting explicit __dict__ and weakref.

* Restore 3.10 behavior for multiple inheritance of C extension classes that store their dictionary at the end of the struct.

4 files changed, 65 insertions, 3 deletions

diff --git a/Lib/test/test_capi.py b/Lib/test/test_capi.py
index a88a17d..013229a 100644
--- a/Lib/test/test_capi.py
+++ b/Lib/test/test_capi.py
@@ -619,6 +619,25 @@ class CAPITest(unittest.TestCase):
         with self.assertRaisesRegex(TypeError, msg):
             t = _testcapi.pytype_fromspec_meta(_testcapi.HeapCTypeMetaclassCustomNew)
 
+    def test_multiple_inheritance_ctypes_with_weakref_or_dict(self):
+
+        class Both1(_testcapi.HeapCTypeWithWeakref, _testcapi.HeapCTypeWithDict):
+            pass
+        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:
+                    class S(cls, cls2):
+                        pass
+            class B1(Both1, cls):
+                pass
+            class B2(Both1, cls):
+                pass
+
     def test_pytype_fromspec_with_repeated_slots(self):
         for variant in range(2):
             with self.subTest(variant=variant):
diff --git a/Misc/NEWS.d/next/C API/2022-08-03-14-39-08.gh-issue-92678.ozFTEx.rst b/Misc/NEWS.d/next/C API/2022-08-03-14-39-08.gh-issue-92678.ozFTEx.rst
new file mode 100644
index 0000000..6bf3d4b
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2022-08-03-14-39-08.gh-issue-92678.ozFTEx.rst
@@ -0,0 +1,2 @@
+Restore the 3.10 behavior for multiple inheritance of C extension classes
+that store their dictionary at the end of the struct.
diff --git a/Modules/_testcapi/heaptype.c b/Modules/_testcapi/heaptype.c
index 9fb0051..12889e8 100644
--- a/Modules/_testcapi/heaptype.c
+++ b/Modules/_testcapi/heaptype.c
@@ -737,6 +737,14 @@ static PyType_Spec HeapCTypeWithDict_spec = {
     HeapCTypeWithDict_slots
 };
 
+static PyType_Spec HeapCTypeWithDict2_spec = {
+    "_testcapi.HeapCTypeWithDict2",
+    sizeof(HeapCTypeWithDictObject),
+    0,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    HeapCTypeWithDict_slots
+};
+
 static struct PyMemberDef heapctypewithnegativedict_members[] = {
     {"dictobj", T_OBJECT, offsetof(HeapCTypeWithDictObject, dict)},
     {"__dictoffset__", T_PYSSIZET, -(Py_ssize_t)sizeof(void*), READONLY},
@@ -796,6 +804,14 @@ static PyType_Spec HeapCTypeWithWeakref_spec = {
     HeapCTypeWithWeakref_slots
 };
 
+static PyType_Spec HeapCTypeWithWeakref2_spec = {
+    "_testcapi.HeapCTypeWithWeakref2",
+    sizeof(HeapCTypeWithWeakrefObject),
+    0,
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    HeapCTypeWithWeakref_slots
+};
+
 PyDoc_STRVAR(heapctypesetattr__doc__,
 "A heap type without GC, but with overridden __setattr__.\n\n"
 "The 'value' attribute is set to 10 in __init__ and updated via attribute setting.");
@@ -919,6 +935,12 @@ _PyTestCapi_Init_Heaptype(PyObject *m) {
     }
     PyModule_AddObject(m, "HeapCTypeWithDict", HeapCTypeWithDict);
 
+    PyObject *HeapCTypeWithDict2 = PyType_FromSpec(&HeapCTypeWithDict2_spec);
+    if (HeapCTypeWithDict2 == NULL) {
+        return -1;
+    }
+    PyModule_AddObject(m, "HeapCTypeWithDict2", HeapCTypeWithDict2);
+
     PyObject *HeapCTypeWithNegativeDict = PyType_FromSpec(&HeapCTypeWithNegativeDict_spec);
     if (HeapCTypeWithNegativeDict == NULL) {
         return -1;
@@ -931,6 +953,12 @@ _PyTestCapi_Init_Heaptype(PyObject *m) {
     }
     PyModule_AddObject(m, "HeapCTypeWithWeakref", HeapCTypeWithWeakref);
 
+    PyObject *HeapCTypeWithWeakref2 = PyType_FromSpec(&HeapCTypeWithWeakref2_spec);
+    if (HeapCTypeWithWeakref2 == NULL) {
+        return -1;
+    }
+    PyModule_AddObject(m, "HeapCTypeWithWeakref2", HeapCTypeWithWeakref2);
+
     PyObject *HeapCTypeWithBuffer = PyType_FromSpec(&HeapCTypeWithBuffer_spec);
     if (HeapCTypeWithBuffer == NULL) {
         return -1;
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index d33befc0..1980fcb 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -2316,6 +2316,11 @@ 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)
 {
@@ -2328,10 +2333,18 @@ extra_ivars(PyTypeObject *type, PyTypeObject *base)
         return t_size != b_size ||
             type->tp_itemsize != base->tp_itemsize;
     }
-    if (type->tp_weaklistoffset && base->tp_weaklistoffset == 0 &&
-        type->tp_weaklistoffset + sizeof(PyObject *) == t_size &&
-        type->tp_flags & Py_TPFLAGS_HEAPTYPE)
+    /* 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;
 }