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.

1 files changed, 16 insertions, 3 deletions

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;
 }