bpo-38140: Make dict and weakref offsets opaque for C heap types (#16076)

* Make dict and weakref offsets opaque for C heap types

* Add news

1 files changed, 25 insertions, 2 deletions

diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index dfdac9e..94a4da2 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -2853,15 +2853,27 @@ PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
     PyTypeObject *type, *base;
 
     PyType_Slot *slot;
-    Py_ssize_t nmembers;
+    Py_ssize_t nmembers, weaklistoffset, dictoffset;
     char *s, *res_start;
 
-    nmembers = 0;
+    nmembers = weaklistoffset = dictoffset = 0;
     for (slot = spec->slots; slot->slot; slot++) {
         if (slot->slot == Py_tp_members) {
             nmembers = 0;
             for (memb = slot->pfunc; memb->name != NULL; memb++) {
                 nmembers++;
+                if (strcmp(memb->name, "__weaklistoffset__") == 0) {
+                    // The PyMemberDef must be a Py_ssize_t and readonly
+                    assert(memb->type == T_PYSSIZET);
+                    assert(memb->flags == READONLY);
+                    weaklistoffset = memb->offset;
+                }
+                if (strcmp(memb->name, "__dictoffset__") == 0) {
+                    // The PyMemberDef must be a Py_ssize_t and readonly
+                    assert(memb->type == T_PYSSIZET);
+                    assert(memb->flags == READONLY);
+                    dictoffset = memb->offset;
+                }
             }
         }
     }
@@ -2990,6 +3002,17 @@ PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
         res->ht_cached_keys = _PyDict_NewKeysForClass();
     }
 
+    if (weaklistoffset) {
+        type->tp_weaklistoffset = weaklistoffset;
+        if (PyDict_DelItemString((PyObject *)type->tp_dict, "__weaklistoffset__") < 0)
+            goto fail;
+    }
+    if (dictoffset) {
+        type->tp_dictoffset = dictoffset;
+        if (PyDict_DelItemString((PyObject *)type->tp_dict, "__dictoffset__") < 0)
+            goto fail;
+    }
+
     /* Set type.__module__ */
     s = strrchr(spec->name, '.');
     if (s != NULL) {