bpo-44184: Apply GH-26274 to the non-GC-type branch of subtype_dealloc (GH-27165) (GH-27174)

The non-GC-type branch of subtype_dealloc is using the type of an object after freeing in the same unsafe way as GH-26274 fixes. (I believe the old news entry covers this change well enough.)

https://bugs.python.org/issue44184
(cherry picked from commit 074e765)

Co-authored-by: T. Wouters <[email protected]>

Author: miss-islington

Objects/typeobject.c

@@ -1344,14 +1344,22 @@ subtype_dealloc(PyObject *self)
         /* Extract the type again; tp_del may have changed it */
         type = Py_TYPE(self);
 
+        // Don't read type memory after calling basedealloc() since basedealloc()
+        // can deallocate the type and free its memory.
+        int type_needs_decref = (type->tp_flags & Py_TPFLAGS_HEAPTYPE
+                                 && !(base->tp_flags & Py_TPFLAGS_HEAPTYPE));
+
         /* Call the base tp_dealloc() */
         assert(basedealloc);
         basedealloc(self);
 
-       /* Only decref if the base type is not already a heap allocated type.
-          Otherwise, basedealloc should have decref'd it already */
-        if (type->tp_flags & Py_TPFLAGS_HEAPTYPE && !(base->tp_flags & Py_TPFLAGS_HEAPTYPE))
+        /* Can't reference self beyond this point. It's possible tp_del switched
+           our type from a HEAPTYPE to a non-HEAPTYPE, so be careful about
+           reference counting. Only decref if the base type is not already a heap
+           allocated type. Otherwise, basedealloc should have decref'd it already */
+        if (type_needs_decref) {
             Py_DECREF(type);
+        }
 
         /* Done */
         return;