[3.12] GH-117195: Avoid assertion error in object.__sizeof__ (GH-117220) (#127605)

GH-117195: Avoid assertion error in `object.__sizeof__` (GH-117220)
(cherry picked from commit 406ffb5)

Co-authored-by: Mark Shannon <[email protected]>

Author: miss-islington

Lib/test/test_long.py

@@ -1639,6 +1639,8 @@ class MyInt(int):
                     MyInt.__basicsize__ + MyInt.__itemsize__ * ndigits
                 )
 
+        # GH-117195 -- This shouldn't crash
+        object.__sizeof__(1)
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2024-03-25-15-07-01.gh-issue-117195.OWakgD.rst

@@ -0,0 +1,2 @@
+Avoid assertion failure for debug builds when calling
+``object.__sizeof__(1)``

Objects/typeobject.c

@@ -6462,8 +6462,11 @@ object___sizeof___impl(PyObject *self)
 
     res = 0;
     isize = Py_TYPE(self)->tp_itemsize;
-    if (isize > 0)
-        res = Py_SIZE(self) * isize;
+    if (isize > 0) {
+        /* This assumes that ob_size is valid if tp_itemsize is not 0,
+         which isn't true for PyLongObject. */
+        res = _PyVarObject_CAST(self)->ob_size * isize;
+    }
     res += Py_TYPE(self)->tp_basicsize;
 
     return PyLong_FromSsize_t(res);