Issue #20440: Massive replacing unsafe attribute setting code with special

macro Py_SETREF.

Include/object.h

@@ -846,6 +846,32 @@ PyAPI_FUNC(void) _Py_Dealloc(PyObject *);
             Py_DECREF(_py_xdecref_tmp);               \
     } while (0)
 
+#ifndef Py_LIMITED_API
+/* Safely decref `op` and set `op` to `op2`.
+ *
+ * As in case of Py_CLEAR "the obvious" code can be deadly:
+ *
+ *     Py_XDECREF(op);
+ *     op = op2;
+ *
+ * The safe way is:
+ *
+ *      Py_SETREF(op, op2);
+ *
+ * That arranges to set `op` to `op2` _before_ decref'ing, so that any code
+ * triggered as a side-effect of `op` getting torn down no longer believes
+ * `op` points to a valid object.
+ */
+
+#define Py_SETREF(op, op2)                      \
+    do {                                        \
+        PyObject *_py_tmp = (PyObject *)(op);   \
+        (op) = (op2);                           \
+        Py_XDECREF(_py_tmp);                    \
+    } while (0)
+
+#endif /* ifndef Py_LIMITED_API */
+
 /*
 These are provided as conveniences to Python runtime embedders, so that
 they can have object code that is not dependent on Python compilation flags.