gh-99537: Use Py_SETREF() function in C code (#99657)

Fix potential race condition in code patterns:

* Replace "Py_DECREF(var); var = new;" with "Py_SETREF(var, new);"
* Replace "Py_XDECREF(var); var = new;" with "Py_XSETREF(var, new);"
* Replace "Py_CLEAR(var); var = new;" with "Py_XSETREF(var, new);"

Other changes:

* Replace "old = var; var = new; Py_DECREF(var)"
  with "Py_SETREF(var, new);"
* Replace "old = var; var = new; Py_XDECREF(var)"
  with "Py_XSETREF(var, new);"
* And remove the "old" variable.

1 files changed, 2 insertions, 4 deletions

diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 9d868d51f0..312406993c 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -5968,8 +5968,7 @@ object___dir___impl(PyObject *self)
     else {
         /* Copy __dict__ to avoid mutating it. */
         PyObject *temp = PyDict_Copy(dict);
-        Py_DECREF(dict);
-        dict = temp;
+        Py_SETREF(dict, temp);
     }
 
     if (dict == NULL)
@@ -9377,8 +9376,7 @@ super_getattro(PyObject *self, PyObject *name)
                        (See SF ID #743627)  */
                     (su->obj == (PyObject *)starttype) ? NULL : su->obj,
                     (PyObject *)starttype);
-                Py_DECREF(res);
-                res = res2;
+                Py_SETREF(res, res2);
             }
 
             Py_DECREF(mro);