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, 5 deletions

diff --git a/Python/errors.c b/Python/errors.c
index d74ac34..6a42f59 100644
--- a/Python/errors.c
+++ b/Python/errors.c
@@ -353,16 +353,13 @@ _PyErr_NormalizeException(PyThreadState *tstate, PyObject **exc,
             if (fixed_value == NULL) {
                 goto error;
             }
-            Py_DECREF(value);
-            value = fixed_value;
+            Py_SETREF(value, fixed_value);
         }
         /* If the class of the instance doesn't exactly match the
            class of the type, believe the instance.
         */
         else if (inclass != type) {
-            Py_INCREF(inclass);
-            Py_DECREF(type);
-            type = inclass;
+            Py_SETREF(type, Py_NewRef(inclass));
         }
     }
     *exc = type;