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, 1 insertions, 2 deletions

diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index 70872222eb..35292b6478 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -718,8 +718,7 @@ _Py_HandleSystemExit(int *exitcode_p)
         /* The error code should be in the `code' attribute. */
         PyObject *code = PyObject_GetAttr(value, &_Py_ID(code));
         if (code) {
-            Py_DECREF(value);
-            value = code;
+            Py_SETREF(value, code);
             if (value == Py_None)
                 goto done;
         }