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/traceback.c b/Python/traceback.c
index 356e643..da26c9b 100644
--- a/Python/traceback.c
+++ b/Python/traceback.c
@@ -136,9 +136,7 @@ tb_next_set(PyTracebackObject *self, PyObject *new_next, void *Py_UNUSED(_))
         cursor = cursor->tb_next;
     }
 
-    PyObject *old_next = (PyObject*)self->tb_next;
-    self->tb_next = (PyTracebackObject *)Py_XNewRef(new_next);
-    Py_XDECREF(old_next);
+    Py_XSETREF(self->tb_next, (PyTracebackObject *)Py_XNewRef(new_next));
 
     return 0;
 }
@@ -533,8 +531,7 @@ display_source_line_with_margin(PyObject *f, PyObject *filename, int lineno, int
         PyObject *truncated;
         truncated = PyUnicode_Substring(lineobj, i, PyUnicode_GET_LENGTH(lineobj));
         if (truncated) {
-            Py_DECREF(lineobj);
-            lineobj = truncated;
+            Py_SETREF(lineobj, truncated);
         } else {
             PyErr_Clear();
         }