code: def _gssapi_authenticate(token):
      state = None
      ctx = stack.top
      try:
          return_code, state = kerberos.authGSSServerInit(
              _KERBEROS_SERVICE.service_name
          )
          if return_code != kerberos.AUTH_GSS_COMPLETE:
              return None
          return_code = kerberos.authGSSServerStep(state, token)
          if return_code == kerberos.AUTH_GSS_COMPLETE:
              ctx.kerberos_token = kerberos.authGSSServerResponse(state)
              ctx.kerberos_user = kerberos.authGSSServerUserName(state)
              return return_code
          if return_code == kerberos.AUTH_GSS_CONTINUE:
              return kerberos.AUTH_GSS_CONTINUE
          return None
      except kerberos.GSSError:
          return None
      finally:
          if state:
              kerberos.authGSSServerClean(state)
  
file_path: airflow/airflow/api/auth/backend/kerberos_auth.py
imports: from __future__ import annotations
  import warnings
  from airflow.exceptions import RemovedInAirflow3Warning
  from airflow.utils.airflow_flask_app import get_airflow_app
  import logging
  import os
  from functools import wraps
  from typing import TYPE_CHECKING, Any, Callable, TypeVar, cast
  import kerberos
  from flask import Response, _request_ctx_stack, g, make_response, request
  from requests_kerberos import HTTPKerberosAuth
  from airflow.configuration import conf
  from airflow.utils.net import getfqdn
label_desc: Use image node_id 4 for calling a global function with example usage: _gssapi_authenticate(token) and returns: None, None, return_code, kerberos, None