docs: def _main_ctxmgr(func):
      '''
      A decorator wrapper for :class:`ServerMainContextManager`
  
      Usage example:
  
      .. code:: python
  
         @aiotools.main
         def mymain():
             server_args = do_init()
             stop_sig = yield server_args
             if stop_sig == signal.SIGINT:
                 do_graceful_shutdown()
             else:
                 do_forced_shutdown()
  
         aiotools.start_server(..., main_ctxmgr=mymain, ...)
      '''
      @functools.wraps(func)
      def helper(*args, **kwargs):
          return ServerMainContextManager(func, args, kwargs)
      return helper