code: def check_channel_subsystem_request(self, channel, name):
          
          transport = channel.get_transport()
          handler_class, larg, kwarg = transport._get_subsystem_handler(name)
          if handler_class is None:
              return False
          handler = handler_class(channel, name, self, *larg, **kwarg)
          handler.start()
          return True
docs: Determine if a requested subsystem will be provided to the client on
          the given channel.  If this method returns ``True``, all future I/O
          through this channel will be assumed to be connected to the requested
          subsystem.  An example of a subsystem is ``sftp``.
  
          The default implementation checks for a subsystem handler assigned via
          `.Transport.set_subsystem_handler`.
          If one has been set, the handler is invoked and this method returns
          ``True``.  Otherwise it returns ``False``.
  
          .. note:: Because the default implementation uses the `.Transport` to
              identify valid subsystems, you probably won't need to override this
              method.
  
          :param .Channel channel: the `.Channel` the pty request arrived on.
          :param str name: name of the requested subsystem.
          :return:
              ``True`` if this channel is now hooked up to the requested
              subsystem; ``False`` if that subsystem can't or won't be provided.