code: func (s *Server) Available(stop <-chan struct{}) {
  	s.cur = s.api
  	<-stop
  	s.cur = unavailable
  }
docs: // Available switches the Server's HTTP handler from a generic 503 Unavailable
  // response to the actual API. Once the provided channel is closed, the API is
  // torn back down and 503 responses are served.