code: func (srv *Server) checkpoint(c *conn, stage chan<- *conn) error {
  	select {
  	case stage <- c:
  	case <-srv.quit:
  		return errServerStopped
  	}
  	select {
  	case err := <-c.cont:
  		return err
  	case <-srv.quit:
  		return errServerStopped
  	}
  }
docs: // checkpoint sends the conn to run, which performs the
  // post-handshake checks for the stage (posthandshake, addpeer).