code: func (s *Server) shutdown() error {
  	s.shutdownLock.Lock()
  	defer s.shutdownLock.Unlock()
  
  	s.unregister()
  
  	if s.shouldShutdown {
  		return nil
  	}
  	s.shouldShutdown = true
  
  	if s.ipv4conn != nil {
  		s.ipv4conn.Close()
  	}
  	if s.ipv6conn != nil {
  		s.ipv6conn.Close()
  	}
  	return nil
  }
docs: // Shutdown server will close currently open connections & channel