code: func (b *Backend) Close() error {
  	log.Info("gateway/gcp_pub_sub: closing backend")
  	b.cancel()
  	close(b.uplinkFrameChan)
  	close(b.gatewayStatsChan)
  	close(b.downlinkTXAckChan)
  	return b.client.Close()
  }
docs: // Close closes the backend.