code: func (n *Notifier) activate() error {
  	n.mu.Lock()
  	defer n.mu.Unlock()
  
  	for _, data := range n.buffer {
  		if err := n.send(n.sub, data); err != nil {
  			return err
  		}
  	}
  	n.activated = true
  	return nil
  }
docs: // acticate is called after the subscription ID was sent to client. Notifications are
  // buffered before activation. This prevents notifications being sent to the client before
  // the subscription ID is sent to the client.