code: func (rs *retryingSink) Close() error {
  	rs.mu.Lock()
  	defer rs.mu.Unlock()
  
  	if rs.closed {
  		return fmt.Errorf("retryingsink: already closed")
  	}
  
  	rs.closed = true
  	return rs.sink.Close()
  }
docs: // Close closes the sink and the underlying sink.