code: func (wf *WriteFlusher) Close() error {
  	wf.closeLock.Lock()
  	defer wf.closeLock.Unlock()
  
  	select {
  	case <-wf.closed:
  		return errWriteFlusherClosed
  	default:
  		close(wf.closed)
  	}
  	return nil
  }
docs: // Close closes the write flusher, disallowing any further writes to the
  // target. After the flusher is closed, all calls to write or flush will
  // result in an error.