code: func (e *Engine) Close() error {
  	e.SetCompactionsEnabled(false)
  
  	// Lock now and close everything else down.
  	e.mu.Lock()
  	defer e.mu.Unlock()
  	e.done = nil // Ensures that the channel will not be closed again.
  
  	if err := e.FileStore.Close(); err != nil {
  		return err
  	}
  	return e.WAL.Close()
  }
docs: // Close closes the engine. Subsequent calls to Close are a nop.