code: func (d *driver) Writer(ctx context.Context, path string, append bool) (storagedriver.FileWriter, error) {
  	d.mutex.Lock()
  	defer d.mutex.Unlock()
  
  	normalized := normalize(path)
  
  	f, err := d.root.mkfile(normalized)
  	if err != nil {
  		return nil, fmt.Errorf("not a file")
  	}
  
  	if !append {
  		f.truncate()
  	}
  
  	return d.newWriter(f), nil
  }
docs: // Writer returns a FileWriter which will store the content written to it
  // at the location designated by "path" after the call to Commit.