code: func (d *driver) Reader(ctx context.Context, path string, offset int64) (io.ReadCloser, error) {
  	d.mutex.RLock()
  	defer d.mutex.RUnlock()
  
  	return d.reader(ctx, path, offset)
  }
docs: // Reader retrieves an io.ReadCloser for the content stored at "path" with a
  // given byte offset.