code: func (d *driver) Reader(ctx context.Context, path string, offset int64) (io.ReadCloser, error) {
  	file, err := os.OpenFile(d.fullPath(path), os.O_RDONLY, 0644)
  	if err != nil {
  		if os.IsNotExist(err) {
  			return nil, storagedriver.PathNotFoundError{Path: path}
  		}
  
  		return nil, err
  	}
  
  	seekPos, err := file.Seek(offset, io.SeekStart)
  	if err != nil {
  		file.Close()
  		return nil, err
  	} else if seekPos < offset {
  		file.Close()
  		return nil, storagedriver.InvalidOffsetError{Path: path, Offset: offset}
  	}
  
  	return file, nil
  }
docs: // Reader retrieves an io.ReadCloser for the content stored at "path" with a
  // given byte offset.