code: func NewFileReader(path string) (*Reader, error) {
  	f, err := fileutil.OpenMmapFile(path)
  	if err != nil {
  		return nil, err
  	}
  	r, err := newReader(realByteSlice(f.Bytes()), f)
  	if err != nil {
  		var merr tsdb_errors.MultiError
  		merr.Add(err)
  		merr.Add(f.Close())
  		return nil, merr
  	}
  
  	return r, nil
  }
docs: // NewFileReader returns a new index reader against the given index file.