code: func (s *storage) fetchMeta(ctx context.Context, b blob.Ref) (plainSize uint32, encBR blob.Ref, err error) {
  	v, err := s.index.Get(b.String())
  	if err == sorted.ErrNotFound {
  		err = os.ErrNotExist
  	}
  	if err != nil {
  		return 0, blob.Ref{}, err
  	}
  	return unpackIndexEntry(v)
  }
docs: // fetchMeta returns os.ErrNotExist if the plaintext blob is not in the index.