code: func (ut *UploadTracker) doHashFile(path string) (isolated.HexDigest, error) {
  	f, err := ut.lOS.Open(path)
  	if err != nil {
  		return "", err
  	}
  	defer f.Close()
  	return isolated.Hash(ut.checker.Hash(), f)
  }
docs: // doHashFile returns the hash of the contents of path. This should not be
  // called directly; call hashFile instead.