code: func SetFd(err error, fd storage.FileDesc) error {
  	switch x := err.(type) {
  	case *ErrCorrupted:
  		x.Fd = fd
  		return x
  	}
  	return err
  }
docs: // SetFd sets 'file info' of the given error with the given file.
  // Currently only ErrCorrupted is supported, otherwise will do nothing.