func closeBitrotWriters(ws []io.Writer) {
	for _, w := range ws {
		if w != nil {
			if bw, ok := w.(io.Closer); ok {
				bw.Close()
			}
		}
	}
}

// Returns hash sum for whole-bitrot, nil for streaming-bitrot.
func bitrotWriterSum(w io.Writer) []byte {
	if bw, ok := w.(*wholeBitrotWriter); ok {
		return bw.Sum(nil)
	}
	return nil
}

// Returns the size of the file with bitrot protection