fdSem <- true
	defer func() { <-fdSem }()

	nextRandom := func() string {
		return strconv.Itoa(rand.Int())
	}

	dir, base := filepath.Split(filename)
	var (
		bakname string
		f       *os.File
	)
	for {
		bakname = filepath.Join(dir, base+"."+nextRandom())
		var err error
		f, err = os.OpenFile(bakname, os.O_RDWR|os.O_CREATE|os.O_EXCL, perm)