func tempFile(ctx context.Context, dir, prefix string, perm fs.FileMode) (f *os.File, err error) {
	for i := 0; i < 10000; i++ {
		name := filepath.Join(dir, prefix+strconv.Itoa(rand.Intn(1000000000))+".tmp")
		f, err = os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, perm)
		if os.IsExist(err) {
			if ctx.Err() != nil {
				return nil, ctx.Err()
			}
			continue
		}
		break