trying to copy a file concurrenlty

copy.go

func copy() error {
	src, size, err := open(dd)
	if err != nil {
		return err
	}
	defer src.Close()

	dst, err := os.Create(dd)
	if err != nil {
		return err
	}
	defer func() {
		dst.Sync()
		dst.Close()
	}()
	w := NewCustomBuff(dst, dd.buffer)

	_, err = io.Copy(w, src)
	return err
}

func open(dd opts) (r io.ReadCloser, size int64, err error) {
	r, err = os.Open(dd.src)
	if err != nil {
		return nil, 0, err
	}

	return r, size, nil
}

type CustomBuff struct {
	w   io.Writer
	buf []byte
}

// NewFixedBuffer is a function
func NewCustomBuff(w io.Writer, size int64) *CustomBuff {
	return &CustomBuff{
		w:   w,
		buf: make([]byte, size),
	}
}