rob.go

type errWriter struct {
	w io.Writer
	err error
}

func (e *errWriter) Write(p []byte) {
	if e.err != nil {
		return
	}
	_, e.err = e.w.Write(p)
}

func (e *errWriter) Err() error {
	return e.err
}

func do() {
	ew := &errWriter{w: ...}
	ew.Write(buf)
	ew.Write(buf)
	...
	...
	if ew.Err() != nil {
			return ew.Err()
	}
	return nil
}