golang： multiple http.writeHeader calls

func DownloadFile(w http.ResponseWriter, r *http.Request, sequence uint64, userid string) {
    userkey := userid
    filename := r.FormValue("filename")
    if len(userkey) <= 0 || len(filename) <= 0 {
        //........
    }
    fp := fmt.Sprintf("%s%s/%s", g_sc.Rootdir, userkey, filename)
    fd, err := os.Open(fp)
    if err == nil {
        defer fd.Close()
        buf, err := ioutil.ReadAll(fd)
        if err != nil {
            //.........
        } else {
            size := len(buf)
            w.Header().Add("Content-Length", fmt.Sprintf("%d", size))
            fmt.Fprint(w, string(buf))
        }
    }
}

buf := new(bytes.Buffer)
buf.Write(...)
 
if err {
  // you can use http.Error here, no response has been written yet
  http.Error(w, err.String, http.StatusInternalServerError)
  return
}
 
if err := buf.WriteTo(w); err != nil {
  log.Printf("WriteTo: %v", err)
  // you can not use http.Error here anymore. So just log the message (e.g. "broken pipe...")
}