Submitting data from front end to back end in Go - http

Question

I have simple website. Go with JavaScript. Now I get this message "http: multiple response.WriteHeader calls" and I know that a have another header open. But I don't know where and I'm struggling to find a solution.
func (t *Server) RootHandler(w http.ResponseWriter, r *http.Request) {
var c *entities.Korisnik
var k *entities.Kilometri
var a *entities.Auto
if c = t.authentication(w, r); c == nil {
return
}
gk, err := t.store.GetKilometri(c)
if errorEval(w, err, http.StatusInternalServerError) {
return
}
if err := r.ParseForm(); err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
if r.FormValue("debug") == "true" {
s, _ := json.MarshalIndent(&Bla{c, gk}, "", " ")
w.Write(s)
return
} else {
w.Header().Set("Content-Type", "application/json")
errorEval(w, json.NewEncoder(w).Encode(&Bla{c, gk}), http.StatusInternalServerError)
}
log.Println("0")
if errorEval(w, json.NewDecoder(r.Body).Decode(&Input{a, k}), http.StatusBadRequest) {
log.Println("1")
return
}
err = t.store.NewKilometri(k, c, a)
log.Println("2")
if errorEval(w, err, http.StatusInternalServerError) {
return
}
}
I get this in my terminal
015/10/20 16:12:32 0
2015/10/20 16:12:32 EOF
2015/10/20 16:12:32 http: multiple response.WriteHeader calls
2015/10/20 16:12:32 1
2015/10/20 16:12:32 0
2015/10/20 16:12:32 EOF
2015/10/20 16:12:32 http: multiple response.WriteHeader calls
2015/10/20 16:12:32 1
func errorEval(w http.ResponseWriter, err error, status int) bool {
if err == nil {
return false
}
log.Println(err)
http.Error(w, errorString[status], status)
return true
}

Answer 1

It looks to me like errorEval likely writes the status code and maybe a body if an error is encountered. Most places you call it you check the return and return from your handler if it handles an error.
In the case of errorEval(w, json.NewEncoder(w).Encode(&Bla{c, gk}), http.StatusInternalServerError) you are not checking the return.
My guess is, there is some json error and the handler is writing a 500 response, and you are continuing to do other things which in turn try to write additional responses.