Redirects return http: multiple response.WriteHeader calls

I am using Jon Calhoun's Go MVC framework from github.

The framework uses julienschmidt/httprouter as its only dependency.

I have a similar main method as found in the example:

  func main() {
    	//register routes
    	router := httprouter.New()
    
    	//default
    	router.GET("/", controllers.Login.Perform(controllers.Login.Index))
    
    	//login
    	router.GET("/login", controllers.Login.Perform(controllers.Login.Login))
    	router.POST("/login", controllers.Login.Perform(controllers.Login.PostLogin))
    
    	//dashboard
    	router.GET("/dashboard", controllers.Dashboard.Perform(controllers.Dashboard.Index))
    	
    
    	//listen and handle requests
    	log.Fatal(http.ListenAndServe(":"+helpers.ReadConfig("port_http"), router))
    }

I make a post to the login url, and it calls the following method:

func (self LoginController) PostLogin(w http.ResponseWriter, r *http.Request, ps httprouter.Params) error {
	//create our api url
	var url = helpers.ReadConfig("api") + "login"
	//fill model to post
	login := models.LoginModel{
		Password: r.FormValue("password"),
		Email:    r.FormValue("username"),
	}
	//render json from model
	bytes, err := json.Marshal(login)
	if err != nil {
		panic(err)
	}
	//post to the API helpers
	var resp = helpers.ApiPost(url, r, string(bytes))
	//check response if successful
	if resp.Code != constants.ApiResp_Success {
		//TODO: Handle API Errors
		login.Password = ""
		errors := make(map[int]string)
		errors[1] = "Please provide valid credntials."
		login.Common = models.CommonModel{
			ErrorList: errors,
		}
		return views.Login.Index.Render(w, login, helpers.AcceptsGzip(r))
	}
	

	log.Println("---Redirect--")
	http.Redirect(w, r, "/dashboard", 307)
	log.Println("-----")
	return views.Dashboard.Index.Render(w, login, helpers.AcceptsGzip(r))
}

Basically, if the login was not correct I return the same view. If the login is correct I want to redirect to another method in a different controller.

However when I call http.Redirect(w, r, "/dashboard", 307), it returns the following error:

http: multiple response.WriteHeader calls  

I'm not sure exactly why this is happening, but I suspect that it has something to do with my listener calling the Perform function, which creates a http.handler, as shown below.

func (c *Controller) Perform(a Action) httprouter.Handle {
	return httprouter.Handle(
		func(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
			//set response headers
			//TODO: set appropriate responce headers
			w.Header().Set("Access-Control-Allow-Origin", "*")
			w.Header().Set("Cache-Control", "public, max-age=0")
			w.Header().Set("Token", "NOT-A-VALID-TOKEN")
			w.WriteHeader(200)
			if err := a(w, r, ps); err != nil {
				http.Error(w, err.Error(), http.StatusInternalServerError)
			}

		})
}

Does anyone have any idea how to redirect using this MVC framework? Or have a one off solution?

http.ResponseWriter's WriteHeader method can only be called once per HTTP response, for obvious reasons: You can only have a single response code, and you can only send the headers once.

The error you see means that it is called a second time on the same response.

Your middleware calls:

        w.WriteHeader(200)

Then your handler also calls:

http.Redirect(w, r, "/dashboard", 307)
log.Println("-----")
return views.Dashboard.Index.Render(w, login, helpers.AcceptsGzip(r))

Your middleware should never call WriteHeader, until after the fate of the response is known.

Further, without knowing about your particular MVC framework, it seems possible that after you send the 307 status, then you also tell the MVC framework to render a response, which may also call WriteHeader again.