Go multiple response.WriteHeader calls

Question

So I'm writing a basic webapp and I'm having trouble redirecting the user after a sucessfull login. The code is:

t, err := template.ParseFiles("home.html")
if err != nil {
    log.Fatal("template.ParseFiles: ", err)
}

err = t.Execute(w, nil)
if err != nil {
    log.Fatal("t.Execute: ", err)
}

if r.Method == "POST" {
    r.ParseForm()
    user := r.FormValue("username")
    pass := r.FormValue("password")

    if checkLogin(user, pass) {
        loggedIn = true
        http.Redirect(w, r, "/home", 302)
    }
}

The error message is: "http: multiple response.WriteHeader calls".

My problem is that I don't see a way to serve the html file containing the login-form without calling t.Execute which sets the header.

How can I display the login page and still be able to redirect to a different page?

Answer 1

You are writing (using w ) and then later trying to redirect (also using w ) using 302 header redirection.

You can only send headers once, and if you start writing to w it assumes a 200 header (OK)

Also, Its best if you check the http.Method before writing to the ResponseWriter ( w )

And, Remember to return after a redirection or handing over the ResponseWriter and Request pair to another function!

Hope this helps.

Answer 2

How can I display the login page and still be able to redirect to a different page?

Have a different route for authentication. Make the login form submit to the authentication route. Have a separate handler for authentication as well.

For example, your login form:

<form method="post" action="/auth">

your Go main:

http.HandleFunc("/", homeHandler)
http.HandleFunc("/auth", authHandler)

When authentication processing is complete you can redirect the user to the appropriate page. You could pass a parameter in the query string that contains the destination path for the redirect.