Redirection to a page issue in Go

Question

I am serving index.html through go. However, depending upon certain parameters that will be send through the page, go should redirect successfully to a different page. I am getting the below error while trying to execute the code.

http: multiple response.WriteHeader calls

func main() {
http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {

    http.ServeFile(w, r, r.URL.Path[1:])
    fmt.Println(r.FormValue("login"))

    if r.FormValue("signup") == "signup" {
        signup(w, r)
    } else if r.FormValue("login") == "login" {
        if login(w, r) {
            if r.Method == "POST" {
                fmt.Println("I m here")
                http.Redirect(w, r, "http://localhost:8080/home.html" (http://localhost:8080/home.html') , http.StatusSeeOther)
            }

        }

    }

})
var port string
if port = os.Getenv("PORT"); len(port) == 0 {
    port = DEFAULT_PORT
}
log.Fatal(http.ListenAndServe(":"+port, nil))
}

You cannot ServeFile and afterwards try to redirect. Each HTTP request can be answered exactly once and sending a redirect is an answer. — Volker, Aug 31 '17 at 07:51
Search for your error (in bold above) on stackoverflow and you'll find lots of illustrations of the same problem. — Kenny Grant, Aug 31 '17 at 08:13

boaz_shuster · Accepted Answer · 2017-08-31T13:37:49.197

As already mentioned in the comments and implied in the error message, you can't change the response header twice:

When http.ServeFile(w, r, r.URL.Path[1:]) is called in some point w.WriteHeader(statusCode) will be called. In other words, an HTTP response will be sent with statusCode as status code.
singup or http.Redirect sends an HTTP response after calling w.WriteHeader.

So it is very confusing which response should be sent. You may want to check signup and login first and if none then call http.ServeFile

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    fmt.Println(r.FormValue("login"))
    switch {
    case r.FormValue("signup") == "signup":
        signup(w, r)
    case r.FormValue("login") == "login" && login(w,r):
        if r.Method == "POST" {
            fmt.Println("I m here")
            http.Redirect(w, r, "http://localhost:8080/home.html" (http://localhost:8080/home.html') , http.StatusSeeOther)
    default:
        http.ServeFile(w, r, r.URL.Path[1:])
    }
})

More about why WriteHeader is warning you about this from a probably duplicated thread

Redirection to a page issue in Go

1 Answers1