Go http panic serving

Question

I got this panic:

C:\Users\loow\Desktop\USBWebserver v8.5\duplicate_submissions>go run server.go
2015/10/23 13:00:39 http: panic serving [::1]:63867: runtime error: invalid memo
ry address or nil pointer dereference
goroutine 5 [running]:
net/http.(*conn).serve.func1(0xc0820a1810, 0x3b55b8, 0xc082024040)
        c:/go/src/net/http/server.go:1287 +0xbc
main.login(0x2990058, 0xc0820d4000, 0xc0820be1c0)
        C:/Users/loow/Desktop/USBWebserver v8.5/duplicate_submissions/server.go:
27 +0x5a5
net/http.HandlerFunc.ServeHTTP(0x8326a8, 0x2990058, 0xc0820d4000, 0xc0820be1c0)
        c:/go/src/net/http/server.go:1422 +0x41
net/http.(*ServeMux).ServeHTTP(0xc082062360, 0x2990058, 0xc0820d4000, 0xc0820be1
c0)
        c:/go/src/net/http/server.go:1699 +0x184
net/http.serverHandler.ServeHTTP(0xc08200c360, 0x2990058, 0xc0820d4000, 0xc0820b
e1c0)
        c:/go/src/net/http/server.go:1862 +0x1a5
net/http.(*conn).serve(0xc0820a1810)
        c:/go/src/net/http/server.go:1361 +0xbf5
created by net/http.(*Server).Serve
        c:/go/src/net/http/server.go:1910 +0x3fd
2015/10/23 13:00:39 http: panic serving [::1]:63868: runtime error: invalid memo
ry address or nil pointer dereference
goroutine 33 [running]:
net/http.(*conn).serve.func1(0xc082114000, 0x3b55b8, 0xc082112000)
        c:/go/src/net/http/server.go:1287 +0xbc
main.login(0x2990058, 0xc0821140b0, 0xc0821200e0)
        C:/Users/loow/Desktop/USBWebserver v8.5/duplicate_submissions/server.go:
27 +0x5a5
net/http.HandlerFunc.ServeHTTP(0x8326a8, 0x2990058, 0xc0821140b0, 0xc0821200e0)
        c:/go/src/net/http/server.go:1422 +0x41
net/http.(*ServeMux).ServeHTTP(0xc082062360, 0x2990058, 0xc0821140b0, 0xc0821200
e0)
        c:/go/src/net/http/server.go:1699 +0x184
net/http.serverHandler.ServeHTTP(0xc08200c360, 0x2990058, 0xc0821140b0, 0xc08212
00e0)
        c:/go/src/net/http/server.go:1862 +0x1a5
net/http.(*conn).serve(0xc082114000)
        c:/go/src/net/http/server.go:1361 +0xbf5
created by net/http.(*Server).Serve
        c:/go/src/net/http/server.go:1910 +0x3fd
exit status 2

Whit this code:

package main

import(
    "fmt"
    "net/http"
    "html/template"
    "log"
    "time"
    "crypto/md5"
    "io"
    "strconv"
)

func loginForm(w http.ResponseWriter, r *http.Request){

}

func login(w http.ResponseWriter, r *http.Request){
    fmt.Println(r.Method)
    if r.Method == "GET"{
        cruTime := time.Now().Unix()
        h := md5.New()
        io.WriteString(h,strconv.FormatInt(cruTime,10))
        token := fmt.Sprintf("%x", h.Sum(nil))
        fmt.Println(token)
        t, err := template.ParseFiles("templates/index.gtpl")
        fmt.Println(err.Error())
        err = t.Execute(w, token)
        fmt.Println(err.Error())
    } else{
        r.ParseForm()
        token := r.Form.Get("token")
        if token != ""{
            fmt.Println(token)
        } else{
            fmt.Println("There is no token")
        }
        fmt.Println("username length: ", len(r.Form["username"][0]))
        fmt.Println("username: ", template.HTMLEscapeString(r.Form.Get("username")))
        fmt.Println("password: ", template.HTMLEscapeString(r.Form.Get("password")))
        template.HTMLEscape(w, []byte(r.Form.Get("username")))

    }
}

func main(){
    http.HandleFunc("/", loginForm)
    http.HandleFunc("/login", login)
    err := http.ListenAndServe(":9090", nil)
    if err != nil {
        log.Fatal("ListenAndServe: ", err)
    }
}

I cant fix it, I tried everything what I found in the stackoverflow. What is the problem? There is no error, and the panic said that the problem in t, err := template.ParseFiles("templates/index.gtpl")..

There is the template file:

<input type="checkbox" name="interest" value="football">Football
<input type="checkbox" name="interest" value="basketball">Basketball
<input type="checkbox" name="interest" value="tennis">Tennis
Username: <input type="text" name="username">
Password: <input type="password" name="password">
<input type="hidden" name="token" value="{{.}}">
<input type="submit" value="Login">

Answer 1

The panic stacktrace gives you this info :

2015/10/23 13:00:39 http: panic serving [::1]:63868: runtime error: invalid memory address or nil pointer dereference goroutine 33 [running]:

It means you're trying to access something that does not exist (nil pointer).

Then the first line that comes from your file is this one :

v8.5/duplicate_submissions/server.go:27

Which is there :

26: t, err := template.ParseFiles("templates/index.gtpl")
27: fmt.Println(err.Error())
28: err = t.Execute(w, token)

It means err is nil.

Solution

If you get the error, you cannot continue the process. That's the reason why you cannot just print out the error. In order to stop gracefully the process, you need to return an HTTP status code and then return. For the case above, you can return a code 500 (internal server error).

t, err := template.ParseFiles("templates/index.gtpl")
if err != nil {
  fmt.Println(err) // Ugly debug output
  w.WriteHeader(http.StatusInternalServerError) // Proper HTTP response
  return
}

That has to be done for a template.ParseFiles and t.Execute too.

By the way, that is called the "comma ok" pattern