How to test reverse proxy with martini in go

I'm writing test code for martini app working as a reverse proxy in go, and want to test it using httptest.ResponseRecorder, but I got an error the following.

[martini] PANIC: interface conversion: *httptest.ResponseRecorder is not http.CloseNotifier: missing method CloseNotify

httptest.ResponseRecorder has no method CloseNotify()

How should I test it?

package main

import (
        "github.com/go-martini/martini"
        "github.com/stretchr/testify/assert"
        "net/http"
        "net/http/httptest"
        "net/http/httputil"
        "net/url"
        "testing"
)

func TestReverseProxy(t *testing.T) {
        // Mock backend
        backendResponse := "I am the backend"
        backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                w.Write([]byte(backendResponse))
        }))
        defer backend.Close()
        backendURL, _ := url.Parse(backend.URL)

        // Frontend
        m := martini.Classic()
        m.Get("/", func(w http.ResponseWriter, r *http.Request) {
                proxy := httputil.NewSingleHostReverseProxy(backendURL)
                proxy.ServeHTTP(w, r)
        })

        // Testing
        req, _ := http.NewRequest("GET", "/", nil)
        res := httptest.NewRecorder()
        m.ServeHTTP(res, req)

        assert.Equal(t, 200, res.Code, "should be equal")
}

First, please note that the martini framework is no longer maintained as said in their README.

Then, about your issue, it's because Martini does something that looks pretty bad to me: it takes an http.ResponseWriter and assumes it is also an http.CloseNotifier, while there is absolutely no guarantee of this. They should take a custom interface wrapping both of them, something like that:

type ResponseWriterCloseNotifier interface {
http.ResponseWriter
http.CloseNotifier
}

You can see in their source code that they had the same issue for their own tests, and used some workaround: https://github.com/go-martini/martini/commit/063dfcd8b0f64f4e2c97f0bc27fa422969baa23b#L13

Here is some working code made with it:

package main
import (
"net/http"
"net/http/httptest"
"net/http/httputil"
"net/url"
"testing"
"github.com/go-martini/martini"
"github.com/stretchr/testify/assert"
)
type closeNotifyingRecorder struct {
*httptest.ResponseRecorder
closed chan bool
}
func newCloseNotifyingRecorder() *closeNotifyingRecorder {
return &closeNotifyingRecorder{
httptest.NewRecorder(),
make(chan bool, 1),
}
}
func (c *closeNotifyingRecorder) close() {
c.closed <- true
}
func (c *closeNotifyingRecorder) CloseNotify() <-chan bool {
return c.closed
}
func TestReverseProxy(t *testing.T) {
// Mock backend
backendResponse := "I am the backend"
backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
w.Write([]byte(backendResponse))
}))
defer backend.Close()
backendURL, _ := url.Parse(backend.URL)
// Frontend
m := martini.Classic()
m.Get("/", func(w http.ResponseWriter, r *http.Request) {
proxy := httputil.NewSingleHostReverseProxy(backendURL)
proxy.ServeHTTP(w, r)
})
// Testing
req, _ := http.NewRequest("GET", "/", nil)
res := newCloseNotifyingRecorder()
m.ServeHTTP(res, req)
assert.Equal(t, 200, res.Code, "should be equal")
}