Golang mocking functions for http handler tests

I am writing a unit test for my PostLoginHandler and need to mock a session middleware function. In my handler it calls session.Update() that I would like to mock to return nil.

My first instinct after reading various answers was to make a SessionManager interface but even then I am unclear how to proceed.

main.go:

func PostLoginHandler(c web.C, w http.ResponseWriter, r *http.Request) {
    r.ParseForm()
    user, pass := r.PostFormValue("username"), r.PostFormValue("password")
    ctx := context.GetContext(c)

    if !authorizeUser(user, pass) {
        http.Error(w, "Wrong username or password", http.StatusBadRequest)
        return
    }

    ctx.IsLogin = true
    err := session.Update(ctx) \\ mock this function call.
    if err != nil {
        log.Println(err)
        return
    }
    http.Redirect(w, r, "/admin/", http.StatusFound)
}

main_test:

var loginTests = []struct {
    username string
    password string
    code     int
}{
    {"admin", "admin", http.StatusFound},
    {"", "", http.StatusBadRequest},
    {"", "admin", http.StatusBadRequest},
    {"admin", "", http.StatusBadRequest},
    {"admin", "badpassword", http.StatusBadRequest},
}

func TestPostLoginHandler(t *testing.T) {
    //  setup()
    ctx := &context.Context{IsLogin: false, Data: make(map[string]interface{})}
    c := newC()
    c.Env["context"] = ctx

    for k, test := range loginTests {
        v := url.Values{}
        v.Set("username", test.username)
        v.Set("password", test.password)
        r, _ := http.NewRequest("POST", "/login", nil)
        r.PostForm = v
        resp := httptest.NewRecorder()
        m.ServeHTTPC(c, resp, r)

        if resp.Code != test.code {
            t.Fatalf("TestPostLoginHandler #%v failed. Expected: %v\tReceived: %v", k, test.code, resp.Code)
        }
    }
}