go-oauth2无法获取令牌：错误的请求

I write a callback handler to login with Google account:

func GoogleCallbackHandler(w http.ResponseWriter, r *http.Request) {
    conf:=&oauth2.Config{
        ClientID:"700740834863-m4om9r91htn19htq2b6a05fu6vu4j7i5.apps.googleusercontent.com",
        ClientSecret:"...-rB",
        RedirectURL:"http://localhost:3000/auth/google/callback",
        Scopes:[]string{"profile"},
        Endpoint:google.Endpoint,
    }

    fmt.Println(conf.AuthCodeURL("state"))

    code := r.URL.Query().Get("code")

    token, err := conf.Exchange(oauth2.NoContext, code)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    client := conf.Client(oauth2.NoContext, token)
    resp, err := client.Get("https://www.googleapis.com/userinfo/v2/me")
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    raw, err := ioutil.ReadAll(resp.Body)
    defer resp.Body.Close()
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    var profile map[string]interface{}
    if err := json.Unmarshal(raw, &profile); err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    session, _ := util.GlobalSessions.SessionStart(w, r)
    defer session.SessionRelease(w)

    session.Set("id_token", token.Extra("id_token"))
    session.Set("access_token", token.AccessToken)
    session.Set("profile", profile)

    // // Redirect to logged in page

    http.Redirect(w, r, "/user", http.StatusMovedPermanently)
}

In main, I serve handlers

http.HandleFunc("/",route.IndexHandler)
http.HandleFunc("/auth/google/",route.GoogleCallbackHandler)
http.Handle("/user",negroni.New(
    negroni.HandlerFunc(route.IsAuthenticated),
    negroni.Wrap(http.HandlerFunc(route.UserHandler)),
))

I encounter the same problem I got with my old question: oauth2 cannot fetch token: bad request

oauth2: cannot fetch token: 400 Bad Request
Response: {
  "error" : "invalid_request",
  "error_description" : "Missing required parameter: code"
}

After I asked this question last time, I managed to fix it with my above code, accessing and getting data successfully, but last night it suddenly got this error again, and I don't get why it no longer works after last few days working with totally same code

How can I get the code from AuthCodeURL? Is there any way to get the code for exchanging without redirecting to any other handler? I want to handle all things in just one this handler

I can solve it by adding one more handler

var (
    conf *oauth2.Config
)

func GoogleCallbackHandler(w http.ResponseWriter, r *http.Request) {
    conf=&oauth2.Config{
        ClientID:"700740834863-m4om9r91htn19htq2b6a05fu6vu4j7i5.apps.googleusercontent.com",
        ClientSecret:"...-rB",
        RedirectURL:"http://localhost:3000/auth/google/callback",
        Scopes:[]string{"profile"},
        Endpoint:google.Endpoint,
    }

    http.Redirect(w, r, conf.AuthCodeURL("state"), http.StatusMovedPermanently)
}

func GoogleLoginHandler(w http.ResponseWriter, r *http.Request) {
    code := r.URL.Query().Get("code")

    token, err := conf.Exchange(oauth2.NoContext, code)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    client := conf.Client(oauth2.NoContext, token)
    resp, err := client.Get("https://www.googleapis.com/userinfo/v2/me")
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    raw, err := ioutil.ReadAll(resp.Body)
    defer resp.Body.Close()
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    var profile map[string]interface{}
    if err := json.Unmarshal(raw, &profile); err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    session, _ := util.GlobalSessions.SessionStart(w, r)
    defer session.SessionRelease(w)

    session.Set("id_token", token.Extra("id_token"))
    session.Set("access_token", token.AccessToken)
    session.Set("profile", profile)

    http.Redirect(w, r, "/user", http.StatusMovedPermanently)
}