Go Google Oauth2 example using the tokeninfo endpoint

google_oauth.go

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"os"

	"golang.org/x/oauth2"
	"golang.org/x/oauth2/google"

	"google.golang.org/appengine"
)

// TODO: This must be randomized per request
const todoBadStateImplementation = "9bd752171ba8fd0590fe5c26c5f03c23"

var oauthConfig = oauth2.Config{
	ClientID:     "TODO",
	ClientSecret: "TODO",
	Endpoint:     google.Endpoint,
	Scopes:       []string{"email"},
	RedirectURL:  "http://localhost:8080/oauth_callback",
}

func init() {
	http.HandleFunc("/", handler)
	http.HandleFunc("/start", startOauth)
	http.HandleFunc("/oauth_callback", oauthCallback)
}

func handler(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/plain;charset=utf-8")

	ctx := appengine.NewContext(r)
	fmt.Fprintln(w, "REQUEST ID:", appengine.RequestID(ctx))
	fmt.Fprintln(w)

	fmt.Fprintln(w, "HEADERS:")
	for k, values := range r.Header {
		fmt.Fprintln(w, k, ":")
		for _, s := range values {
			fmt.Fprintln(w, "   ", s)
		}
	}
	fmt.Fprintln(w)

	fmt.Fprintln(w, "ENVIRONMENT:")
	for _, s := range os.Environ() {
		fmt.Fprintln(w, s)
	}
}

func startOauth(w http.ResponseWriter, r *http.Request) {
	url := oauthConfig.AuthCodeURL(todoBadStateImplementation)
	log.Println("startOauth: redirecting to", url)
	http.Redirect(w, r, url, http.StatusFound)
}

func oauthCallback(w http.ResponseWriter, r *http.Request) {
	errorString := r.FormValue("error")
	if errorString != "" {
		panic(errorString)
	}

	stateString := r.FormValue("state")
	if stateString != todoBadStateImplementation {
		panic("state does not match")
	}

	// things look like they might be valid! Let's get the token
	code := r.FormValue("code")
	ctx := appengine.NewContext(r)
	token, err := oauthConfig.Exchange(ctx, code)
	if err != nil {
		panic(err)
	}

	// it worked! get the tokeninfo
	log.Printf("access_token: %s refresh_token: %s", token.AccessToken, token.RefreshToken)
	tokeninfoUrl := "https://www.googleapis.com/oauth2/v3/tokeninfo?access_token=" + token.AccessToken
	log.Println("URL: ", tokeninfoUrl)
	resp, err := http.Get(tokeninfoUrl)
	if err != nil {
		panic(err)
	}
	data, err := ioutil.ReadAll(resp.Body)
	err2 := resp.Body.Close()
	if err != nil {
		panic(err)
	}
	if err2 != nil {
		panic(err2)
	}

	w.Header().Set("Content-Type", "text/plain;charset=utf-8")
	fmt.Fprintln(w, "Tokeninfo:")
	w.Write(data)
}

func main() {
	appengine.Main()
}