AppEngine Go User API Authentication

app.go

package app

import (
	"fmt"
	"net/http"
	"strings"

	"google.golang.org/appengine"
	"google.golang.org/appengine/user"
)

func init() {
	http.HandleFunc("/", welcome)
}

func welcome(w http.ResponseWriter, r *http.Request) {
	println("-----")
	for k, values := range r.Header {
		println(k, strings.Join(values, ","))
	}
	println("-----")

	w.Header().Set("Content-type", "text/html; charset=utf-8")
	ctx := appengine.NewContext(r)
	u := user.Current(ctx)
	if u == nil {
		url, _ := user.LoginURL(ctx, "/")
		fmt.Fprintf(w, `<a href="%s">Sign in or register</a>`, url)
		return
	}
	url, _ := user.LogoutURL(ctx, "/")
	fmt.Fprintf(w, `Welcome, %s! (<a href="%s">sign out</a>)`, u, url)
}

app.yml

service: default
runtime: go
api_version: go1.8
handlers:
- url: /.*
  script: _go_app