Firebase Authentication のトークンを使ってバックエンドとフロントエンドの間の認証を行う

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import React, { useState } from "react";
import "./App.css";
import { app } from './firebase';
import { getAuth, onAuthStateChanged, signOut } from "firebase/auth";

function Form() {
  const [text, setText] = useState("");

  const handleChange = (event) => {
    // 中略
  };

  const XHR = new XMLHttpRequest();
  XHR.addEventListener("load", function (event) {
    setText('');
  });

  XHR.addEventListener("error", function (event) {
    alert('エラーが発生しました');
  });

  const handleSubmit = (event) => {
    event.preventDefault();

    if (text === '') {
      return;
    }

    const auth = getAuth();
    var unsubscribe = onAuthStateChanged(auth, (user) => {
      if (user) {
        user.getIdToken(true).then(function(idToken) {
          const FD = new FormData(document.getElementById('input-form'));
          XHR.open("POST", process.env.REACT_APP_API_BASE_URL + '/input');
          XHR.setRequestHeader('Authorization', 'Bearer ' + idToken);
          XHR.send(FD);
        }).catch(function(error) {
          alert(error);
        });
      }
    })
    unsubscribe();
  };

{/*  以下省略 */}

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package main

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"strings"
	"time"

	"cloud.google.com/go/firestore"
	firebase "firebase.google.com/go"
	auth "firebase.google.com/go/auth"
	"github.com/pkg/errors"

	ae "github.com/k1350/amicroblog/internal/errors"
	"github.com/k1350/amicroblog/internal/repository"
)

var (
	client *firestore.Client
	ac     *auth.Client
	jst    *time.Location
)

// 中略

func inputHaldler(w http.ResponseWriter, r *http.Request) {
	if r.Method == "POST" {
		h := r.Header["Authorization"]
		idToken := strings.Split(h[0], " ")

		ctx := context.Background()
		_, err := ac.VerifyIDToken(ctx, idToken[1])
		if err != nil {
			w.WriteHeader(http.StatusUnauthorized)
			return
		}

		err = input(w, r)
		if err != nil {
			w.WriteHeader(http.StatusInternalServerError)
			log.Printf("%+v", err)
			return
		}
		w.WriteHeader(http.StatusOK)
	} else {
		w.WriteHeader(http.StatusMethodNotAllowed)
	}
}

func input(w http.ResponseWriter, r *http.Request) error {
	err := r.ParseMultipartForm(1024 * 5)
	if err != nil {
		return errors.WithStack(errors.WithMessage(ae.ParseError, err.Error()))
	}
	return repository.Add(r.MultipartForm.Value["text"][0], client)
}

func main() {
	var err error
	time.Local = time.FixedZone("Local", 9*60*60)
	jst, err = time.LoadLocation("Local")
	if err != nil {
		panic(err)
	}
	ctx := context.Background()
	app, err := firebase.NewApp(ctx, nil)
	if err != nil {
		panic(err)
	}
	client, err = app.Firestore(ctx)
	if err != nil {
		panic(err)
	}
	ac, err = app.Auth(ctx)
	if err != nil {
		panic(err)
	}
	defer client.Close()

	http.HandleFunc("/", rootHandler)
	http.HandleFunc("/input", inputHaldler)
	http.ListenAndServe(":8000", nil)
}