コールバック用のAPIを準備

func (h *GoogleAuthHandler) HandleGoogleCallback(c *gin.Context) {
	code := c.Query("code")
	err := c.Query("error")
	ctx := c.Request.Context()

	if err != "" {
		c.Redirect(http.StatusFound, os.Getenv("APP_URL")+"/login")
		return
	}

	googleUser, authError := h.GoogleAuthService.GetGoogleAuthUser(code)
	if authError != nil {
		c.Redirect(http.StatusFound, os.Getenv("APP_URL")+"/login")
		return
	}

    // 後継処理を書く
}

コールバックで受け取ったcodeをdecodeし、accessTokenを使ってOpenID Connectへリクエストする 受け取ったresponseからuser情報を取得

package google

import (
	"context"
	"encoding/json"
	"net/http"

	"github.com/mizuko-dev/paput-api/config"
	"golang.org/x/oauth2"
	"golang.org/x/oauth2/google"
)

type GoogleAuthUser struct {
	Sub           string `json:"sub"`
	Email         string `json:"email"`
	VerifiedEmail bool   `json:"verified_email"`
	Name          string `json:"name"`
	GivenName     string `json:"given_name"`
	FamilyName    string `json:"family_name"`
	Picture       string `json:"picture"`
	Locale        string `json:"locale"`
}

type GoogleAuthService struct {
	OAuthConfig *oauth2.Config
}

func NewGoogleAuthService() GoogleAuthService {
	config, _ := config.LoadConfig()

	oauthConfig := &oauth2.Config{
		RedirectURL:  config.Google.RedirectURL,
		ClientID:     config.Google.ClientID,
		ClientSecret: config.Google.ClientSecret,
		Scopes: []string{
			"email",
			"profile",
		},
		Endpoint: google.Endpoint,
	}

	return GoogleAuthService{
		OAuthConfig: oauthConfig,
	}
}

func (r *GoogleAuthService) GetGoogleAuthUser(code string) (*GoogleAuthUser, error) {
	token, err := r.OAuthConfig.Exchange(context.Background(), code)
	if err != nil {
		return nil, err
	}

	client := &http.Client{}
	req, _ := http.NewRequest("GET", "https://openidconnect.googleapis.com/v1/userinfo", nil)
	req.Header.Set("Authorization", "Bearer "+token.AccessToken)

	response, err := client.Do(req)
	if err != nil {
		return nil, err
	}
	defer response.Body.Close()

	var GoogleAuthUser GoogleAuthUser
	if err := json.NewDecoder(response.Body).Decode(&GoogleAuthUser); err != nil {
		return nil, err
	}

	return &GoogleAuthUser, nil
}