Snip/backend/internal/adapter/identity/oidc.go

// Package identity adapts OpenID Connect providers (Google, generic OIDC) to
// the IdentityProvider port. Google is just an OIDC issuer, so one
// implementation covers both required login methods.
package identity

import (
	"context"
	"errors"

	"github.com/coreos/go-oidc/v3/oidc"
	"golang.org/x/oauth2"

	"github.com/snip/backend/internal/port"
)

// GoogleIssuer is Google's OIDC discovery issuer.
const GoogleIssuer = "https://accounts.google.com"

type OIDCProvider struct {
	name     string
	oauth    *oauth2.Config
	verifier *oidc.IDTokenVerifier
}

// NewOIDCProvider performs OIDC discovery against the issuer (one network call
// at startup) and returns a ready provider.
func NewOIDCProvider(ctx context.Context, name, issuer, clientID, clientSecret, redirectURL string, extraScopes []string) (*OIDCProvider, error) {
	provider, err := oidc.NewProvider(ctx, issuer)
	if err != nil {
		return nil, err
	}
	scopes := append([]string{oidc.ScopeOpenID, "email", "profile"}, extraScopes...)
	return &OIDCProvider{
		name: name,
		oauth: &oauth2.Config{
			ClientID:     clientID,
			ClientSecret: clientSecret,
			Endpoint:     provider.Endpoint(),
			RedirectURL:  redirectURL,
			Scopes:       scopes,
		},
		verifier: provider.Verifier(&oidc.Config{ClientID: clientID}),
	}, nil
}

func (p *OIDCProvider) Name() string { return p.name }

func (p *OIDCProvider) AuthURL(state string) string {
	return p.oauth.AuthCodeURL(state, oauth2.AccessTypeOffline)
}

func (p *OIDCProvider) Exchange(ctx context.Context, code string) (*port.Identity, error) {
	tok, err := p.oauth.Exchange(ctx, code)
	if err != nil {
		return nil, err
	}
	rawID, ok := tok.Extra("id_token").(string)
	if !ok {
		return nil, errors.New("oidc: response missing id_token")
	}
	idToken, err := p.verifier.Verify(ctx, rawID)
	if err != nil {
		return nil, err
	}
	var claims struct {
		Sub   string `json:"sub"`
		Email string `json:"email"`
		Name  string `json:"name"`
	}
	if err := idToken.Claims(&claims); err != nil {
		return nil, err
	}
	return &port.Identity{Subject: claims.Sub, Email: claims.Email, Name: claims.Name}, nil
}