1// Copyright 2021 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// Package authhandler implements a TokenSource to support
6// "three-legged OAuth 2.0" via a custom AuthorizationHandler.
7package authhandler
8
9import (
10	"context"
11	"errors"
12
13	"golang.org/x/oauth2"
14)
15
16// AuthorizationHandler is a 3-legged-OAuth helper that prompts
17// the user for OAuth consent at the specified auth code URL
18// and returns an auth code and state upon approval.
19type AuthorizationHandler func(authCodeURL string) (code string, state string, err error)
20
21// TokenSource returns an oauth2.TokenSource that fetches access tokens
22// using 3-legged-OAuth flow.
23//
24// The provided context.Context is used for oauth2 Exchange operation.
25//
26// The provided oauth2.Config should be a full configuration containing AuthURL,
27// TokenURL, and Scope.
28//
29// An environment-specific AuthorizationHandler is used to obtain user consent.
30//
31// Per the OAuth protocol, a unique "state" string should be specified here.
32// This token source will verify that the "state" is identical in the request
33// and response before exchanging the auth code for OAuth token to prevent CSRF
34// attacks.
35func TokenSource(ctx context.Context, config *oauth2.Config, state string, authHandler AuthorizationHandler) oauth2.TokenSource {
36	return oauth2.ReuseTokenSource(nil, authHandlerSource{config: config, ctx: ctx, authHandler: authHandler, state: state})
37}
38
39type authHandlerSource struct {
40	ctx         context.Context
41	config      *oauth2.Config
42	authHandler AuthorizationHandler
43	state       string
44}
45
46func (source authHandlerSource) Token() (*oauth2.Token, error) {
47	url := source.config.AuthCodeURL(source.state)
48	code, state, err := source.authHandler(url)
49	if err != nil {
50		return nil, err
51	}
52	if state != source.state {
53		return nil, errors.New("state mismatch in 3-legged-OAuth flow")
54	}
55	return source.config.Exchange(source.ctx, code)
56}
57