// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package authhandler implements a TokenSource to support
// "three-legged OAuth 2.0" via a custom AuthorizationHandler.
package authhandler

import (
	"context"
	"errors"

	"golang.org/x/oauth2"
)

// AuthorizationHandler is a 3-legged-OAuth helper that
// prompts the user for OAuth consent at the specified Auth URL
// and returns an auth code and state upon approval.
type AuthorizationHandler func(string) (string, string, error)

// TokenSource returns an oauth2.TokenSource that fetches access tokens
// using 3-legged-OAuth flow.
//
// The provided oauth2.Config should be a full configuration containing AuthURL,
// TokenURL, and scope. An environment-specific AuthorizationHandler is used to
// obtain user consent.
//
// Per OAuth protocol, a unique "state" string should be sent and verified
// before exchanging auth code for OAuth token to prevent CSRF attacks.
func TokenSource(ctx context.Context, config *oauth2.Config, authHandler AuthorizationHandler, state string) oauth2.TokenSource {
	return oauth2.ReuseTokenSource(nil, authHandlerSource{config: config, ctx: ctx, authHandler: authHandler, state: state})
}

type authHandlerSource struct {
	ctx         context.Context
	config      *oauth2.Config
	authHandler AuthorizationHandler
	state       string
}

func (source authHandlerSource) Token() (*oauth2.Token, error) {
	url := source.config.AuthCodeURL(source.state)
	code, state, err := source.authHandler(url)
	if err != nil {
		return nil, err
	}
	if state == source.state {
		return source.config.Exchange(source.ctx, code)
	}
	return nil, errors.New("State mismatch in 3-legged-OAuth flow.")
}