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// Copyright 2018 The LUCI Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// Package firebase implements an auth server that allows firebase-tools
// to use an exposed OAuth2 TokenSource for auth.
// See firebase auth documentation at
// and auth implementation
package firebase
import (
// Server runs a local server that handles requests to token_uri.
type Server struct {
// Source is used to obtain OAuth2 tokens.
Source oauth2.TokenSource
// Port is a local TCP port to bind to or 0 to allow the OS to pick one.
Port int
srv localsrv.Server
// Start launches background goroutine with the serving loop.
// The provided context is used as base context for request handlers and for
// logging. The server must be eventually stopped with Stop().
// Returns the root URL of a local server to use as FIREBASE_TOKEN_URL.
func (s *Server) Start(ctx context.Context) (string, error) {
// Launch the server to get the port number.
addr, err := s.srv.Start(ctx, "firebase-auth", s.Port, func(c context.Context, l net.Listener, wg *sync.WaitGroup) error {
return s.serve(c, l, wg)
if err != nil {
return "", errors.Annotate(err, "failed to start the server").Err()
return fmt.Sprintf("http://%s", addr), nil
// Stop closes the listening socket, notifies pending requests to abort and
// stops the internal serving goroutine.
// Safe to call multiple times. Once stopped, the server cannot be started again
// (make a new instance of Server instead).
// Uses the given context for the deadline when waiting for the serving loop
// to stop.
func (s *Server) Stop(ctx context.Context) error {
return s.srv.Stop(ctx)
// serve runs the serving loop.
func (s *Server) serve(ctx context.Context, l net.Listener, wg *sync.WaitGroup) error {
mux := http.NewServeMux()
mux.Handle("/oauth2/v3/token", &handler{ctx, wg, func(rw http.ResponseWriter, r *http.Request) {
err := s.handleTokenRequest(rw, r)
code := 0
msg := ""
if transient.Tag.In(err) {
code = http.StatusInternalServerError
msg = fmt.Sprintf("Transient error - %s", err)
} else if err != nil {
code = http.StatusBadRequest
msg = fmt.Sprintf("Bad request - %s", err)
if code != 0 {
logging.Errorf(ctx, "%s", msg)
http.Error(rw, msg, code)
srv := http.Server{Handler: mux}
return srv.Serve(l)
// handleTokenRequest handles the OAuth2 flow.
// The body of the request is documented here (among many other places):
// We ignore client_id and client_secret, since we aren't really running OAuth2.
func (s *Server) handleTokenRequest(rw http.ResponseWriter, r *http.Request) error {
ctx := r.Context()
// We support only refreshing access token via 'refresh_token' grant.
if r.PostFormValue("grant_type") != "refresh_token" {
return fmt.Errorf("expecting 'refresh_token' grant type")
// Grab an access token through the source and return it.
tok, err := s.Source.Token()
if err != nil {
return err
rw.Header().Set("Content-Type", "application/json")
return json.NewEncoder(rw).Encode(map[string]interface{}{
"access_token": tok.AccessToken,
"expires_in": clock.Until(ctx, tok.Expiry) / time.Second,
"token_type": "Bearer",
// handler implements http.Handler by wrapping the given handler with some
// housekeeping stuff.
type handler struct {
ctx context.Context
wg *sync.WaitGroup
handler http.HandlerFunc
func (h *handler) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
defer h.wg.Done()
defer paniccatcher.Catch(func(p *paniccatcher.Panic) {
"panic.error": p.Reason,
}.Errorf(h.ctx, "Caught panic during handling of %q: %s\n%s", r.RequestURI, p.Reason, p.Stack)
http.Error(rw, "Internal Server Error. See logs.", http.StatusInternalServerError)
logging.Debugf(h.ctx, "Handling %s %s", r.Method, r.RequestURI)
h.handler(rw, r.WithContext(h.ctx))