oauthutil: shut down the oauth webserver when the context closes

This patch ensures that we pass the context from the CreateBackend
call all the way to the creation of the oauth web server.

This means that when the command has finished the webserver will
definitely be shut down, and if the user abandons it (eg via an rc
call timing out or being cancelled) then it will be shut down too.

lib/oauthutil/oauthutil.go

@@ -706,7 +706,7 @@ func configSetup(ctx context.Context, id, name string, m configmap.Mapper, oauth
 
 	// Prepare webserver
 	server := newAuthServer(opt, bindAddress, state, authURL)
-	err = server.Init()
+	err = server.Init(ctx)
 	if err != nil {
 		return "", fmt.Errorf("failed to start auth webserver: %w", err)
 	}
@@ -758,6 +758,7 @@ type authServer struct {
 	authURL     string
 	server      *http.Server
 	result      chan *AuthResult
+	quit        chan struct{}
 }
 
 // newAuthServer makes the webserver for collecting auth
@@ -768,6 +769,7 @@ func newAuthServer(opt *Options, bindAddress, state, authURL string) *authServer
 		bindAddress: bindAddress,
 		authURL:     authURL, // http://host/auth redirects to here
 		result:      make(chan *AuthResult, 1),
+		quit:        make(chan struct{}),
 	}
 }
 
@@ -830,15 +832,32 @@ func (s *authServer) handleAuth(w http.ResponseWriter, req *http.Request) {
 }
 
 // Init gets the internal web server ready to receive config details
-func (s *authServer) Init() error {
+//
+// The web server will listen until ctx is cancelled or the Stop()
+// method is called
+func (s *authServer) Init(ctx context.Context) error {
 	fs.Debugf(nil, "Starting auth server on %s", s.bindAddress)
 	mux := http.NewServeMux()
 	s.server = &http.Server{
-		Addr:    s.bindAddress,
-		Handler: mux,
+		Addr:        s.bindAddress,
+		Handler:     mux,
+		BaseContext: func(net.Listener) context.Context { return ctx },
 	}
 	s.server.SetKeepAlivesEnabled(false)
 
+	// Error the server if the context is cancelled
+	go func() {
+		select {
+		case <-ctx.Done():
+			s.result <- &AuthResult{
+				Name:        "Cancelled",
+				Description: ctx.Err().Error(),
+				Err:         ctx.Err(),
+			}
+		case <-s.quit:
+		}
+	}()
+
 	mux.HandleFunc("/auth", func(w http.ResponseWriter, req *http.Request) {
 		state := req.FormValue("state")
 		if state != s.state {
@@ -852,7 +871,8 @@ func (s *authServer) Init() error {
 	mux.HandleFunc("/", s.handleAuth)
 
 	var err error
-	s.listener, err = net.Listen("tcp", s.bindAddress)
+	var lc net.ListenConfig
+	s.listener, err = lc.Listen(ctx, "tcp", s.bindAddress)
 	if err != nil {
 		return err
 	}
@@ -869,6 +889,7 @@ func (s *authServer) Serve() {
 func (s *authServer) Stop() {
 	fs.Debugf(nil, "Closing auth server")
 	close(s.result)
+	close(s.quit)
 	_ = s.listener.Close()
 
 	// close the server