Listen on web early in startup

Avoid starting up components like the TSDB if we can't bind
to the web listening port.
Signed-off-by: NBen Kochie <superq@gmail.com>

cmd/prometheus/main.go

@@ -558,6 +558,12 @@ func main() {
 	}
 	defer closer.Close()
 
+	listener, err := webHandler.Listener()
+	if err != nil {
+		level.Error(logger).Log("msg", "Unable to start web listener", "err", err)
+		os.Exit(1)
+	}
+
 	var g run.Group
 	{
 		// Termination handler.
@@ -772,7 +778,7 @@ func main() {
 		// Web handler.
 		g.Add(
 			func() error {
-				if err := webHandler.Run(ctxWeb); err != nil {
+				if err := webHandler.Run(ctxWeb, listener); err != nil {
 					return errors.Wrapf(err, "error starting web server")
 				}
 				return nil

web/web.go

@@ -524,13 +524,13 @@ func (h *Handler) Reload() <-chan chan error {
 	return h.reloadCh
 }
 
-// Run serves the HTTP endpoints.
-func (h *Handler) Run(ctx context.Context) error {
+// Listener creates the TCP listener for web requests.
+func (h *Handler) Listener() (net.Listener, error) {
 	level.Info(h.logger).Log("msg", "Start listening for connections", "address", h.options.ListenAddress)
 
 	listener, err := net.Listen("tcp", h.options.ListenAddress)
 	if err != nil {
-		return err
+		return listener, err
 	}
 	listener = netutil.LimitListener(listener, h.options.MaxConnections)
 
@@ -539,6 +539,11 @@ func (h *Handler) Run(ctx context.Context) error {
 		conntrack.TrackWithName("http"),
 		conntrack.TrackWithTracing())
 
+	return listener, nil
+}
+
+// Run serves the HTTP endpoints.
+func (h *Handler) Run(ctx context.Context, listener net.Listener) error {
 	operationName := nethttp.OperationNameFunc(func(r *http.Request) string {
 		return fmt.Sprintf("%s %s", r.Method, r.URL.Path)
 	})

web/web_test.go

@@ -143,11 +143,15 @@ func TestReadyAndHealthy(t *testing.T) {
 
 	webHandler.config = &config.Config{}
 	webHandler.notifier = &notifier.Manager{}
+	l, err := webHandler.Listener()
+	if err != nil {
+		panic(fmt.Sprintf("Unable to start web listener: %s", err))
+	}
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	go func() {
-		err := webHandler.Run(ctx)
+		err := webHandler.Run(ctx, l)
 		if err != nil {
 			panic(fmt.Sprintf("Can't start web handler:%s", err))
 		}
@@ -252,10 +256,14 @@ func TestRoutePrefix(t *testing.T) {
 	opts.Flags = map[string]string{}
 
 	webHandler := New(nil, opts)
+	l, err := webHandler.Listener()
+	if err != nil {
+		panic(fmt.Sprintf("Unable to start web listener: %s", err))
+	}
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	go func() {
-		err := webHandler.Run(ctx)
+		err := webHandler.Run(ctx, l)
 		if err != nil {
 			panic(fmt.Sprintf("Can't start web handler:%s", err))
 		}
@@ -420,12 +428,16 @@ func TestShutdownWithStaleConnection(t *testing.T) {
 
 	webHandler.config = &config.Config{}
 	webHandler.notifier = &notifier.Manager{}
+	l, err := webHandler.Listener()
+	if err != nil {
+		panic(fmt.Sprintf("Unable to start web listener: %s", err))
+	}
 
 	closed := make(chan struct{})
 
 	ctx, cancel := context.WithCancel(context.Background())
 	go func() {
-		err := webHandler.Run(ctx)
+		err := webHandler.Run(ctx, l)
 		if err != nil {
 			panic(fmt.Sprintf("Can't start web handler:%s", err))
 		}
@@ -467,10 +479,14 @@ func TestHandleMultipleQuitRequests(t *testing.T) {
 	webHandler := New(nil, opts)
 	webHandler.config = &config.Config{}
 	webHandler.notifier = &notifier.Manager{}
+	l, err := webHandler.Listener()
+	if err != nil {
+		panic(fmt.Sprintf("Unable to start web listener: %s", err))
+	}
 	ctx, cancel := context.WithCancel(context.Background())
 	closed := make(chan struct{})
 	go func() {
-		err := webHandler.Run(ctx)
+		err := webHandler.Run(ctx, l)
 		if err != nil {
 			panic(fmt.Sprintf("Can't start web handler:%s", err))
 		}