[FIXED] Gateway: possible panic if monitor endpoint inspected too soon

The monitoring http server is started early and the gateway setup
(when configured) may not be fully ready when the `/gatewayz`
endpoint is inspected and could cause a panic.

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>

server/gateway.go

@@ -603,7 +603,7 @@ func (g *srvGateway) generateInfoJSON() {
 	// We could be here when processing a route INFO that has a gateway URL,
 	// but this server is not configured for gateways, so simply ignore here.
 	// The configuration mismatch is reported somewhere else.
-	if !g.enabled {
+	if !g.enabled || g.info == nil {
 		return
 	}
 	g.info.GatewayURLs = g.URLs.getAsStringSlice()

server/gateway_test.go

@@ -6749,3 +6749,34 @@ func TestGatewayDuplicateServerName(t *testing.T) {
 	// to cluster "A"
 	checkForDupError(scl.errCh)
 }
+
+func TestGatewayNoPanicOnStartupWithMonitoring(t *testing.T) {
+	o := testDefaultOptionsForGateway("B")
+	o.HTTPHost = "127.0.0.1"
+	o.HTTPPort = 8888
+	s, err := NewServer(o)
+	require_NoError(t, err)
+
+	wg := sync.WaitGroup{}
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		time.Sleep(50 * time.Millisecond)
+		s.Start()
+		s.WaitForShutdown()
+	}()
+
+	for {
+		g, err := s.Gatewayz(nil)
+		if err != nil {
+			continue
+		}
+		if g.Port != 0 && g.Port != s.GatewayAddr().Port {
+			t.Fatalf("Unexpected port: %v vs %v", g.Port, s.GatewayAddr().Port)
+		}
+		break
+	}
+	s.Shutdown()
+	wg.Wait()
+}

server/monitor.go

@@ -1777,7 +1777,7 @@ func (s *Server) Gatewayz(opts *GatewayzOptions) (*Gatewayz, error) {
 	now := time.Now().UTC()
 	gw := s.gateway
 	gw.RLock()
-	if !gw.enabled {
+	if !gw.enabled || gw.info == nil {
 		gw.RUnlock()
 		gwz := &Gatewayz{
 			ID:               srvID,