fix of endless loop while doing Consul service discovery. (#4044)

Reloading Prometheus configs doesn't make loop end.
It produced a goroutine leak

discovery/consul/consul.go

@@ -302,17 +302,22 @@ func (d *Discovery) Run(ctx context.Context, ch chan<- []*targetgroup.Group) {
 	if len(d.watchedServices) == 0 || d.watchedTag != "" {
 		// We need to watch the catalog.
 		ticker := time.NewTicker(d.refreshInterval)
-		go func() {
-			// Watched services and their cancellation functions.
-			services := make(map[string]func())
-			var lastIndex uint64
 
-			for ; true; <-ticker.C {
+		// Watched services and their cancellation functions.
+		services := make(map[string]func())
+		var lastIndex uint64
+
+		for {
+			select {
+			case <-ctx.Done():
+				ticker.Stop()
+				return
+			default:
 				d.watchServices(ctx, ch, &lastIndex, services)
+				<-ticker.C
 			}
-		}()
-		<-ctx.Done()
-		ticker.Stop()
+		}
+
 	} else {
 		// We only have fully defined services.
 		for _, name := range d.watchedServices {
@@ -417,11 +422,16 @@ func (d *Discovery) watchService(ctx context.Context, ch chan<- []*targetgroup.G
 		ticker := time.NewTicker(d.refreshInterval)
 		var lastIndex uint64
 		catalog := srv.client.Catalog()
-		for ; true; <-ticker.C {
-			srv.watch(ctx, ch, catalog, &lastIndex)
+		for {
+			select {
+			case <-ctx.Done():
+				ticker.Stop()
+				return
+			default:
+				srv.watch(ctx, ch, catalog, &lastIndex)
+				<-ticker.C
+			}
 		}
-		<-ctx.Done()
-		ticker.Stop()
 	}()
 }