fix(trafficrouting): patch VirtualService when there is only one named route

rollout/trafficrouting/istio/istio.go

@@ -903,8 +903,13 @@ func (r *Reconciler) VerifyWeight(desiredWeight int32, additionalDestinations ..
 // getHttpRouteIndexesToPatch returns array indices of the httpRoutes which need to be patched when updating weights
 func getHttpRouteIndexesToPatch(routeNames []string, httpRoutes []VirtualServiceHTTPRoute) ([]int, error) {
 	//We have no routes listed in spec.strategy.canary.trafficRouting.istio.virtualService.routes so find index
-	//of the first empty named route
+	//of the first empty named route.
+	// If there is only one HTTPRoute defined in the VirtualService, then we can patch it without a name.
 	if len(routeNames) == 0 {
+		if len(httpRoutes) == 1 {
+			return []int{0}, nil
+		}
+
 		for i, route := range httpRoutes {
 			if route.Name == "" {
 				return []int{i}, nil

rollout/trafficrouting/istio/istio_test.go

@@ -3156,3 +3156,47 @@ spec:
 	actions := client.Actions()
 	assert.Len(t, actions, 0)
 }
+
+func TestGetHttpRouteIndexesToPatch(t *testing.T) {
+	t.Run("the rollout has no managed routes defined", func(t *testing.T) {
+		httpRoutes := []VirtualServiceHTTPRoute{
+			{Name: "foo", Match: nil},
+			{Name: "", Match: nil},
+		}
+
+		indexes, err := getHttpRouteIndexesToPatch([]string{}, httpRoutes)
+		assert.NoError(t, err)
+		assert.Equal(t, []int{1}, indexes)
+	})
+
+	t.Run("the rollout has managed routes defined", func(t *testing.T) {
+		httpRoutes := []VirtualServiceHTTPRoute{
+			{Name: "foo", Match: nil},
+			{Name: "bar", Match: nil},
+		}
+
+		indexes, err := getHttpRouteIndexesToPatch([]string{"foo", "bar"}, httpRoutes)
+		assert.NoError(t, err)
+		assert.Equal(t, []int{0, 1}, indexes)
+	})
+
+	t.Run("the rollout has only one managed route defined", func(t *testing.T) {
+		httpRoutes := []VirtualServiceHTTPRoute{
+			{Name: "foo", Match: nil},
+		}
+
+		indexes, err := getHttpRouteIndexesToPatch([]string{}, httpRoutes)
+		assert.NoError(t, err)
+		assert.Equal(t, []int{0}, indexes)
+	})
+
+	t.Run("http route is not found", func(t *testing.T) {
+		httpRoutes := []VirtualServiceHTTPRoute{
+			{Name: "foo", Match: nil},
+		}
+
+		indexes, err := getHttpRouteIndexesToPatch([]string{"bar"}, httpRoutes)
+		assert.Equal(t, "HTTP Route 'bar' is not found in the defined Virtual Service.", err.Error())
+		assert.Nil(t, indexes)
+	})
+}