internal/envoy: fix wrong check for filter/router (#5864)

a single Router can be added to an existing non-empty list of filters without failure

Signed-off-by: gang.liu <[email protected]>

internal/envoy/v3/listener.go

@@ -418,7 +418,7 @@ func (b *httpConnectionManagerBuilder) AddFilter(f *http.HttpFilter) *httpConnec
 		// If this happens, it has to be programmer error, so we panic to tell them
 		// it needs to be fixed. Note that in hitting this case, it doesn't matter we added
 		// the second one earlier, because we're panicking anyway.
-		if f.GetTypedConfig().MessageIs(&envoy_router_v3.Router{}) {
+		if f.GetTypedConfig().MessageIs(&envoy_router_v3.Router{}) && routerIndex != lastIndex {
 			panic("Can't add more than one router to a filter chain")
 		}
 		if routerIndex != lastIndex {

internal/envoy/v3/listener_test.go

@@ -1785,6 +1785,35 @@ func TestAddFilter(t *testing.T) {
 				},
 			},
 		},
+		"Add a single router filter to non-empty builder": {
+			builder: HTTPConnectionManagerBuilder().AddFilter(&http.HttpFilter{
+				Name: "grpcweb",
+				ConfigType: &http.HttpFilter_TypedConfig{
+					TypedConfig: protobuf.MustMarshalAny(&envoy_grpc_web_v3.GrpcWeb{}),
+				},
+			}),
+			add: &http.HttpFilter{
+				Name: "router",
+				ConfigType: &http.HttpFilter_TypedConfig{
+					TypedConfig: protobuf.MustMarshalAny(&envoy_router_v3.Router{}),
+				},
+			},
+			want: []*http.HttpFilter{
+				{
+					Name: "grpcweb",
+					ConfigType: &http.HttpFilter_TypedConfig{
+						TypedConfig: protobuf.MustMarshalAny(&envoy_grpc_web_v3.GrpcWeb{}),
+					},
+				},
+				{
+					Name: "router",
+					ConfigType: &http.HttpFilter_TypedConfig{
+						TypedConfig: protobuf.MustMarshalAny(&envoy_router_v3.Router{}),
+					},
+				},
+			},
+		},
+
 		"Add a filter to a builder with a router": {
 			builder: HTTPConnectionManagerBuilder().AddFilter(&http.HttpFilter{
 				Name: "router",