xdsclient: ignore routes with cluster_specifier_plugin when GRPC_EXPE…

…RIMENTAL_XDS_RLS_LB is off (#5670)

xds/internal/xdsclient/xdsresource/unmarshal_rds.go

@@ -361,8 +361,22 @@ func routesProtoToSlice(routes []*v3routepb.Route, csps map[string]clusterspecif
 					return nil, nil, fmt.Errorf("route %+v, action %+v, has no valid cluster in WeightedCluster action", r, a)
 				}
 			case *v3routepb.RouteAction_ClusterSpecifierPlugin:
+				// gRFC A28 was updated to say the following:
+				//
+				// The route’s action field must be route, and its
+				// cluster_specifier:
+				// - Can be Cluster
+				// - Can be Weighted_clusters
+				//   - The sum of weights must add up to the total_weight.
+				// - Can be unset or an unsupported field. The route containing
+				//   this action will be ignored.
+				//
+				// This means that if this env var is not set, we should treat
+				// it as if it we didn't know about the cluster_specifier_plugin
+				// at all.
 				if !envconfig.XDSRLS {
-					return nil, nil, fmt.Errorf("route %+v, has an unknown ClusterSpecifier: %+v", r, a)
+					logger.Infof("route %+v contains route_action with unsupported field: cluster_specifier_plugin, the route will be ignored", r)
+					continue
 				}
 				if _, ok := csps[a.ClusterSpecifierPlugin]; !ok {
 					// "When processing RouteActions, if any action includes a

xds/internal/xdsclient/xdsresource/unmarshal_rds_test.go

@@ -738,7 +738,7 @@ func (s) TestRDSGenerateRDSUpdateFromRouteConfiguration(t *testing.T) {
 			rlsEnabled: true,
 		},
 		{
-			name: "ignore-error-in-cluster-specifier-plugin",
+			name: "ignore-error-in-cluster-specifier-plugin-env-var-off",
 			rc: goodRouteConfigWithClusterSpecifierPlugins([]*v3routepb.ClusterSpecifierPlugin{
 				clusterSpecifierPlugin("cspA", configOfClusterSpecifierDoesntExist, false),
 			}, []string{}),
@@ -749,7 +749,7 @@ func (s) TestRDSGenerateRDSUpdateFromRouteConfiguration(t *testing.T) {
 			rc: goodRouteConfigWithClusterSpecifierPlugins([]*v3routepb.ClusterSpecifierPlugin{
 				clusterSpecifierPlugin("cspA", mockClusterSpecifierConfig, false),
 			}, []string{"cspA"}),
-			wantError: true,
+			wantUpdate: goodUpdate,
 		},
 		// This tests a scenario where a cluster specifier plugin is not found
 		// and is optional. Any routes referencing that not found optional
@@ -1545,7 +1545,7 @@ func (s) TestRoutesProtoToSlice(t *testing.T) {
 							ClusterSpecifier: &v3routepb.RouteAction_ClusterSpecifierPlugin{}}},
 				},
 			},
-			wantErr: true,
+			wantRoutes: []*Route{},
 		},
 		{
 			name: "default totalWeight is 100 in weighted clusters action",