xDS retry policy should be considered present even if there are no supported policies in retry_on (#27315)

* Ensure that per route retry policy (even when there are no supported
retry_on statuses) still takes precedence over virtual host level retry
policy.

Added a test to guard this case.

* Taking care of code review comments and removing unnecessary block

src/core/ext/filters/client_channel/resolver/xds/xds_resolver.cc

@@ -469,7 +469,7 @@ grpc_error_handle XdsResolver::XdsConfigSelector::CreateMethodConfig(
     RefCountedPtr<ServiceConfig>* method_config) {
   std::vector<std::string> fields;
   // Set retry policy if any.
-  if (route.retry_policy.has_value()) {
+  if (route.retry_policy.has_value() && !route.retry_policy->retry_on.Empty()) {
     std::vector<std::string> retry_parts;
     retry_parts.push_back(absl::StrFormat(
         "\"retryPolicy\": {\n"

src/core/ext/xds/xds_api.cc

@@ -1552,10 +1552,6 @@ grpc_error_handle RetryPolicyParse(
       }
     }
   }
-  // TODO(donnadionne): when we add support for per_try_timeout, we will need to
-  // return a policy if per_try_timeout is set even if retry_on specified no
-  // supported policies.
-  if (retry_to_return.retry_on.Empty()) return GRPC_ERROR_NONE;
   const google_protobuf_UInt32Value* num_retries =
       envoy_config_route_v3_RetryPolicy_num_retries(retry_policy);
   if (num_retries != nullptr) {

test/cpp/end2end/xds_end2end_test.cc

@@ -5894,6 +5894,39 @@ TEST_P(LdsRdsTest, XdsRetryPolicyUnsupportedStatusCode) {
   EXPECT_EQ(1, backends_[0]->backend_service()->request_count());
 }
 
+TEST_P(LdsRdsTest,
+       XdsRetryPolicyUnsupportedStatusCodeWithVirtualHostLevelRetry) {
+  const size_t kNumRetries = 3;
+  SetNextResolution({});
+  SetNextResolutionForLbChannelAllBalancers();
+  // Populate new EDS resources.
+  AdsServiceImpl::EdsResourceArgs args({
+      {"locality0", CreateEndpointsForBackends(0, 1)},
+  });
+  balancers_[0]->ads_service()->SetEdsResource(BuildEdsResource(args));
+  // Construct route config to set retry policy with no supported retry_on
+  // statuses.
+  RouteConfiguration new_route_config = default_route_config_;
+  auto* route1 = new_route_config.mutable_virtual_hosts(0)->mutable_routes(0);
+  auto* retry_policy = route1->mutable_route()->mutable_retry_policy();
+  retry_policy->set_retry_on("5xx");
+  retry_policy->mutable_num_retries()->set_value(kNumRetries);
+  // Construct a virtual host level retry policy with supported statuses.
+  auto* virtual_host_retry_policy =
+      new_route_config.mutable_virtual_hosts(0)->mutable_retry_policy();
+  virtual_host_retry_policy->set_retry_on(
+      "cancelled,deadline-exceeded,internal,resource-exhausted,unavailable");
+  virtual_host_retry_policy->mutable_num_retries()->set_value(kNumRetries);
+  SetRouteConfiguration(0, new_route_config);
+  // We expect no retry.
+  CheckRpcSendFailure(
+      CheckRpcSendFailureOptions()
+          .set_rpc_options(RpcOptions().set_server_expected_error(
+              StatusCode::DEADLINE_EXCEEDED))
+          .set_expected_error_code(StatusCode::DEADLINE_EXCEEDED));
+  EXPECT_EQ(1, backends_[0]->backend_service()->request_count());
+}
+
 TEST_P(LdsRdsTest, XdsRetryPolicyInvalidNumRetriesZero) {
   SetNextResolution({});
   SetNextResolutionForLbChannelAllBalancers();