Fix fake xDS server bug that caused unnecessary round trips in NACK tests.

test/cpp/end2end/xds_end2end_test.cc

@@ -834,15 +834,14 @@ class AdsServiceImpl : public std::enable_shared_from_this<AdsServiceImpl> {
                         SubscriptionMap* subscription_map,
                         SentState* sent_state,
                         absl::optional<DiscoveryResponse>* response) {
-      // Determine client resource type version.
+      // Check the nonce sent by the client, if any.
+      // (This will be absent on the first request on a stream.)
+      if (request.response_nonce().empty()) {
         int client_resource_type_version = 0;
         if (!request.version_info().empty()) {
           GPR_ASSERT(absl::SimpleAtoi(request.version_info(),
                                       &client_resource_type_version));
         }
-      // Check the nonce sent by the client, if any.
-      // (This will be absent on the first request on a stream.)
-      if (request.response_nonce().empty()) {
         EXPECT_GE(client_resource_type_version,
                   parent_->resource_type_min_versions_[v3_resource_type])
             << "resource_type: " << v3_resource_type;
@@ -896,7 +895,7 @@ class AdsServiceImpl : public std::enable_shared_from_this<AdsServiceImpl> {
                                     &subscription_state, &resource_state,
                                     update_queue) ||
             ClientNeedsResourceUpdate(resource_type_state, resource_state,
-                                      client_resource_type_version)) {
+                                      sent_state->resource_type_version)) {
           gpr_log(GPR_INFO, "ADS[%p]: Sending update for type=%s name=%s", this,
                   request.type_url().c_str(), resource_name.c_str());
           resources_added_to_response.emplace(resource_name);