Fixing the transient failure that occurs during the case where (Porting

src/core/ext/filters/client_channel/xds/xds_client.cc

@@ -1034,6 +1034,13 @@ void XdsClient::ChannelState::AdsCallState::AcceptCdsUpdate(
     const std::string& cluster_name = p.first;
     if (cds_update_map.find(cluster_name) == cds_update_map.end()) {
       ClusterState& cluster_state = xds_client()->cluster_map_[cluster_name];
+      // If the resource was newly requested but has not yet been received,
+      // we don't want to generate an error for the watchers, because this CDS
+      // response may be in reaction to an earlier request that did not yet
+      // request the new resource, so its absence from the response does not
+      // necessarily indicate that the resource does not exist.
+      // For that case, we rely on the request timeout instead.
+      if (!cluster_state.update.has_value()) continue;
       cluster_state.update.reset();
       for (const auto& p : cluster_state.watchers) {
         p.first->OnResourceDoesNotExist();

test/cpp/end2end/xds_end2end_test.cc

@@ -1356,6 +1356,7 @@ class XdsEnd2endTest : public ::testing::TestWithParam<TestType> {
             "Performed %d warm up requests against the backends. "
             "%d succeeded, %d failed, %d dropped.",
             num_total, num_ok, num_failure, num_drops);
+    EXPECT_EQ(num_failure, 0);
     return std::make_tuple(num_ok, num_failure, num_drops);
   }