[xds] xDS client per data plane target (#35730)

Closes #35730

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35730 from eugeneo:tasks/multiple-xds-clients 30b459658e
PiperOrigin-RevId: 610511488

src/core/ext/xds/xds_client_grpc.cc

@@ -92,7 +92,9 @@ namespace {
 
 Mutex* g_mu = new Mutex;
 const grpc_channel_args* g_channel_args ABSL_GUARDED_BY(*g_mu) = nullptr;
-GrpcXdsClient* g_xds_client ABSL_GUARDED_BY(*g_mu) = nullptr;
+// Key bytes live in clients so they outlive the entries in this map
+NoDestruct<std::map<absl::string_view, GrpcXdsClient*>> g_xds_client_map
+    ABSL_GUARDED_BY(*g_mu);
 char* g_fallback_bootstrap_config ABSL_GUARDED_BY(*g_mu) = nullptr;
 
 }  // namespace
@@ -139,9 +141,9 @@ absl::StatusOr<std::string> GetBootstrapContents(const char* fallback_config) {
 std::vector<RefCountedPtr<GrpcXdsClient>> GetAllXdsClients() {
   MutexLock lock(g_mu);
   std::vector<RefCountedPtr<GrpcXdsClient>> xds_clients;
-  if (g_xds_client != nullptr) {
+  for (const auto& key_client : *g_xds_client_map) {
     auto xds_client =
-        g_xds_client->RefIfNonZero(DEBUG_LOCATION, "DumpAllClientConfigs");
+        key_client.second->RefIfNonZero(DEBUG_LOCATION, "DumpAllClientConfigs");
     if (xds_client != nullptr) {
       xds_clients.emplace_back(xds_client.TakeAsSubclass<GrpcXdsClient>());
     }
@@ -152,7 +154,7 @@ std::vector<RefCountedPtr<GrpcXdsClient>> GetAllXdsClients() {
 }  // namespace
 
 absl::StatusOr<RefCountedPtr<GrpcXdsClient>> GrpcXdsClient::GetOrCreate(
-    const ChannelArgs& args, const char* reason) {
+    absl::string_view key, const ChannelArgs& args, const char* reason) {
   // If getting bootstrap from channel args, create a local XdsClient
   // instance for the channel or server instead of using the global instance.
   absl::optional<absl::string_view> bootstrap_config = args.GetString(
@@ -164,13 +166,14 @@ absl::StatusOr<RefCountedPtr<GrpcXdsClient>> GrpcXdsClient::GetOrCreate(
         GRPC_ARG_TEST_ONLY_DO_NOT_USE_IN_PROD_XDS_CLIENT_CHANNEL_ARGS);
     auto channel_args = ChannelArgs::FromC(xds_channel_args);
     return MakeRefCounted<GrpcXdsClient>(
-        std::move(*bootstrap), channel_args,
+        key, std::move(*bootstrap), channel_args,
         MakeOrphanable<GrpcXdsTransportFactory>(channel_args));
   }
   // Otherwise, use the global instance.
   MutexLock lock(g_mu);
-  if (g_xds_client != nullptr) {
-    auto xds_client = g_xds_client->RefIfNonZero(DEBUG_LOCATION, reason);
+  auto it = g_xds_client_map->find(key);
+  if (it != g_xds_client_map->end()) {
+    auto xds_client = it->second->RefIfNonZero(DEBUG_LOCATION, reason);
     if (xds_client != nullptr) {
       return xds_client.TakeAsSubclass<GrpcXdsClient>();
     }
@@ -188,9 +191,13 @@ absl::StatusOr<RefCountedPtr<GrpcXdsClient>> GrpcXdsClient::GetOrCreate(
   // Instantiate XdsClient.
   auto channel_args = ChannelArgs::FromC(g_channel_args);
   auto xds_client = MakeRefCounted<GrpcXdsClient>(
-      std::move(*bootstrap), channel_args,
+      key, std::move(*bootstrap), channel_args,
       MakeOrphanable<GrpcXdsTransportFactory>(channel_args));
-  g_xds_client = xds_client.get();
+  g_xds_client_map->emplace(xds_client->key(), xds_client.get());
+  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_client_trace)) {
+    gpr_log(GPR_INFO, "xDS client for key: %s was created",
+            std::string(key).c_str());
+  }
   return xds_client;
 }
 
@@ -211,6 +218,8 @@ grpc_slice GrpcXdsClient::DumpAllClientConfigs()
                                                                 arena.ptr());
     xds_client->mu()->Lock();
     xds_client->DumpClientConfig(&string_pool, arena.ptr(), client_config);
+    envoy_service_status_v3_ClientConfig_set_client_scope(
+        client_config, StdStringToUpbString(xds_client->key()));
   }
   // Serialize the upb message to bytes
   size_t output_length;
@@ -223,7 +232,8 @@ grpc_slice GrpcXdsClient::DumpAllClientConfigs()
 }
 
 GrpcXdsClient::GrpcXdsClient(
-    std::unique_ptr<GrpcXdsBootstrap> bootstrap, const ChannelArgs& args,
+    absl::string_view key, std::unique_ptr<GrpcXdsBootstrap> bootstrap,
+    const ChannelArgs& args,
     OrphanablePtr<XdsTransportFactory> transport_factory)
     : XdsClient(
           std::move(bootstrap), std::move(transport_factory),
@@ -237,13 +247,18 @@ GrpcXdsClient::GrpcXdsClient(
                    args.GetDurationFromIntMillis(
                            GRPC_ARG_XDS_RESOURCE_DOES_NOT_EXIST_TIMEOUT_MS)
                        .value_or(Duration::Seconds(15)))),
+      key_(key),
       certificate_provider_store_(MakeOrphanable<CertificateProviderStore>(
           static_cast<const GrpcXdsBootstrap&>(this->bootstrap())
               .certificate_providers())) {}
 
-GrpcXdsClient::~GrpcXdsClient() {
+void GrpcXdsClient::Orphan() {
   MutexLock lock(g_mu);
-  if (g_xds_client == this) g_xds_client = nullptr;
+  auto it = g_xds_client_map->find(key_);
+  if (it != g_xds_client_map->end() && it->second == this) {
+    g_xds_client_map->erase(it);
+  }
+  XdsClient::Orphan();
 }
 
 grpc_pollset_set* GrpcXdsClient::interested_parties() const {
@@ -258,9 +273,9 @@ void SetXdsChannelArgsForTest(grpc_channel_args* args) {
   g_channel_args = args;
 }
 
-void UnsetGlobalXdsClientForTest() {
+void UnsetGlobalXdsClientsForTest() {
   MutexLock lock(g_mu);
-  g_xds_client = nullptr;
+  g_xds_client_map->clear();
 }
 
 void SetXdsFallbackBootstrapConfig(const char* config) {

src/core/ext/xds/xds_client_grpc.h

@@ -43,7 +43,7 @@ class GrpcXdsClient : public XdsClient {
  public:
   // Factory function to get or create the global XdsClient instance.
   static absl::StatusOr<RefCountedPtr<GrpcXdsClient>> GetOrCreate(
-      const ChannelArgs& args, const char* reason);
+      absl::string_view key, const ChannelArgs& args, const char* reason);
 
   // Builds ClientStatusResponse containing all resources from all XdsClients
   static grpc_slice DumpAllClientConfigs();
@@ -60,10 +60,12 @@ class GrpcXdsClient : public XdsClient {
   // work for callers that use interested_parties() but not for callers
   // that also use certificate_provider_store(), but we should consider
   // alternatives for that case as well.
-  GrpcXdsClient(std::unique_ptr<GrpcXdsBootstrap> bootstrap,
+  GrpcXdsClient(absl::string_view key,
+                std::unique_ptr<GrpcXdsBootstrap> bootstrap,
                 const ChannelArgs& args,
                 OrphanablePtr<XdsTransportFactory> transport_factory);
-  ~GrpcXdsClient() override;
+
+  void Orphan() override;
 
   // Helpers for encoding the XdsClient object in channel args.
   static absl::string_view ChannelArgName() {
@@ -79,13 +81,16 @@ class GrpcXdsClient : public XdsClient {
     return *certificate_provider_store_;
   }
 
+  absl::string_view key() const { return key_; }
+
  private:
+  std::string key_;
   OrphanablePtr<CertificateProviderStore> certificate_provider_store_;
 };
 
 namespace internal {
 void SetXdsChannelArgsForTest(grpc_channel_args* args);
-void UnsetGlobalXdsClientForTest();
+void UnsetGlobalXdsClientsForTest();
 // Sets bootstrap config to be used when no env var is set.
 // Does not take ownership of config.
 void SetXdsFallbackBootstrapConfig(const char* config);

src/core/ext/xds/xds_server_config_fetcher.cc

@@ -94,6 +94,8 @@ namespace {
 
 using ReadDelayHandle = XdsClient::ReadDelayHandle;
 
+constexpr absl::string_view kServerXdsClientKey = "#server";
+
 TraceFlag grpc_xds_server_config_fetcher_trace(false,
                                                "xds_server_config_fetcher");
 
@@ -1372,7 +1374,7 @@ grpc_server_config_fetcher* grpc_server_config_fetcher_xds_create(
       "update=%p, user_data=%p}, args=%p)",
       3, (notifier.on_serving_status_update, notifier.user_data, args));
   auto xds_client = grpc_core::GrpcXdsClient::GetOrCreate(
-      channel_args, "XdsServerConfigFetcher");
+      grpc_core::kServerXdsClientKey, channel_args, "XdsServerConfigFetcher");
   if (!xds_client.ok()) {
     gpr_log(GPR_ERROR, "Failed to create xds client: %s",
             xds_client.status().ToString().c_str());

src/core/resolver/xds/xds_resolver.cc

@@ -880,7 +880,8 @@ void XdsResolver::ClusterSelectionFilter::Call::OnClientInitialMetadata(
 //
 
 void XdsResolver::StartLocked() {
-  auto xds_client = GrpcXdsClient::GetOrCreate(args_, "xds resolver");
+  auto xds_client =
+      GrpcXdsClient::GetOrCreate(uri_.ToString(), args_, "xds resolver");
   if (!xds_client.ok()) {
     gpr_log(GPR_ERROR,
             "Failed to create xds client -- channel will remain in "

test/cpp/end2end/xds/xds_core_end2end_test.cc

@@ -13,8 +13,10 @@
 // limitations under the License.
 //
 
+#include <algorithm>
 #include <memory>
 #include <string>
+#include <type_traits>
 #include <vector>
 
 #include <gmock/gmock.h>
@@ -24,9 +26,11 @@
 
 #include "src/core/client_channel/backup_poller.h"
 #include "src/core/lib/config/config_vars.h"
+#include "src/proto/grpc/testing/xds/v3/listener.pb.h"
 #include "test/core/util/resolve_localhost_ip46.h"
 #include "test/core/util/scoped_env_var.h"
 #include "test/cpp/end2end/xds/xds_end2end_test_lib.h"
+#include "test/cpp/end2end/xds/xds_server.h"
 
 namespace grpc {
 namespace testing {
@@ -167,7 +171,21 @@ INSTANTIATE_TEST_SUITE_P(XdsTest, GlobalXdsClientTest,
                              XdsTestType::kBootstrapFromEnvVar)),
                          &XdsTestType::Name);
 
-TEST_P(GlobalXdsClientTest, MultipleChannelsShareXdsClient) {
+TEST_P(GlobalXdsClientTest, MultipleChannelsSameTargetShareXdsClient) {
+  CreateAndStartBackends(1);
+  EdsResourceArgs args({{"locality0", CreateEndpointsForBackends()}});
+  balancer_->ads_service()->SetEdsResource(BuildEdsResource(args));
+  WaitForAllBackends(DEBUG_LOCATION);
+  // Create second channel and tell it to connect to the same server.
+  auto channel2 = CreateChannel(/*failover_timeout_ms=*/0, kServerName);
+  channel2->GetState(/*try_to_connect=*/true);
+  ASSERT_TRUE(channel2->WaitForConnected(grpc_timeout_seconds_to_deadline(1)));
+  // Make sure there's only one client connected.
+  EXPECT_EQ(1UL, balancer_->ads_service()->clients().size());
+}
+
+TEST_P(GlobalXdsClientTest,
+       MultipleChannelsDifferentTargetDoNotShareXdsClient) {
   CreateAndStartBackends(1);
   const char* kNewServerName = "new-server.example.com";
   Listener listener = default_listener_;
@@ -181,8 +199,8 @@ TEST_P(GlobalXdsClientTest, MultipleChannelsShareXdsClient) {
   auto channel2 = CreateChannel(/*failover_timeout_ms=*/0, kNewServerName);
   channel2->GetState(/*try_to_connect=*/true);
   ASSERT_TRUE(channel2->WaitForConnected(grpc_timeout_seconds_to_deadline(1)));
-  // Make sure there's only one client connected.
-  EXPECT_EQ(1UL, balancer_->ads_service()->clients().size());
+  // Make sure there are two clients connected.
+  EXPECT_EQ(2UL, balancer_->ads_service()->clients().size());
 }
 
 TEST_P(
@@ -219,64 +237,62 @@ TEST_P(
 }
 
 // Tests that the NACK for multiple bad LDS resources includes both errors.
-// This needs to be in GlobalXdsClientTest because the only way to request
-// two LDS resources in the same XdsClient is for two channels to share
-// the same XdsClient.
+// This needs to use xDS server as this is the only scenario when XdsClient
+// is shared.
 TEST_P(GlobalXdsClientTest, MultipleBadLdsResources) {
-  CreateAndStartBackends(1);
-  constexpr char kServerName2[] = "server.other.com";
-  constexpr char kServerName3[] = "server.another.com";
-  auto listener = default_listener_;
-  listener.clear_api_listener();
-  balancer_->ads_service()->SetLdsResource(listener);
-  listener.set_name(kServerName2);
-  balancer_->ads_service()->SetLdsResource(listener);
-  listener = default_listener_;
-  listener.set_name(kServerName3);
-  SetListenerAndRouteConfiguration(balancer_.get(), listener,
-                                   default_route_config_);
-  EdsResourceArgs args({{"locality0", CreateEndpointsForBackends()}});
+  CreateBackends(2, true);
+  EdsResourceArgs args({{"locality0", CreateEndpointsForBackends(0, 2)}});
   balancer_->ads_service()->SetEdsResource(BuildEdsResource(args));
-  const auto response_state = WaitForLdsNack(DEBUG_LOCATION);
-  ASSERT_TRUE(response_state.has_value()) << "timed out waiting for NACK";
-  EXPECT_EQ(response_state->error_message,
-            "xDS response validation errors: ["
-            "resource index 0: server.example.com: "
-            "INVALID_ARGUMENT: Listener has neither address nor ApiListener]");
-  // Need to create a second channel to subscribe to a second LDS resource.
-  auto channel2 = CreateChannel(0, kServerName2);
-  auto stub2 = grpc::testing::EchoTestService::NewStub(channel2);
-  {
-    ClientContext context;
-    EchoRequest request;
-    request.set_message(kRequestMessage);
-    EchoResponse response;
-    grpc::Status status = stub2->Echo(&context, request, &response);
-    EXPECT_FALSE(status.ok());
-    // Wait for second NACK to be reported to xDS server.
-    const auto response_state = WaitForLdsNack(DEBUG_LOCATION);
+  auto listener = default_server_listener_;
+  listener.clear_address();
+  listener.set_name(GetServerListenerName(backends_[0]->port()));
+  balancer_->ads_service()->SetLdsResource(listener);
+  backends_[0]->Start();
+  auto response_state = WaitForLdsNack(DEBUG_LOCATION);
   ASSERT_TRUE(response_state.has_value()) << "timed out waiting for NACK";
   EXPECT_EQ(
       response_state->error_message,
+      absl::StrFormat(
           "xDS response validation errors: ["
-        "resource index 0: server.other.com: "
-        "INVALID_ARGUMENT: Listener has neither address nor ApiListener; "
-        "resource index 1: server.example.com: "
-        "INVALID_ARGUMENT: Listener has neither address nor ApiListener]");
+          "resource index 0: "
+          "grpc/server?xds.resource.listening_address=127.0.0.1:%lu: "
+          "INVALID_ARGUMENT: Listener has neither address nor ApiListener]",
+          backends_[0]->port()));
+  listener = default_server_listener_;
+  listener.clear_address();
+  listener.set_name(GetServerListenerName(backends_[1]->port()));
+  balancer_->ads_service()->SetLdsResource(listener);
+  backends_[1]->Start();
+  constexpr absl::string_view kMessageFormat =
+      "xDS response validation errors: ["
+      "resource index 0: "
+      "grpc/server?xds.resource.listening_address=127.0.0.1:%d: "
+      "INVALID_ARGUMENT: Listener has neither address nor "
+      "ApiListener; "
+      "resource index 1: "
+      "grpc/server?xds.resource.listening_address=127.0.0.1:%d: "
+      "INVALID_ARGUMENT: Listener has neither address nor "
+      "ApiListener"
+      "]";
+  const std::string expected_message1 = absl::StrFormat(
+      kMessageFormat, backends_[0]->port(), backends_[1]->port());
+  const std::string expected_message2 = absl::StrFormat(
+      kMessageFormat, backends_[1]->port(), backends_[0]->port());
+  response_state = WaitForNack(
+      DEBUG_LOCATION, [&]() -> absl::optional<AdsServiceImpl::ResponseState> {
+        auto response = balancer_->ads_service()->lds_response_state();
+        if (response.has_value() &&
+            response->state == AdsServiceImpl::ResponseState::NACKED) {
+          if (response->error_message == expected_message1 ||
+              response->error_message == expected_message2) {
+            return response;
           }
-  // Now start a new channel with a third server name, this one with a
-  // valid resource.
-  auto channel3 = CreateChannel(0, kServerName3);
-  auto stub3 = grpc::testing::EchoTestService::NewStub(channel3);
-  {
-    ClientContext context;
-    EchoRequest request;
-    request.set_message(kRequestMessage);
-    EchoResponse response;
-    grpc::Status status = stub3->Echo(&context, request, &response);
-    EXPECT_TRUE(status.ok()) << "code=" << status.error_code()
-                             << " message=" << status.error_message();
+          gpr_log(GPR_INFO, "non-matching NACK message: %s",
+                  response->error_message.c_str());
         }
+        return absl::nullopt;
+      });
+  EXPECT_TRUE(response_state.has_value()) << "timed out waiting for NACK";
 }
 
 // Tests that we don't trigger does-not-exist callbacks for a resource

test/cpp/end2end/xds/xds_csds_end2end_test.cc

@@ -22,6 +22,7 @@
 
 #include "absl/memory/memory.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/strip.h"
 
 #include <grpcpp/create_channel.h>
 #include <grpcpp/security/credentials.h>
@@ -344,7 +345,7 @@ TEST_P(ClientStatusDiscoveryServiceTest, XdsConfigDumpVanilla) {
   // Fetches the client config
   auto csds_response = FetchCsdsResponse();
   gpr_log(GPR_INFO, "xDS config dump: %s", csds_response.DebugString().c_str());
-  EXPECT_EQ(1, csds_response.config_size());
+  ASSERT_EQ(1, csds_response.config_size());
   const auto& client_config = csds_response.config(0);
   // Validate the Node information
   EXPECT_THAT(client_config.node(),
@@ -615,6 +616,82 @@ TEST_P(ClientStatusDiscoveryServiceTest, XdsConfigDumpClusterRequested) {
                       ClientResourceStatus::REQUESTED, ::testing::_))));
 }
 
+TEST_P(ClientStatusDiscoveryServiceTest, XdsConfigDumpMultiClient) {
+  Listener listener = default_listener_;
+  const char* kServer2Name = "server2.example.com";
+  listener.set_name(kServer2Name);
+  balancer_->ads_service()->SetLdsResource(listener);
+  SetListenerAndRouteConfiguration(balancer_.get(), listener,
+                                   default_route_config_);
+  CreateAndStartBackends(1);
+  const size_t kNumRpcs = 5;
+  EdsResourceArgs args({{"locality0", CreateEndpointsForBackends(0, 1)}});
+  balancer_->ads_service()->SetEdsResource(BuildEdsResource(args));
+  // Send several RPCs to ensure the xDS setup works
+  CheckRpcSendOk(DEBUG_LOCATION, kNumRpcs);
+  // Connect to a second server
+  auto channel2 = CreateChannel(0, kServer2Name);
+  channel2->GetState(/*try_to_connect=*/true);
+  ASSERT_TRUE(channel2->WaitForConnected(grpc_timeout_seconds_to_deadline(1)));
+  // Fetches the client config
+  auto csds_response = FetchCsdsResponse();
+  ASSERT_EQ(2, csds_response.config_size());
+  std::vector<std::string> scopes;
+  for (const auto& client_config : csds_response.config()) {
+    // Validate the Node information
+    EXPECT_THAT(client_config.node(),
+                EqNode("xds_end2end_test", ::testing::HasSubstr("C-core"),
+                       ::testing::HasSubstr(grpc_version_string()),
+                       ::testing::ElementsAre(
+                           "envoy.lb.does_not_support_overprovisioning")));
+    scopes.emplace_back(client_config.client_scope());
+    absl::string_view server = client_config.client_scope();
+    // Listener matcher depends on whether RDS is enabled.
+    ::testing::Matcher<google::protobuf::Any> api_listener_matcher;
+    if (GetParam().enable_rds_testing()) {
+      api_listener_matcher = IsRdsEnabledHCM();
+    } else {
+      api_listener_matcher =
+          EqNoRdsHCM(kDefaultRouteConfigurationName, kDefaultClusterName);
+    }
+    // Construct list of all matchers.
+    std::vector<::testing::Matcher<
+        envoy::service::status::v3::ClientConfig_GenericXdsConfig>>
+        matchers = {
+            // Listener
+            EqGenericXdsConfig(
+                kLdsTypeUrl, absl::StripPrefix(server, "xds:"), "3",
+                UnpackListener(EqListener(absl::StripPrefix(server, "xds:"),
+                                          api_listener_matcher)),
+                ClientResourceStatus::ACKED, ::testing::_),
+            // Cluster
+            EqGenericXdsConfig(kCdsTypeUrl, kDefaultClusterName, "1",
+                               UnpackCluster(EqCluster(kDefaultClusterName)),
+                               ClientResourceStatus::ACKED, ::testing::_),
+            // ClusterLoadAssignment
+            EqGenericXdsConfig(
+                kEdsTypeUrl, kDefaultEdsServiceName, "1",
+                UnpackClusterLoadAssignment(EqClusterLoadAssignment(
+                    kDefaultEdsServiceName, backends_[0]->port(),
+                    kDefaultLocalityWeight)),
+                ClientResourceStatus::ACKED, ::testing::_),
+        };
+    // If RDS is enabled, add matcher for RDS resource.
+    if (GetParam().enable_rds_testing()) {
+      matchers.push_back(EqGenericXdsConfig(
+          kRdsTypeUrl, kDefaultRouteConfigurationName, "2",
+          UnpackRouteConfiguration(EqRouteConfiguration(
+              kDefaultRouteConfigurationName, kDefaultClusterName)),
+          ClientResourceStatus::ACKED, ::testing::_));
+    }
+    // Validate the dumped xDS configs
+    EXPECT_THAT(client_config.generic_xds_configs(),
+                ::testing::UnorderedElementsAreArray(matchers));
+  }
+  EXPECT_THAT(scopes, ::testing::UnorderedElementsAre(
+                          "xds:server.example.com", "xds:server2.example.com"));
+}
+
 class CsdsShortAdsTimeoutTest : public ClientStatusDiscoveryServiceTest {
  protected:
   void SetUp() override {

test/cpp/end2end/xds/xds_end2end_test_lib.cc

@@ -523,7 +523,7 @@ void XdsEnd2endTest::InitClient(XdsBootstrapBuilder builder,
     // because it's not expecting the client to connect.  It also
     // ensures that each test can independently set the global channel
     // args for the xDS channel.
-    grpc_core::internal::UnsetGlobalXdsClientForTest();
+    grpc_core::internal::UnsetGlobalXdsClientsForTest();
   }
   // Create channel and stub.
   ResetStub();
@@ -558,7 +558,14 @@ std::shared_ptr<Channel> XdsEnd2endTest::CreateChannel(
         GRPC_ARG_TEST_ONLY_DO_NOT_USE_IN_PROD_XDS_CLIENT_CHANNEL_ARGS,
         &xds_channel_args_, &kChannelArgsArgVtable);
   }
-  std::string uri = absl::StrCat("xds://", xds_authority, "/", server_name);
+  std::vector<absl::string_view> parts = {"xds:"};
+  if (xds_authority != nullptr && xds_authority[0] != '\0') {
+    parts.emplace_back("//");
+    parts.emplace_back(xds_authority);
+    parts.emplace_back("/");
+  }
+  parts.emplace_back(server_name);
+  std::string uri = absl::StrJoin(parts, "");
   std::shared_ptr<ChannelCredentials> channel_creds =
       GetParam().use_xds_credentials()
           ? XdsCredentials(CreateTlsFallbackCredentials())