[xDS] make transport factory injectable in GrpcXdsClient (#33328)

src/core/ext/xds/xds_client_grpc.cc

@@ -145,8 +145,10 @@ absl::StatusOr<RefCountedPtr<GrpcXdsClient>> GrpcXdsClient::GetOrCreate(
     if (!bootstrap.ok()) return bootstrap.status();
     grpc_channel_args* xds_channel_args = args.GetPointer<grpc_channel_args>(
         GRPC_ARG_TEST_ONLY_DO_NOT_USE_IN_PROD_XDS_CLIENT_CHANNEL_ARGS);
-    return MakeRefCounted<GrpcXdsClient>(std::move(*bootstrap),
-                                         ChannelArgs::FromC(xds_channel_args));
+    auto channel_args = ChannelArgs::FromC(xds_channel_args);
+    return MakeRefCounted<GrpcXdsClient>(
+        std::move(*bootstrap), channel_args,
+        MakeOrphanable<GrpcXdsTransportFactory>(channel_args));
   }
   // Otherwise, use the global instance.
   MutexLock lock(g_mu);
@@ -165,16 +167,19 @@ absl::StatusOr<RefCountedPtr<GrpcXdsClient>> GrpcXdsClient::GetOrCreate(
   auto bootstrap = GrpcXdsBootstrap::Create(*bootstrap_contents);
   if (!bootstrap.ok()) return bootstrap.status();
   // Instantiate XdsClient.
+  auto channel_args = ChannelArgs::FromC(g_channel_args);
   auto xds_client = MakeRefCounted<GrpcXdsClient>(
-      std::move(*bootstrap), ChannelArgs::FromC(g_channel_args));
+      std::move(*bootstrap), channel_args,
+      MakeOrphanable<GrpcXdsTransportFactory>(channel_args));
   g_xds_client = xds_client.get();
   return xds_client;
 }
 
-GrpcXdsClient::GrpcXdsClient(std::unique_ptr<GrpcXdsBootstrap> bootstrap,
-                             const ChannelArgs& args)
+GrpcXdsClient::GrpcXdsClient(
+    std::unique_ptr<GrpcXdsBootstrap> bootstrap, const ChannelArgs& args,
+    OrphanablePtr<XdsTransportFactory> transport_factory)
     : XdsClient(
-          std::move(bootstrap), MakeOrphanable<GrpcXdsTransportFactory>(args),
+          std::move(bootstrap), std::move(transport_factory),
           grpc_event_engine::experimental::GetDefaultEventEngine(),
           absl::StrCat("gRPC C-core ", GPR_PLATFORM_STRING,
                        GRPC_XDS_USER_AGENT_NAME_SUFFIX_STRING),

src/core/ext/xds/xds_client_grpc.h

@@ -29,6 +29,7 @@
 #include "src/core/ext/xds/certificate_provider_store.h"
 #include "src/core/ext/xds/xds_bootstrap_grpc.h"
 #include "src/core/ext/xds/xds_client.h"
+#include "src/core/ext/xds/xds_transport.h"
 #include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/gpr/useful.h"
 #include "src/core/lib/gprpp/orphanable.h"
@@ -44,8 +45,20 @@ class GrpcXdsClient : public XdsClient {
       const ChannelArgs& args, const char* reason);
 
   // Do not instantiate directly -- use GetOrCreate() instead.
+  // TODO(roth): The transport factory is injectable here to support
+  // tests that want to use a fake transport factory with code that
+  // expects a GrpcXdsClient instead of an XdsClient, typically because
+  // it needs to call the interested_parties() method.  Once we
+  // finish the EventEngine migration and remove the interested_parties()
+  // method, consider instead changing callers to an approach where the
+  // production code uses XdsClient instead of GrpcXdsClient, and then
+  // passing in a fake XdsClient impl in the tests.  Note that this will
+  // 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,
-                const ChannelArgs& args);
+                const ChannelArgs& args,
+                OrphanablePtr<XdsTransportFactory> transport_factory);
   ~GrpcXdsClient() override;
 
   // Helpers for encoding the XdsClient object in channel args.