Properly implement unimplemented RPCs at callback-only server

include/grpcpp/server.h

@@ -326,6 +326,10 @@ class Server : public ServerInterface, private GrpcLibraryCodegen {
   std::unique_ptr<HealthCheckServiceInterface> health_check_service_;
   bool health_check_service_disabled_;
 
+  // When appropriate, use a default callback generic service to handle
+  // unimplemented methods
+  std::unique_ptr<experimental::CallbackGenericService> unimplemented_service_;
+
   // A special handler for resource exhausted in sync case
   std::unique_ptr<internal::MethodHandler> resource_exhausted_handler_;
 

src/cpp/server/server_cc.cc

@@ -1004,6 +1004,14 @@ void Server::Start(ServerCompletionQueue** cqs, size_t num_cqs) {
     RegisterService(nullptr, default_health_check_service_impl);
   }
 
+  // If this server uses callback methods, then create a callback generic
+  // service to handle any unimplemented methods using the default reactor
+  // creator
+  if (!callback_reqs_to_start_.empty() && !has_callback_generic_service_) {
+    unimplemented_service_.reset(new experimental::CallbackGenericService);
+    RegisterCallbackGenericService(unimplemented_service_.get());
+  }
+
   grpc_server_start(server_);
 
   if (!has_async_generic_service_ && !has_callback_generic_service_) {