From 7e72bba1627f2563c1a8d3c5c8135f7f286ce456 Mon Sep 17 00:00:00 2001
From: Vijay Pai <vpai@google.com>
Date: Thu, 15 Oct 2020 04:41:59 -0700
Subject: [PATCH] Reduce templating for unary callback client RPC

---
 include/grpcpp/impl/codegen/client_callback.h | 16 +++++++++++++---
 src/compiler/cpp_generator.cc                 |  2 ++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/include/grpcpp/impl/codegen/client_callback.h b/include/grpcpp/impl/codegen/client_callback.h
index 90c817ceaa7..6ab0dfbc746 100644
--- a/include/grpcpp/impl/codegen/client_callback.h
+++ b/include/grpcpp/impl/codegen/client_callback.h
@@ -35,15 +35,25 @@ class ClientContext;
 namespace internal {
 class RpcMethod;
 
-/// Perform a callback-based unary call
+/// Perform a callback-based unary call.  May optionally specify the base
+/// class of the Request and Response so that the internal calls and structures
+/// below this may be based on those base classes and thus achieve code reuse
+/// across different RPCs (e.g., for protobuf, MessageLite would be a base
+/// class).
 /// TODO(vjpai): Combine as much as possible with the blocking unary call code
-template <class InputMessage, class OutputMessage>
+template <class InputMessage, class OutputMessage,
+          class BaseInputMessage = InputMessage,
+          class BaseOutputMessage = OutputMessage>
 void CallbackUnaryCall(::grpc::ChannelInterface* channel,
                        const ::grpc::internal::RpcMethod& method,
                        ::grpc::ClientContext* context,
                        const InputMessage* request, OutputMessage* result,
                        std::function<void(::grpc::Status)> on_completion) {
-  CallbackUnaryCallImpl<InputMessage, OutputMessage> x(
+  static_assert(std::is_base_of<BaseInputMessage, InputMessage>::value,
+                "Invalid input message specification");
+  static_assert(std::is_base_of<BaseOutputMessage, OutputMessage>::value,
+                "Invalid output message specification");
+  CallbackUnaryCallImpl<BaseInputMessage, BaseOutputMessage> x(
       channel, method, context, request, result, on_completion);
 }
 
diff --git a/src/compiler/cpp_generator.cc b/src/compiler/cpp_generator.cc
index 9a20a5a5711..7ed74eb2302 100644
--- a/src/compiler/cpp_generator.cc
+++ b/src/compiler/cpp_generator.cc
@@ -1897,6 +1897,8 @@ void PrintSourceClientMethod(grpc_generator::Printer* printer,
                    "std::function<void(::grpc::Status)> f) {\n");
     printer->Print(*vars,
                    "  ::grpc::internal::CallbackUnaryCall"
+                   "< $Request$, $Response$, ::grpc::protobuf::MessageLite, "
+                   "::grpc::protobuf::MessageLite>"
                    "(stub_->channel_.get(), stub_->rpcmethod_$Method$_, "
                    "context, request, response, std::move(f));\n}\n\n");