[C++] Improve flatbuffers + gRPC integration (#4310)

* Rework flatbuffers + gRPC integration

- Introduce `flatbuffers::grpc::Message<T>`, a `grpc_slice`-backed
message buffer that handles refcounting and allows flatbuffers to
transfer ownership to gRPC efficiently. This replaces
`flatbuffers::BufferRef<T>`, which required a copy call and was also
unsafe w.r.t. buffer lifetime.
- Introduce `flatbuffers::grpc::MessageBuilder`, a gRPC-specific builder
that forces a `grpc_slice`-backed allocator and also adds some helpful
`Message<T>`-related methods.
- Update serializers accordingly (now zero-copy between flatbuffers and
gRPC).

* gRPC: verify messages by default, but allow user to override

* gRPC: fix some formatting issues

* Disable verification by default, but add helper method

* Make FlatBufferBuilder fields protected + remove vec accessor

* Use bool add_ref parameter to toggle refcount incr

* Remove unnecessary inline specifiers

* Fix formatting

* Use auto

* Remove empty lines

* Use grpc_slice helper macros

* Simplify reset code

* Disable Message copy ctor and assignment by default

* Remove unused member

* Enable gRPC verification by default

* Use auto

* Bake in message verification (remove template specialization)

* Add RoundUp func

* Consolidate gRPC message copy flag

* Make vector_downward allocations fully lazy

* Test message verification failure code/message

* Add grpctest verification test comments

* Simplify reallocate implementation

* Make initial_size a size_t

* Use ternary op for growth_policy

* Use truthiness rather than dont explicit nullptr check

* Indent preprocessor directives

* Remove grpc message copy/assignment

* Fix a few bugs

* Add gRPC example

* Add basic gRPC docs

* Use doxygen EXAMPLE_PATH + @include

* Reference example fbs in grpc docs

* Move gRPC examples into grpc/samples

* Fix pointer/reference formatting

* Use std::function rather than templated callback func

* Create fresh message builder for each request

* Use Clear() in Reset() impl

* Use FLATBUFFERS_CONSTEXPR

grpc/samples/greeter/Makefile

@@ -0,0 +1,14 @@
+CXXFLAGS ?= -I../../../include
+LDFLAGS ?=
+
+.PHONY: all
+all: server client
+
+greeter_generated.h: greeter.fbs
+	flatc --grpc --cpp $<
+
+server: server.cpp greeter.grpc.fb.cc greeter_generated.h greeter.grpc.fb.h
+	g++ -std=c++11 -O2 $(CXXFLAGS) $(LDFLAGS) -lgpr -lgrpc -lgrpc++ server.cpp greeter.grpc.fb.cc -o $@
+
+client: client.cpp greeter.grpc.fb.cc greeter_generated.h greeter.grpc.fb.h
+	g++ -std=c++11 -O2 $(CXXFLAGS) $(LDFLAGS) -lgpr -lgrpc -lgrpc++ client.cpp greeter.grpc.fb.cc -o $@

grpc/samples/greeter/client.cpp

@@ -0,0 +1,85 @@
+#include "greeter.grpc.fb.h"
+#include "greeter_generated.h"
+
+#include <grpc++/grpc++.h>
+
+#include <iostream>
+#include <memory>
+#include <string>
+
+class GreeterClient {
+ public:
+  GreeterClient(std::shared_ptr<grpc::Channel> channel)
+    : stub_(Greeter::NewStub(channel)) {}
+
+  std::string SayHello(const std::string &name) {
+    flatbuffers::grpc::MessageBuilder mb;
+    auto name_offset = mb.CreateString(name);
+    auto request_offset = CreateHelloRequest(mb, name_offset);
+    mb.Finish(request_offset);
+    auto request_msg = mb.ReleaseMessage<HelloRequest>();
+
+    flatbuffers::grpc::Message<HelloReply> response_msg;
+
+    grpc::ClientContext context;
+
+    auto status = stub_->SayHello(&context, request_msg, &response_msg);
+    if (status.ok()) {
+      const HelloReply *response = response_msg.GetRoot();
+      return response->message()->str();
+    } else {
+      std::cerr << status.error_code() << ": " << status.error_message()
+                << std::endl;
+      return "RPC failed";
+    }
+  }
+
+  void SayManyHellos(const std::string &name, int num_greetings,
+                     std::function<void(const std::string &)> callback) {
+    flatbuffers::grpc::MessageBuilder mb;
+    auto name_offset = mb.CreateString(name);
+    auto request_offset =
+        CreateManyHellosRequest(mb, name_offset, num_greetings);
+    mb.Finish(request_offset);
+    auto request_msg = mb.ReleaseMessage<ManyHellosRequest>();
+
+    flatbuffers::grpc::Message<HelloReply> response_msg;
+
+    grpc::ClientContext context;
+
+    auto stream = stub_->SayManyHellos(&context, request_msg);
+    while (stream->Read(&response_msg)) {
+      const HelloReply *response = response_msg.GetRoot();
+      callback(response->message()->str());
+    }
+    auto status = stream->Finish();
+    if (!status.ok()) {
+      std::cerr << status.error_code() << ": " << status.error_message()
+                << std::endl;
+      callback("RPC failed");
+    }
+  }
+
+ private:
+  std::unique_ptr<Greeter::Stub> stub_;
+};
+
+int main(int argc, char **argv) {
+  std::string server_address("localhost:50051");
+
+  auto channel =
+      grpc::CreateChannel(server_address, grpc::InsecureChannelCredentials());
+  GreeterClient greeter(channel);
+
+  std::string name("world");
+
+  std::string message = greeter.SayHello(name);
+  std::cerr << "Greeter received: " << message << std::endl;
+
+  int num_greetings = 10;
+  greeter.SayManyHellos(name, num_greetings, [](const std::string &message) {
+    std::cerr << "Greeter received: " << message << std::endl;
+  });
+
+  return 0;
+}

grpc/samples/greeter/greeter.fbs

@@ -0,0 +1,17 @@
+table HelloReply {
+  message:string;
+}
+
+table HelloRequest {
+  name:string;
+}
+
+table ManyHellosRequest {
+  name:string;
+  num_greetings:int;
+}
+
+rpc_service Greeter {
+  SayHello(HelloRequest):HelloReply;
+  SayManyHellos(ManyHellosRequest):HelloReply (streaming: "server");
+}

grpc/samples/greeter/server.cpp

@@ -0,0 +1,80 @@
+#include "greeter.grpc.fb.h"
+#include "greeter_generated.h"
+
+#include <grpc++/grpc++.h>
+
+#include <iostream>
+#include <memory>
+#include <string>
+
+class GreeterServiceImpl final : public Greeter::Service {
+  virtual grpc::Status SayHello(
+      grpc::ServerContext *context,
+      const flatbuffers::grpc::Message<HelloRequest> *request_msg,
+      flatbuffers::grpc::Message<HelloReply> *response_msg) override {
+    // flatbuffers::grpc::MessageBuilder mb_;
+    // We call GetRoot to "parse" the message. Verification is already
+    // performed by default. See the notes below for more details.
+    const HelloRequest *request = request_msg->GetRoot();
+
+    // Fields are retrieved as usual with FlatBuffers
+    const std::string &name = request->name()->str();
+
+    // `flatbuffers::grpc::MessageBuilder` is a `FlatBufferBuilder` with a
+    // special allocator for efficient gRPC buffer transfer, but otherwise
+    // usage is the same as usual.
+    auto msg_offset = mb_.CreateString("Hello, " + name);
+    auto hello_offset = CreateHelloReply(mb_, msg_offset);
+    mb_.Finish(hello_offset);
+
+    // The `ReleaseMessage<T>()` function detaches the message from the
+    // builder, so we can transfer the resopnse to gRPC while simultaneously
+    // detaching that memory buffer from the builer.
+    *response_msg = mb_.ReleaseMessage<HelloReply>();
+    assert(response_msg->Verify());
+
+    // Return an OK status.
+    return grpc::Status::OK;
+  }
+
+  virtual grpc::Status SayManyHellos(
+      grpc::ServerContext *context,
+      const flatbuffers::grpc::Message<ManyHellosRequest> *request_msg,
+      grpc::ServerWriter<flatbuffers::grpc::Message<HelloReply>> *writer)
+      override {
+    // The streaming usage below is simply a combination of standard gRPC
+    // streaming with the FlatBuffers usage shown above.
+    const ManyHellosRequest *request = request_msg->GetRoot();
+    const std::string &name = request->name()->str();
+    int num_greetings = request->num_greetings();
+
+    for (int i = 0; i < num_greetings; i++) {
+      auto msg_offset = mb_.CreateString("Many hellos, " + name);
+      auto hello_offset = CreateHelloReply(mb_, msg_offset);
+      mb_.Finish(hello_offset);
+      writer->Write(mb_.ReleaseMessage<HelloReply>());
+    }
+
+    return grpc::Status::OK;
+  }
+
+  flatbuffers::grpc::MessageBuilder mb_;
+};
+
+void RunServer() {
+  std::string server_address("0.0.0.0:50051");
+  GreeterServiceImpl service;
+
+  grpc::ServerBuilder builder;
+  builder.AddListeningPort(server_address, grpc::InsecureServerCredentials());
+  builder.RegisterService(&service);
+  std::unique_ptr<grpc::Server> server(builder.BuildAndStart());
+  std::cerr << "Server listening on " << server_address << std::endl;
+
+  server->Wait();
+}
+
+int main(int argc, const char *argv[]) {
+  RunServer();
+  return 0;
+}