grpc callbackService support added

Signed-off-by: shankeleven <[email protected]>

Author: shankeleven

CHANGELOG.md

@@ -4,6 +4,10 @@ All major or breaking changes will be documented in this file, as well as any
 new features that should be highlighted. Minor fixes or improvements are not
 necessarily listed.
 
+## Unreleased
+
+* flatc: `--grpc-callback-api` flag generates C++ gRPC Callback API server `CallbackService` skeletons AND client native callback/async stubs (unary + all streaming reactor forms) (opt-in, non-breaking, issue #8596).
+
 ## [25.2.10] (February 10 2025)(https://github.com/google/flatbuffers/releases/tag/v25.2.10)
 
 * Removed the old documentation pages. The new one is live at https://flatbuffers.dev

CMakeLists.txt

@@ -278,6 +278,7 @@ set(FlatBuffers_GRPCTest_SRCS
   tests/test_builder.cpp
   grpc/tests/grpctest.cpp
   grpc/tests/message_builder_test.cpp
+  grpc/tests/grpctest_callback_compile.cpp
 )
 
 # TODO(dbaileychess): Figure out how this would now work. I posted a question on

grpc/README.md

@@ -1,5 +1,4 @@
 GRPC implementation and test
-============================
 
 NOTE: files in `src/` are shared with the GRPC project, and maintained there
 (any changes should be submitted to GRPC instead). These files are copied
@@ -39,4 +38,64 @@ For Bazel users:
 
 ```shell
 $bazel test tests/...
-```
+```
+
+## C++ Callback API Generation
+
+FlatBuffers gRPC C++ code generation now optionally supports the modern gRPC Callback API.
+
+To enable generation of a `CallbackService` skeleton alongside the existing `Service` and async mixins, invoke `flatc` with both `--grpc` and `--grpc-callback-api`:
+
+```shell
+flatc --cpp --grpc --grpc-callback-api your_service.fbs
+```
+
+This adds (guarded by `#if defined(GRPC_CALLBACK_API_NONEXPERIMENTAL)`) a class:
+
+```cpp
+class YourService::CallbackService : public ::grpc::Service { /* reactor virtuals */ };
+```
+
+Each RPC shape maps to the appropriate reactor return type:
+
+- Unary -> `::grpc::ServerUnaryReactor*` Method(...)
+- Client streaming -> `::grpc::ServerReadReactor<Request>*`
+- Server streaming -> `::grpc::ServerWriteReactor<Response>*`
+- Bidi streaming -> `::grpc::ServerBidiReactor<Request, Response>*`
+
+Default generated implementations return `nullptr`; override in your derived class and return a reactor instance you manage (see gRPC docs for lifecycle patterns).
+
+If your gRPC library predates the stable callback API macro, the code inside the guard will be skipped (no breaking changes). Ensure you build against a recent gRPC (1.38+; verify current minimum in grpc repo) to use this feature.
+
+### Client Callback Stubs
+
+When `--grpc-callback-api` is supplied, the generated C++ client stub gains native callback / reactor based async methods in addition to the existing synchronous / generic async flavors, guarded by the same macro. For each RPC named `Foo`:
+
+Unary:
+
+```
+void async_Foo(::grpc::ClientContext*, const Request&, Response*, std::function<void(::grpc::Status)>);
+void async_Foo(::grpc::ClientContext*, const Request&, Response*, ::grpc::ClientUnaryReactor*);
+```
+
+Client streaming:
+
+```
+::grpc::ClientWriteReactor<Request>* async_Foo(::grpc::ClientContext*, Response*, ::grpc::ClientWriteReactor<Request>*);
+```
+
+Server streaming:
+
+```
+::grpc::ClientReadReactor<Response>* async_Foo(::grpc::ClientContext*, const Request&, ::grpc::ClientReadReactor<Response>*);
+```
+
+Bidirectional streaming:
+
+```
+::grpc::ClientBidiReactor<Request, Response>* async_Foo(::grpc::ClientContext*, ::grpc::ClientBidiReactor<Request, Response>*);
+```
+
+These map directly onto the native gRPC callback API factories (e.g. `CallbackUnaryCall`, `ClientCallbackWriterFactory::Create`, etc.) and do not spawn threads. Override the appropriate reactor callbacks per gRPC's documentation to drive I/O.
+
+If your build uses an older gRPC lacking the non-experimental macro, these symbols will not be emitted, preserving backwards compatibility.