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gRPC callbackService support added (#8666)

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* grpc callbackService support added

Signed-off-by: shankeleven <shashanksati11@gmail.com>

* tests: regenerate C++ gRPC golden with --grpc-callback-api (CallbackService & async_ reactor APIs); update formatting and method placement

---------

Signed-off-by: shankeleven <shashanksati11@gmail.com>
Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>
This commit is contained in:
Shashank
2025-08-29 04:19:27 +05:30
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parent b87d04af8c
commit deb3d93454
21 changed files with 617 additions and 51 deletions
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grpc/README.md
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@@ -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.