fix: make GRPC.Stub generation opt-out for server-only protoc runs

[mvanhorn]

Summary

protoc-gen-elixir (the gRPC generator) now supports opting out of GRPC.Stub client generation for server-only runs. A new gen_stubs=false option skips emitting the client stub module while still generating the service module, so projects that only implement servers no longer carry unused client stubs.

Why this matters

Issue #549 asks for a way to generate server code without the client Stub. Today the generator always emits both the service behaviour and a GRPC.Stub client module; a server-only project ends up with a client stub it never uses, which is dead surface area and can clash with a separately generated or hand-written client. There was no switch to turn it off.

This adds a gen_stubs plugin option, defaulting to true so existing behavior is unchanged. Passing --elixir_out=gen_stubs=false:... (parsed in Grpc.Protoc.CLI) threads the flag into the service generator, and the service.ex.eex template conditionally omits the stub module. Default output is byte-identical to before.

Testing

mix test test/grpc/protoc/generator/service_test.exs — 4 tests pass, covering default generation (stub present) and gen_stubs: false (stub omitted, service retained). mix format --check-formatted clean on the changed source files.

Fixes #549

[sleipnir]

I will take a look but how this is compatible with protobuf_generate hex package? We support both protoc-gen-elixir and probuf_generate.

[mvanhorn]

Good question. The change is entirely inside GRPC.Protoc.CLI (the protoc-gen-elixir plugin), and it's fully backward compatible: gen_stubs defaults to true, so the .Stub module is only omitted when someone explicitly passes gen_stubs=false. Nothing changes for existing users of either path.

gen_stubs is a standard protoc plugin parameter, so it flows through anywhere protoc-gen-elixir is invoked with plugin params - direct protoc, Buf, etc. For protobuf_generate specifically: it depends on whether protobuf_generate forwards arbitrary plugin options through to the elixir plugin. I wasn't able to confirm its opt-forwarding from the docs - if it doesn't pass extra plugin params today, then gen_stubs=false wouldn't be reachable via protobuf_generate until that's added, but the default (stubs generated) is unaffected either way. You know that package better than I do - if you can point me at how it forwards plugin opts, I'm happy to add a protobuf_generate example/integration test so both paths are covered.

[sleipnir]

Okay, I believe it will work; we just need to test it. In any case, it doesn't seem like an obstacle to us moving forward.

[mvanhorn]

Appreciate the merge, @sleipnir - server-only proto builds can opt out of stub generation now.

[sleipnir]

Appreciate the merge, @sleipnir - server-only proto builds can opt out of stub generation now.

Thank you for your contribution.