Better plugin support for cmake builds

[Li He]

Hi protobuf community members,

I had an idea about some improvements for the protobuf cmake file. It was discussed over a recent merge request. I agree with Brad King's comment about moving this to the upstream code base but before I send it out, just want to get a sense whether I am on the right track. Right now, the function generate_protobuf is intended to be used like this:

protobuf_generate(

  OUT_VAR helloworld_proto

  PROTOS helloworld.proto

  LANGUAGE cpp

)

protobuf_generate(

  OUT_VAR helloworld_grpc

  PROTOS helloworld.proto

  LANGUAGE grpc

  PLUGIN "protoc-gen-grpc=${_GRPC_CPP_PLUGIN_EXECUTABLE}"

  GENERATE_EXTENSIONS .grpc.pb.h .grpc.pb.cc

)

It seems for each grpc proto file, we will need two calls, so I want to make it also support

protobuf_generate(

  OUT_VAR helloworld_proto

  PLUGIN_OUT_VAR helloworld_gprc

  PROTOS helloworld.proto

  LANGUAGE cpp

  PLUGIN "protoc-gen-grpc=${_GRPC_CPP_PLUGIN_EXECUTABLE}"

  GENERATE_EXTENSIONS .pb.h .pb.cc .grpc.pb.h .grpc.pb.cc

)

Let me know if this would be a good change to make and I am happy to send out a pull request.

[Adam Cozzette]

Thanks for the suggestion and sorry for taking so long to reply. It's an interesting idea but I guess I'm skeptical of whether it's worth the effort to avoid an extra protoc invocation, since in my experience protoc invocations are not usually the bottleneck in build performance. I would also be nervous about adding a special case for gRPC, since that is just one of many plugins that one might want to use.

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[Li He]

Well, it's very simple to add to the existing implementation and is not limited to grpc (which I am just using as an example). According to 

https://protobuf.dev/reference/cpp/api-docs/google.protobuf.compiler.plugin/

We just need to extract the plugin name from PLUGIN using regex and try to append something for the plugin output if the LANGUAGE value isn't the plugin name.

--

Best regards,

HE, Li