Before we use the latest version of protoc-gen-go to get the exported service desc.
We have to modify the go_plugin_install to support higher protoc-gen-go version.
The current protoc-gen-go we used is v1.3.1, and this is getting from github.com/golang/protobuf/protoc-gen-go.
The higher version (> v1.5.2) of protoc-gen-go has been change to google.golang.org/protobuf/cmd/protoc-gen-go.

We also have to introduce google.golang.org/grpc/cmd/protoc-gen-go-grpc into our protogen tool. Because it's separated from protoc-gen-go after v1.20.0.

After protogen supports higher version.
I will pick protoc-gen-go@v1.20.0 and protoc-gen-go-grpc@v1.1.0 as the base version for our protogen v2 proposal (https://github.com/carousell/shared-proto/pull/2736). This is the minimum version to support the exported service desc.

To call the exported service desc from *.orion.pb.go, we have to make protoc-gen-orion support to retrieve go_package from protobufs. The go_package provides the package import path of generated *.pb.go files. protoc-gen-orion can import the package with go_package to get the exported service desc.

// applebomb/v1/applebomb.proto
syntax = "proto3";

package applebomb.v1;

option go_package = "github.com/carousell/shared-proto/gen/go/applebomb/v1;applebombv1";

import "applepie/v2/applepie.proto";
import "appletree/v1/appletree.proto";

service AppleBombService {
  rpc Bomb(appletree.v1.Tree) returns (applepie.v2.Pie);
}

// gen/go/applebomb/v1/applebomb.pb.go
package applebombv1
...
...
var AppleBombService_ServiceDesc = grpc.ServiceDesc{...}

// gen/orion/applebomb/v1/applebomb.orion.pb.go
import applebombv1 "github.com/carousell/shared-proto/gen/go/applebomb/v1"
...
...
func RegisterAppleBombServiceOrionServer(sf interface{}, orionServer orion.Server) error {
  err := orionServer.RegisterService(&applebombv1.AppleBombService_ServiceDesc, sf)
  ...
}

Alright, the approach of having protoc-gen-orion read go_package is what I was intending to do as well.

I'm thinking about how to make the changes backwards compatible too.

1. We'll need to tell protoc-gen-orion to use the exported the grpc.ServiceDesc variable instead of the unexported one.
2. We'll also need to continue supporting the existing use-case of having the *.orion.pb.go file as a sibling of the *.pb.go files.

My proposal is:

1. Add a flag exported-service-desc=true. If true, then the exported variable name is used. Default false.
2. Add a flag standalone-mode=true. If true, then go_package is required and exported-service-desc is overridden to true. The package name from go_package is prepended to the exported variable (e.g. listing_proto.ListingService_ServiceDesc). Default false.
3. Add a deprecation warning when exported-service-desc is left as false. In 6 months (or whatever timeframe) we'll set the default to true. We'll create a ticket for tracking when to change the default.

With these two flags, our protoc command can be either:

protoc --orion_out=. --orion_opt="exported-service-desc=true" --orion_opt="standalone-mode=true" source.proto
protoc --orion_out="exported-service-desc=true,standalone-mode=true:." source.proto

What do you think?

The flag designed in your proposal is good.

How about having a v2 protoc-gen-orion.
It can prevent the breaking changes from migrating protoc-gen-go library to google.golang.org/protobuf/cmd/protoc-gen-go. Support exported-service-desc can be the default value in v2. So we reduce the effort on implementing the exported-service-desc flag.
Adding a new version protoc-gen-orion also help us speed up the v1 deprecation.

@benjaminheng Please check the PR. To make the changes smaller, the PR doesn't contain the versioning upgrading plan. We can have more discussion on the plan later.