I'm trying to get protoc and protoc-gen-go to play nice in a multi-repo codebase using go modules.
I've managed to get things more or less working, until I introduced a major version bump in one of my "api" (i.e. protobuf) repos, and I've hit a bit of a barrier.
Here's a simplified picture of my setup:
Let's say I have two repos, github.com/kpruden/base-api and github.com/kpruden/dep-api.
base-api includes base-api.proto and base-api.pb.go generated from it.
Similarly, dep-api contains dep-api.proto and the corresponding generated code. In addition, dep-api.proto depends on base-api.proto via an import github.com/kpruden/base-api/base-api.proto statement.
Both base-api and dep-api use go modules, with dep-api's go.mod specifying a dependency on base-api.
To make this work, I can run go mod vendor in the dep-api repo to pull all of its dependencies into a vendor directory, then add -I./vendor to my call to protoc.
This all works well until I decide I need to do a major release of base-api. This is done by updating base-api/go.mod's module line to end with /v2. I have no need to keep version 1 around, so I don't create a v2 subdirectory in my repo, and the version 2 code and protobuf definitions exist at the root of the repo.
At first, this didn't seem to be a problem. Since I'm not moving any files, protoc can still find base-api.proto. However, in the generated go code, version 2 of base-api must be imported using import "github.com/kpruden/base-api/v2", but protoc is generating the code to import it at github.com/kpruden/base-api.
My question is: how do I get protoc to generate the go code for dep-api to import base-api at the correct module path? Is this even possible? None of the documentation of either protobuf or gRPC have much of anything to say about go modules. Google has led me to some github issues in these projects that talk about supporting go modules, but they are mostly internal discussions without much in the way of prescriptions, and I've found no mention anywhere of handling major version releases.
I was able to get something working. The setup I describe in the question is actually pretty close to what ultimately ended up working.
In summary, things mostly work as expected:
/v2 to the module line in go.mod. Nothing else is required. In particular, the directory layout in source control doesn't need to have v2 anywhere.v2 of the parent module in go code is handled normally. Using the example above, a simple import "github.com/kpruden/base-api/v2" will do it.import "github.com/kpruden/base-api/v2/base-api.proto"protoc use go mod vendor to copy dependencies into ./vendor which will lay them such that the protobuf definitions can be found and the import path in (3). At this point protoc -I./vendor ... works as expected, the imports in the generated go source are correct, and everyone's happy :)