Golang. Replace module path for package that are not real go packages

There is this package https://github.com/open-telemetry/opentelemetry-proto
which contains protobuf definitions only. To generate golang code one must type:

make gen-go

and go build fails with following message:

build opentel: cannot load github.com/open-telemetry/opentelemetry-proto/gen/go/common/v1: module github.com/open-telemetry/opentelemetry-proto@latest found (v0.11.0), but does not contain package github.com/open-telemetry/opentelemetry-proto/gen/go/common/v

I've tried to replace one path with another in go.mod file but apparently I'm not too good at it. How can I make it work?

I've copied those generated files to

$GOPATH/src/opentelemetry-proto/gen/go

what should I put in import statement inside my main package?

There are a couple of challenges with the way the Go sources are generated. I assume that the repo authors are aiming for consistency across languages and for GOPATH and GO MODULES use-cases with Go.... so, yes, gnarly for them and us.

Here's a (!?) solution:

Assuming you're in /path/to/somedir and it contains a clone of the opentelemetry-proto and ``my-module` at the same level, i.e.:

.
├── my-module
└── opentelemetry-proto

1. make gen-go as before. This should create ./opentelemetry-proto/gen

2. In ./opentelemetry-proto/gen/go/github.com/open-telemetry/opentelemetry-proto go go mod init github.com/open-telemetry/opentelemetry-proto:

.
├── gen
└── go.mod

3. From within my-module, go mod init my-module and then:

go.mod:

module my-module

go 1.17

require (
    github.com/open-telemetry/opentelemetry-proto v0.11.0
)

replace (
    github.com/open-telemetry/opentelemetry-proto => ../opentelemetry-proto/gen/go/github.com/open-telemetry/opentelemetry-proto

> NOTE With GOPATH paths down to packages are required (they're not using Go Modules) and so, if we were using GOPATH, we could GOPATH=${GOPATH}:${PROTO_GEN_GO_DIR}/github.com/open-telemetry/opentelemetry-proto

And then, e.g.

main.go:

package main

import (
	v1 "github.com/open-telemetry/opentelemetry-proto/gen/go/collector/metrics/v1"
)

func main() {
	// E.g.
	_ = v1.ExportMetricsServiceRequest{}
}

Explanation:

make gen-go does not create a Module but we can create one.

The Module is implicit from the path under gen/go i.e. github.com/open-telemetry/opentelemetry-proto

Then, from our project, we can replace to provide a local path to it. The path is the path to the clone, then back down to our newly-created go.mod.

The import path is the path from the Module (i.e. that excessively convoluted replace path) to whichever package we're interested in.

> NOTE Commonly protobuf imports are pb but I've used v1.

What I would do:

I think the Module of generated code should match its repo. If opentelemetry-proto were mine, I'd generate the sources into the repo root without gen/go

As this would -- IMO -- simplify everything to:

github.com/open-telemetry/opentelemetry-proto => ../opentelemetry-proto

And:

import (
	v1 "github.com/open-telemetry/opentelemetry-proto/collector/metrics/v1"
)