Allow customizing visibility of the models generated from third-party protos

[iRevive]

The problem

otel4s provides a pure Scala implementation of the OpenTelemetry specification. The protocol models used by otel4s are generated from the third-party protos defined in opentelemetry-proto.

Currently, these models are generated in a public package, making them accessible on the classpath.

This poses a significant problem because opentelemetry-proto may introduce binary-breaking changes. As a result, any updates to the opentelemetry-proto can cause otel4s-sdk-exporter-proto to break binary compatibility.

https://scalapb.github.io/docs/faq/#how-do-i-mark-a-generated-case-class-private doesn't work because proto models are third-party.

The proposition

Perhaps we can extend the package-scoped options to allow customizing modifiers of a package?

import "scalapb/scalapb.proto";

package com.mypackage;

option (scalapb.options) = {
  scope: PACKAGE
  package_modifier: "private[com.mypackage]" // or "private"
};

The workaround

As a workaround, I customized the PB.generate task to override package and visibility afterward: typelevel/otel4s#860.

[thesamet]

Hi Maksym, have you tried auxiliary options to match all messages in the package using * as a target, and make them private, using the same approach from the FAQ you linked to?

[iRevive]

Hm, I never thought of this option, thanks for the suggestion!

The approach covers almost all cases. Here is the package.proto that I use:

syntax = "proto3";

package opentelemetry.proto;

import "scalapb/scalapb.proto";

option (scalapb.options) = {
  scope: PACKAGE
  aux_message_options: [
    {
      target: "*"
      options: {
        annotations: "private[proto]"
        companion_annotations: "private[proto]"
      }
    }
  ]
  aux_enum_options: [
    {
      target: "*"
      options: {
        base_annotations: "private[proto]"
      }
    }
  ]
  // not really necessary, my attempt to make 'oneof' companion private
  aux_enum_value_options: [
    {
      target: "*"
      options: {
        annotations: "private[proto]"
      }
    }
  ]
};

What is missing:

1) 'oneof' companion object remains public

Definition: https://github.com/open-telemetry/opentelemetry-proto/blob/v1.4.0/opentelemetry/proto/metrics/v1/metrics.proto#L281-L283

Generated code:

private[proto]
sealed abstract class AggregationTemporality(val value: _root_.scala.Int) extends _root_.scalapb.GeneratedEnum {
 ...
}
object AggregationTemporality extends _root_.scalapb.GeneratedEnumCompanion[AggregationTemporality] {

2) Package-specific *Proto object remains public

That's not a big deal, I guess. Generated code:

object MetricsProto extends _root_.scalapb.GeneratedFileObject {
  lazy val dependencies: Seq[_root_.scalapb.GeneratedFileObject] = Seq(
    io.opentelemetry.proto.common.v1.common.CommonProto,
    io.opentelemetry.proto.resource.v1.resource.ResourceProto
  )
  ...
}