Add API validation script to CI

[Lukasa]

SwiftNIO uses a script built on top of the Swift API digester command line tool to validate that PRs do not cause semver major breaking changes. This script could be adapted for the use cases of protobuf as well, and help avoid ambiguity around whether a change is breaking or not.

Note that, while this script is helpful, it does encounter some false positives. In particular it tends to believe that adding a protocol customisation point with a default implementation is a semver major, when so far as we can tell it is not (it's ABI breaking, I think, but not a semver major).

[thomasvl]

Happened to find https://github.com/ltetzlaff/swift-api-diff today. Don't suppose SwiftNIO is looking at make a standard github action out of this?

[thomasvl]

The script also needs jq installed, which isn't on macOS be default.

[Lukasa]

If the community has already done a standard GitHub action I'm more than happy to defer to that!

[thomasvl]

Since it seems to have a copy of the script, i wasn't sure how insync it is staying, etc. Is there any coordination with them?

[thomasvl]

It also looks like it posts back to the comments, which means it needs a github token. That likely means someone on Apple's side would need to sign off on it before we could use it here.

@tbkka I'm guessing the need for a secret to post back means it shouldn't be used here?

[thomasvl]

Also looks like despite the script having xcrun support, the Xcode toolchains don't include swift-api-digester, so it also requires a custom toolchain install for macOS.

(I was looking to wire up a Makefile rule for running it, but it would be linux only.)

[tbkka]

@thomasvl -- Yeah, we should avoid anything that needs a secret to post back.

[thomasvl]

@Lukasa should I open a nio issue asking for a standard action that doesn't require write access?

[Lukasa]

@thomasvl Sure, worst-case we can say we think it's out of scope. But I think an action is a nice approach.

[thomasvl]

Opened apple/swift-nio#1600