Enable GID override in Build

[jsncmgs1]

Summary

Allows group ID to be set via a -gid flag.
#961

Output

Before

After

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

Related

Resolves #961

[jsncmgs1]

I think I'm close here (minus test changes and additions), but I could use a nudge in the right direction on this.

So far I've added the flag and a type representing the flag, and I pass the flag value down into builder.userAndGroupIDs, overriding the value set on CNB_GROUP_ID.

When I run this locally, it picks up the group ID change for /layers, but not /workspace:

$ out/pack build test_img --path ../go_stuff --builder heroku/buildpacks:18 --gid=3 --pull-policy if-not-present

I'm having some trouble locating where I need to make the change for the /workspace group ID and prevent the change for /layers. I think they're created here?

pack/internal/builder/builder.go

Line 547 in 860396d

for _, path := range []string{workspaceDir, layersDir} {

But it seems like at some point /app is copied or symlinked into /workspace. Any help is appreciated :)

[dfreilich]

@jsncmgs1 Thanks for the PR and work on it! Your path right now seems to be changing the user environment of the ephemeral builder, which is definitely interesting! What I'd recommend is rather than changing the builder, pass the gid argument (if provided) to the relevant lifecycle phases (analyze, restore, export, and create). You can see examples of how we pass flags to the lifecycle phases here (in internal/build/lifecycle_execution.go):

pack/internal/build/lifecycle_execution.go

Lines 191 to 212 in c911002

	flags := addTags([]string{
	"-cache-dir", l.mountPaths.cacheDir(),
	"-run-image", runImage,
	}, additionalTags)
	if clearCache {
	flags = append(flags, "-skip-restore")
	}
	processType := determineDefaultProcessType(l.platformAPI, l.opts.DefaultProcessType)
	if processType != "" {
	flags = append(flags, "-process-type", processType)
	}
	var cacheOpts PhaseConfigProviderOperation
	switch buildCache.Type() {
	case cache.Image:
	flags = append(flags, "-cache-image", buildCache.Name())
	cacheOpts = WithBinds(volumes...)
	case cache.Volume:
	cacheOpts = WithBinds(append(volumes, fmt.Sprintf("%s:%s", buildCache.Name(), l.mountPaths.cacheDir()))...)
	}

[dfreilich]

Hey @jsncmgs1 - let me know if I can be of help with this (assuming you're still interested). The fix for this would be similar to what's being done in #988, I'd be happy to walk through it with you.