Skylark support for multi-arch ("fat") binaries

[gregestren]

Part of the Skylark build configuration project.

This is a tracking issue for the ability of Skylark writers to design fat binaries, such as fat Android APKs.

This will eventually replace ad hoc flags like --fat_apk_cpu. In particular, it will make it possible for any rule to create multi-arch dependencies with specific compilation / bundling logic of its choosing.

This complements the Bazel Configurability Roadmap. Write questions / comments / status updates here. See there for the bigger picture.

[jin]

/sub

[gregestren]

2018 EOY update:

This feature is essentially done.

• The pieces of Skylark support for custom configuration transitions #5574 needed for multi-arch are done.
• This requires --experimental_starlark_config_transitions, which is a temporary flag going away in the next release or two.
• Proper documentation is TBD.
• This has serious performance / memory consequences, as described at Skylark support for custom configuration transitions #5574 (comment). We encourage cautious adoption (on the experimental side) at this time.
• Support for custom flags is ongoing.

Since we're just easing in this support now in such a tentative way, I'll keep the issue open for now. Interested users / experimenters should ping this thread.

[ixdy]

multi-arch docker manifest lists are a strong use case for this. bazelbuild/rules_docker#300

[Pensu]

Making multi-arch binaries for kubernetes/test-infra repo is also a use case for this. @gregestren Just to get the clarity, according to #5575 (comment), we can use bazel to build multi-arch binaries now? Thanks.

[gregestren]

Sorry I missed this last comment.

Yes, bazel can build multi-arch binaries now, with caveats. Specifically you can apply the split but the API for a consuming rule to filter its deps by how they were split (e.g. "give me all the deps that were built for ARM" ) is still incomplete

Check out https://docs.bazel.build/versions/master/skylark/config.html#defining-12-transitions for an example.

[Pensu]

Thanks @gregestren. Will try these out.

[buchgr]

@gregestren I am curious what exactly is missing from the API. There seems to be ctx.split_attr. Is this not sufficient?

Ahh ctx.split_attr ... it doesn't work for split transitions created via Starlark yet.

[gregestren]

I think it'll still work if you're splitting just on --cpu. Generalizing that is the main challenge.