Add plugin support for Bazel

Bazel is something which has interested me for a while now, so writing a Spotless plugin for it may prove to be a good way to get my feet wet with it. However I’m limited for time with my studies and I suspect I won’t be able to work on something this big until late May 2017 or later.

Here’s what I currently understand about Bazel with regards to plugin-writing:

1. Bazel uses a subset of Python called Skylark for writing extensions (Bazel’s terminology for plugins AFAICT). So it may very well be that Spotless would need to be ported to Skylark first, which isn’t ideal.
2. There seem to be ways of integrating shell scripts and binaries as extensions into Bazel. I’m hoping that we can use one of these options or some another option to call spotless-lib (or a Java binary wrapper around it) without porting it to Skylark.

[t-rad679]

A colleague and I have been thinking about this a lot, and the cleanest way we’ve found is to start with a binary wrapper (as Jonathan suggested early on in this thread) that takes a config that specifies the same stuff you would in a build.gradle, use a custom Bazel rule to generate the config from attributes passed from the target, then write another Bazel rule calling the executable as an action. The config generation is a bit wonky, but this is still more or less in line with how Bazel was intended to work. We unfortunately did not run across any functionality in Starlark that allows interoperation between it and Java libraries in its extensions. Someone please let me know if they find out I missed something.

Seems to me like implementing said binary wrapper would basically amount to building a command line application, so it seems silly not to expose it as such for any users who want that (even if it’s not really a standard use case). Would you guys accept a well written and tested implementation of a command line application for Spotless? It might be a while before I have the time, but we’d be willing to work on such a thing. Let me know and I’ll open a ticket for it. Then I’d also be willing to put in some work on this ticket after that is complete.

[nedtwigg]

Just upload anything you think might be useful to somebody else who might wanna do this 👍