Portable decision tasks (+action tasks, +cron.yml)

[djmitche]

In the Gecko tree, we have a sophisticated decision task that makes the task graph.

We should provide some support for building a similar system for other projects. That should be some basic building blocks on which those projects can build, since their needs will be different from Gecko. For example, perhaps a Github repo with some basic decision-task code, and a docker image that can run it. Then projects can fork that repo and get some tasks running quickly, then modify it to suit their needs.

Associated with decision tasks, we have action tasks and cron.yml. It would be great to support those in general, too.

[ncalexan]

I culted the Android Components TC integration from https://github.com/mozilla-mobile/android-components/blob/master/.taskcluster.yml and didn't find just getting a decision task running to be onerous. But it's not very general. There's some prior art that seems to back the DeepSpeech TC integration at https://github.com/lissyx/taskcluster-github-decision which is a little more general.

My immediate need is for caching build steps, like Gecko's toolchain tasks. I'm not sure that's in scope for this effort.

[escapewindow]

Releng definitely wants portable decision tasks, largely to allow for chain of trust support for android projects on github. Adding chain of trust support allows for automated signing and pushing to Google Play. We have a non-portable copy in the Focus repo, iirc.

[djmitche]

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1252144

[tomprince]

I'm planning on extracting a copy of gecko's taskgraph code into a re-usable form in 2018 Q4.

[djmitche]

I think Tom and the taskgraph work has taken this a long way, and this issue isn't providing use anymore.