Hubcast for Spack CI

[cmelone]

I went through the Spack CI Github-Gitlab sync script to see what features might be good to include in Hubcast, and which would be better served as plugins.

The basic idea behind the script is to pull a list of all open PRs in the Spack repo, perform some filtering, and push the merge commits of those refs to the Spack Gitlab instance. Once the pipeline runs, the statuses are synced back to the Github checks for the PR.

There are some architectural differences with our approach, namely that the sync script is run as a cronjob and does not respond to push events like Hubcast.

I identified some important features implemented in the sync script that would be valuable to include in Hubcast or as an extension:

• Filtering ineligible PRs: draft PRs and those that haven't passed their checks (all-prechecks). This could be a plugin.
• Pipeline deferral: do not sync refs of which the base is newer than the most recently tested commit. this could be a plugin along with the other filters. [1] [2]
• Syncing protected branches and tags: current implementation here. I'm assuming protected branches is for develop pipeline runs; I would need to think more about how that should be set up. Not sure about tags, I don't think Hubcast implements that functionality.
• The merge-commit is made on behalf of the author by spackbot <noreply@spack.io> -- I'm thinking about writing up an account mapper for Hubcast that allows for a wildcard option: any user could push to the source forge, but the pipeline would run on behalf of a bot user on the destination forge, which would be done through impersonation tokens. However, this wouldn't change the author of the commit. This is currently implemented through merge commits, which Hubcast doesn't handle.
• Sync pipeline status to GH: this is already implemented in Hubcast

re prerequisite checks: is there potentially a more efficient way to implement this functionality without needing to make API calls to the status of existing checks? maybe as a dependent job in GitHub actions? not sure how feasible this is

cc: @kwryankrattiger, let me know if I'm missing anything

[kwryankrattiger]

For protected branch sync, those are all just direct sync with no merge commit. So for hubcast there should just be an option to say "sync on update" and pull/push the branch without changes.

Tags can be treated the same way as protected branch sync.

I think the required check can be implemented as a "sync on event" where the event is configurable. In the case of requiring all prechecks to pass, the sync trigger event would be set to that workflow job completing successfully.

sync:
  on:
    - workflow_job:
        conclusion: "success",
        name: "all-prechecks"

Something I don't see on the list is the ability to configure the link/message of the sync status based on criteria other than reporting back the status from gitlab. Deferred and pending pipelines would have different types of statuses to post back to the check.