Post jobs to slack when PR is merged

[jhkennedy]

This updates the post-jobs-slack.yaml workflow to be based on pushes to main, so that jobs are posted to slack when a jobs PR is merged to ensure what's posted to the website, is what's posted to slack.

The command to get the parent commit hash:

PARENT_HASH=$(git log --pretty=%P -n 1 ${CURRENT_SHA} | cut -d ' ' -f 1)

works for both merge commits (e.g., USRSE/usrse.github.io@f4853c5) and squash commits/direct commits (e.g., USRSE/usrse.github.io@284fbb3).

I also checked to make sure it won't post when expired jobs get pruned.

[vsoch]

Awesome! So my thinking is that we can just test this here - the CI should still be hooked up to a dummy slack that I made. When you are ready to do that I’m good to merge! And don’t worry if there are bugs and you need to tweak, perfection is the enemy of the food sometimes.

[vsoch]

@jhkennedy one thing I just thought of - if you want to have this trigger on merge to another branch (your testing branch!) we can test this and get it into working condition before merge into main, just to make the git history a little cleaner. What do you think?

[jhkennedy]

@vsoch This should be ready for review now.

I went ahead and added a slack integration to one of the workspaces I admin to test it out.

To test, I fast-forward merged this to themain branch of jhkennedy/jobs-updater, then

• A direct commit (or squash commit) to main, as in jhkennedy/jobs-updater@b5058f2, successfully ran:
  https://github.com/jhkennedy/jobs-updater/actions/runs/1415394588
• A merge commit from Check PR jhkennedy/jobs-updater#3 successfully ran:
  https://github.com/jhkennedy/jobs-updater/actions/runs/1415407801

Both were successfully posted to my slack workspace:

[vsoch]

This looks great, and thank you for testing! Let’s merge here, and then how about we update your PR to USRSE to use it? I would make sure to add a note to squash and merge (so there is just one new commit on merge to main) to the README. I am not an admin on the USRSE slack so you will need to ask someone to setup the integration for us. Great work tonight, I’m really excited about this!

[jhkennedy]

@vsoch will do! I don't think squash merge is required though -- this should grab the parent on the main branch, so it doesn't matter if it's a merge commit, or just a (squashed) commit.