Set up merge bot for repository

[celskeggs]

You know what's annoying? Having to manually rebase every PR, often multiple times, before it be can merged.

As suggested by @cryslith, let's set up Bors-NG. This will allow us to adjust our workflow to stop being based in rebasing everything -- which is somewhat easy for new contributors to get wrong, beyond being a waste of valuable developer time.

[cryslith]

Unfortunately bors only supports merge and squash-merge, not rebasing right now, as we currently use.

[celskeggs]

As long as we only use merge commits to merge into master, and never use them for conflict resolution, I would be okay with us switching to a system where we use merge commits in a limited fashion.

i.e.

• Before you can submit your PR to the bot for evaluation and merge, you have to:
  • rebase away any "fix [...]" commits or similar
  • rebase against a recent enough version of master that there are no merge conflicts
  • ensure that there are no merge commits in your branch
• Once you get approval from a reviewer, you can submit your PR to the bot (bors r+ mode) -- or the reviewer can do that for you when they approve, if you checked our "ready to merge" box.
• The bot does its testing dance to ensure that your PR -- and any others ready at the same time -- don't conflict, and then merges them into master with an explicit merge commit.

This way, instead of a single linear history, we would have a history with only simple branching. This would still be reasonably easy to bisect and interpret after the fact.

I would also be fine with us finding an alternative that works with rebasing. I am also a fan of the gerrit model where you review and verify individual commits in a PR, and can press a button to cherry-pick them onto master one commit at a time.

What do you think?

[cryslith]

Sounds fine to me :)

[cryslith]

We've decided to go with our own homegrown solution to this, so we can use cherry-picking and also for general stability/quality.