It's been a while since I moved all my projects to GitHub. It's convenient to host Git projects, and the collaboration workflow is smooth.

I love pull requests to merge code. I review them, I send them, I merge them. The fact that you can plug them into a continuous integration system is great and makes sure that you don't merge code that will break your software. I usually have Travis-CI setup and running my unit tests and code style check.

The problem with the GitHub workflow is that it allows merging untested code.

What?

Yes, it does. If you think that your pull requests, all green decorated, are ready to be merged, you're wrong.

This might not be as good as you think

You see, pull requests on GitHub are marked as valid as soon as the continuous integration system passes and indicates that everything is valid. However, if the target branch (let's say, master) is updated while the pull request is opened, nothing forces to retest that pull request with this new master branch. You think that the code in this pull request works while that might have become false.

Master moved, the pull request is not up to date though it's still marked as passing integration.

So it might be that what went into your master branch now breaks this not-yet-merged pull request. You've no clue. You'll trust GitHub and press that green merge button, and you'll break your software. For whatever reason, it's possible that the test will break.

If the pull request has not been updated with the latest version of its target branch, it might break your integration.

The good news is that's something that's solvable with the strict workflow that Mergify provides. There's a nice explanation and example in Mergify's blog post You are merging untested code that I advise you to read. What Mergify provides here is a way to serialize the merge of pull requests while making sure that they are always updated with the latest version of their target branch. It makes sure that there's no way to merge broken code.

That's a workflow I've now adopted and automatized on all my repositories, and we've been using such a workflow for Gnocchi for more than a year, with great success. Once you start using it, it becomes impossible to go back!