Aha, so GitHub merge queue = GitLab merge trains (or at least very similar).

Yes that’s pretty much what it is. Both are replicas of bors, and implementations https://graydon.livejournal.com/186550.html

Bors is also very similar to the Zuul CI system used for OpenStack. It has the equivalent of a merge queue (with additional support for cross repositories dependencies): https://zuul-ci.org/docs/zuul/latest/gating.html You can then have pull requests from different repositories all serialized in the same queue ensuring you don't break tests from any of the repositories participating.

Also continuous integration best practices advance one funeral at a time, it seems.

So does each new PR start new tests that will supersede the previous PR’s tests? If one PR’s tests fail, does it block all PRs behind it in the queue?

I’ve read docs several times and never found them very clear about the details.

Each PR on the queue is tested with whatever commits it would have were it merged to the target branch in queue order. So if the target branch already has commit A and commits B and C are in queue, commit D will be tested on its own temporary branch with commits A B C and D. If the tests for C fail, C is removed from the queue, and D is retested with just commits A B and D (because that's what would be on the target branch by the time it merges).

OK, thank you.