GitHub Merge Queue

Short, unedited note on GitHub merge queue.

TL;DR, delivers a meaningfully better experience, although it suffers from being a third-party integration.

Specific grievances:

Complexity. This is a vague feeling, but merge queue feels like it is built by complexity merchants there are a lot of unclear settings and voluminous and byzantine docs. Good for allocating extra budget towards build engineering, bad for actual build engineering.

GUI-only configuration. Bors is setup using bors.toml in the repository, merge queue is setup by clicking through web GUI. To share config with other maintainers, I resorted to a zoomed-out screenshot of the page.

Unclear set of checks. The purpose of the merge queue is to enforce not rocket science rule of software engineering making sure that the code in the main branch satisfies certain quality invariants (all tests are passing). It is impossible to tell what merge queue actually enforces. Typically, when you enable merge queue, you subsequently find out that it actually merges anything, without any checks whatsoever.

Double latency. One of the biggest benefits of a merge queue for a high velocity project is its asynchrony. After submitting a PR, you can do a review and schedule PR to be merged without waiting for CI to finish. This is massive: it is 2X reduction to human attention required. Without queue, you need to look at a PR twice: once to do a review, and once to click merge after the green checkmark is in. With the queue, you only need a review, and the green checkmark comes in asynchronously. Except that with GitHub merge queue, you cant actually add a PR to the queue until you get a green checkmark. In effect, thats still 2X attention, and then a PR runs through the same CI checks twice (yes, you can have separate checks for merge queue and PR. No, this is not a good idea, this is complexity and busywork).

Lack of delegation. With bors, you can use bors delegate+ to delegate merging of a single, specific pull request to its author. This is helpful to drive contributor engagement, and to formalize LGTM with the nits fixed approval (which again reduces number of human round trips).

You still should use GitHub merge queue, rather than bors-ng, as thats now a first-party feature. Still, its important to understand how things should work, to be able to improve state of the art some other time.