Summary: Keep track of when a pull request is unmergeable, or a draft, and reject them.

I allow this RFC document to be modified and redistributed under the terms of the Apache-2.0 license, or the CC-BY-NC-SA 3.0 license, at your option.

Motivation

Right now, bors checks whether a pull request is "not mergeable" according to GitHub's API, before it tries to merge it. This is good, but it's kinda late at that point. The pull request webhook actually reports whether it's mergeable in advance, so if bors used that, it would be able to reject it earlier and avoid wasting time, and it could show that information on the pull request dashboard.

Guide-level explanation

This change is almost completely transparent to the user, and would basically only change the behavior in very weird corner cases, like if they queued a pull request that wouldn't merge cleanly into the main branch behind another one that actually fixed the merge conflict.

Reference-level explanation

Bors will check whether your pull request is mergeable both before enqueueing it, and after dequeueing it.

Bors will also refuse to merge draft pull requests.

Drawbacks

It adds two new columns to the database, and one new column to the dashboard.

Rationale and alternatives

By adding it to the database, bors can also show it in the dashboard, which makes it a bit more useful (whether a pull request is mergeable or not seems important).
The current one uses slightly less code, but it's probably not what we want.

Prior art

It's the design that homu uses.

Unresolved questions

I'm not really aware of any.

Future possibilities

Right now, we don't pluck unmergeable pull requests out of batches, but probably could change to do that. I don't think it would actually make much difference, performance-wise, but it would make the dashboard a bit more accurate.

The Bors RFC Archive