"branch cannot be merged" on freedesktop.org

Since a gitlab update, we've been seeing "branch cannot be merged" intermittently from marge. One of our gitlab admins looked into it and concluded:

Yeah, as suspected the culprit is Marge not doing the right thing:

Note: Starting in GitLab 12.8, the mergeability (merge_status) of a merge request will be checked asynchronously when a request is made to this endpoint. Poll this API endpoint to get updated status. This affects the has_conflicts property as it is dependent on the merge_status. It’ll return false unless merge_status is cannot_be_merged.

Completing a pipeline will cause a MR to become mergeable, but not instantaneously. Marge assumes that the instant a pipeline reports complete, the MR will be mergeable. That's not actually true, since it still has to check for things like whether or not the head is a fast-forward, etc. This is done in a background job via Sidekiq. The fix seems pretty straightforward: in Marge's job handling, after pipeline completion but before merging, it needs to poll on merge_status and watch it go green. I looked at it briefly this morning, but it involved NixOS and that pushed it over the time budget I have right now. If someone wants to pick up updating Marge then I'm happy to get it deployed.