feat(priority): Ability to push back PRs to the back of the queue

[Erez Rokah]

TLDR: Similar to priority_merge_label but low_priority_merge_label, where a PR is scheduled last.

This is useful for bots PRs, where we want them to always come after humans' PRs.

Found a few priority related issues, but not quite like this

Also not sure if it will work to use priority_merge_label: automerge and automerge_label: low priority as I remember one needs both to be in the queue

[Christopher Dignam]

@erezrokah Thanks for the suggestion. I think the implementation would be similar to priority_merge_label. For pull requests labeled with priority_merge_label, we insert them into the Redis Sorted Set with a score of 0 instead of using the current unix timestamp as the score.

For low_priority_merge_label, we could insert the pull requests into the queue with some very high timestamp, like 4800000000, to prioritize these pull requests last.

kodiak/bot/kodiak/evaluation.py

Lines 1171 to 1172 in ee3cd8e

 priority_merge = config.merge.priority_merge_label in pull_request.labels 
 position_in_queue = await api.queue_for_merge(first=priority_merge) 

kodiak/bot/kodiak/queue.py

Lines 588 to 592 in ee3cd8e

 if first: 
     # place at front of queue. To allow us to always place this PR at 
     # the front, we should not pass only_if_not_exists. 
     await transaction.zadd(queue_name, {event.json(): 1.0}) 
 else: 

[Erez Rokah]

Thanks for the quick response @chdsbd, I can try and submit a PR for it if that makes sense

[Christopher Dignam]

@erezrokah A PR would be great! I can help with tests and polish if you need