ensure job cancellation selection holds a lock

[maxcountryman]

Since we select and then update a set of tasks in a transaction in order to cancel pending tasks relative to a given job, we should also hold a lock over that selection via for update.

[maxcountryman]

@kirillsalykin this should ensure the selected rows are locked as per your suggestion. This could probably be done more efficiently via a single update or e.g. if mark_cancelled took &[&TaskId].

[kirillsalykin]

Indeed, probably having one update would be more efficient and wouldnt require a lock.

[kirillsalykin]

wdyt about just having cancel all by job_id, queue_name and status = pending?

I would be happy to do this change , seems small enough so i can finish :)

[maxcountryman]

wdyt about just having cancel all by job_id, queue_name and status = pending?

It's probably a better approach. 🙂

The reason it wasn't implemented that way is because the queue encapsulates task life cycle. I want to avoid that leaking into other concepts as much as we can.

So maybe we should refactor mark_cancelled to take arbitrary selection criteria for the update (so we can pass through e.g. job_id).

[maxcountryman]

I'm going to merge this for the sake of correctness but we can come back to this at any point and improve it (it likely isn't a breaking change).