potential for out-of-order versions in scheduler?

[sanderr]

Scheduler._new_version is called whenever a new version is released. It then waits for the intent lock before updating the scheduler's internal state. If two requests come in very shortly after another, I believe it's possible for them to race, one passing an older version to _new_version, while it might still acquire the lock later than the other one.

Solution: move the intent lock one level higher up: drop it from _new_version and add it on new_version and _initialize instead, before fetching the latest version from the database. This ensures monotonically increasing versions to the _new_version call.