[andrewfader]

A concurrency-limited job whose class exists where it was dispatched, but not where it runs, keeps its key's semaphore after it fails. That happens during a rolling deploy or a rollback, when a worker on the other release claims the job. ClaimedExecution#perform fails it with ClassMissingError (73602fe), but release_concurrency_lock returns early because concurrency_limited? needs job_class. The slot stays taken and the jobs blocked behind it wait until the semaphore expires (duration, 3 minutes by default).

This releases the lock by concurrency_key alone, and falls back to the default limit and default_concurrency_control_period when the class is missing.

Two trade-offs:

• A job dispatched without its class never took the lock (the existing "dispatch job with missing class and concurrency key skips concurrency controls" test). If it fails while another job of its key holds the lock, this release frees that job's slot, so one extra job of the key can start early. Today, the leaked slot blocks the key for the whole duration instead.
• For a key whose real limit is above 1, the fallback limit of 1 releases nothing unless the semaphore is at 0, so that slot still waits to expire, as it does today.

Happy to narrow it if you'd rather only release when the semaphore shows the lock was taken.

Test: a job dispatched with its class and then run without it releases the next blocked job of its key. It fails without the change.

[rosa]

Thanks a lot for this, @andrewfader, and for spelling out the trade-offs so clearly! While reviewing it, we found that releasing by key alone can let jobs run over the limit: a job whose class was missing when it was dispatched never took a slot, so when it fails it frees a slot that belongs to another running job. That turned out to be the same problem as #827, since the semaphore counts slots but doesn't know who holds them. We're fixing both in #831 by having each job remember the semaphore it got its slot from. That also covers your case, so I'll close this one in favor of it.