Cron might still be running during an update

Steps (inferred)

1. Have a NC cron job run that takes 5+ minutes to run
2. Swap the PHP code to a newer version of NC
3. Run occ upgrade

Expected result

Need to wait for cron to finish.

Actual result

Cron continues running and might cause side effects because it might be operating on data while the data is being migrated.
This is one case that could result in data loss, like https://github.com/nextcloud/spreed/issues/5518#issuecomment-845646929

Note: new cron jobs would not start because an upgrade enables maintenance mode, but existing ones that already started would continue running.

Version

The problem with https://github.com/nextcloud/spreed/issues/5518#issuecomment-845646929 was observed in NC 21 and 22

Possible solution

The upgrade code should wait (sleep) until all cron jobs have finished. To detect completion, the value of "oc_jobs.reserved" might help. And after a timeout of a minute or so, abort the upgrade with an error if a job is still running and did not finish.

Optional: when enabling maintenance mode, display a warning when cron jobs are still running (but don't wait?)

Killing cron jobs is likely not a good idea and is not reliably possible anyway from the PHP code.

Start in the upgrade code and inspect how running cron jobs are represented by the oc_jobs.reserved value. Verify the interaction between maintenance mode and already-started jobs, then define the timeout and error behavior so an upgrade does not proceed while a job remains active.