Create judge tasks for a judging only once.

[meisterT]

unblockJudgeTasks() checks for judgings without judge tasks and creates them, but two concurrent callers both pass the check. The second set fails on judging_run's unique key with the "1062 Duplicate entry" from #2848 and leaves judge tasks behind that no judging run points at. Lock the judging and re-check inside.

[meisterT]

@tuupke heads-up since you have been removing transactions for #2848: this adds one, but of a different kind.

The ones you removed had the shape that snapshot isolation rejects: plain SELECT, someone else commits, then UPDATE the same row. This one starts with SELECT ... FOR UPDATE on the judging and then only inserts, so the row is locked instead of snapshotted and 'record has changed since last read' cannot happen. It is just a critical section around "check for judge tasks, then create them"; without it two concurrent unblockJudgeTasks() calls both insert, the second fails on judging_run's unique key, and orphaned judgetask rows are left for a judgehost to pick up.

If you would rather have no transaction here, a GET_LOCK per judging like calculateScoreRow should give the same guarantee, but is ugly out of its own right.