MySQL get_lock is no longer limited to one lock per session

[vickenty]

Default query for MySQL uses get_lock function in the WHERE clause to find rows that are not locked by any other process and to lock one of them at the same time. E.g.:

select ... where flag=0 and get_lock(concat_ws('=', 'prefix', id))!=0

It relies on the fact that a database session is only allowed to hold one lock at a time, and get_lock is automatically releasing any previously held locks. This behaviour ensures that after this query is executed, worker has a row that isn't locked by anyone else and it has only one row locked.

This no longer works correctly under MySQL 5.7, which removed one-lock-per-session limitation. Now worker can end up locking as much as all rows in a table, preventing all other workers from claiming any items in the queue.