Getting DB lock for lexorank may take more than 4 seconds

Summary

In some rare cases, getting of lexorank row through DB may take more than 4 seconds.

Environment

• Any plugin to create on subtasks on transition
• Some other long-running transition postfunction

Steps to Reproduce

1. Perform long-running transition
2. Simultaneously try to create issue.

Expected Results

rankInitially fails with 4sec timeout

Actual Results

rankInitially waits for transition to finish.

Explanation

LexorankDao#acquireLock is supposed to finish to either succeed or fail immediately. And then there's a lot of error handling about this.
But in case in some other transaction holds an SQL update lock, acquireLock query will wait for that transition to finish.

Proposed solution

The best way would be to use SELECT FOR UPDATE NOWAIT. It will properly represent tryLock nature of acquireLock. Unfortunately, it is not supported by mysql. (https://dev.mysql.com/worklog/task/?id=3597)
A bit worse option: use Statement#setQueryTimeout(int) from JDBC. It won't exit immideately, but at least it will fail after some time instead of waiting infinitely.
We are going with the second option

Workaround

• None