Use concurrent set behind reactive `TransactionSynchronizationManager#registerSynchronization`

[zhou-hao]

Concurrent execution of registerSynchronization and getSynchronization may result in errors .

The context uses LinkedHashSet:

public void initSynchronization() throws IllegalStateException {
   if (isSynchronizationActive()) {
   	throw new IllegalStateException("Cannot activate transaction synchronization - already active");
   }
   this.transactionContext.setSynchronizations(new LinkedHashSet<>());
}

[bclozel]

@zhou-hao this is not actionable. Please share the complete stacktrace in text format and explain how to reproduce this.

[jhoeller]

Looking at this, reactive pipeline operations will usually arriving in a sequential form against a transaction. Most importantly, a commit/rollback is never going to be "interrupted" by a registerSynchronization call since all transactional operations have to conclude before entering the commit phase.

That said, it is nevertheless better to use CopyOnWriteArraySet instead of LinkedHashSet there, even if just for concurrent visibility in case of interleaved registerSynchronization calls during transactional operations. I'll implement this as an improvement for 7.0.2 and 6.2.15.