MailCore/mailcore2 — pull request #883 by foxinushka (2014-09-23 11:49:08+00:00)
Title: Fixed thread safety in OperationQueue

checkRunningOnMainThread is executed on mDispatchQueue.
checkRunningAfterDelay was executed on main dispatch queue.

So, It was possible a situation when checkRunningAfterDelay is started, but not reached line _pendingCheckRunning = false;
and checkRunningOnMainThread reached line
release(); // (4)

So, in this case it makes extra release().