Test for DistributedExecutionCompletionService hangs, if instantiation is done using PriorityBlockingQueue

Hi,

while writing tests for DistributedExecutionCompletionService, the following issue arrised:

I'm creating DistributedExecutionCompletionService with constructor:
DistributedExecutionCompletionService(DistributedExecutorService executor, BlockingQueue<NotifyingFuture<V>> completionQueue)

If I'm passing as a completionQueue instance of ArrayBlockingQueue, then everything works properly. But if as a completionQueue I'm passing instance of PriorityBlockingQueue, then the test hangs.

It hangs on line 61 of DistributedExecutionCompletionService:
completionQueue.add((NotifyingFuture<V>)future);

You can find the test reproducing the issue here:
https://github.com/andyuk1986/infinispan/blob/DIST_EXEC_TESTS/core/src/test/java/org/infinispan/distexec/DistributedExecutionCompletionTest.java (testBasicInvocationWithBlockingQueue()).