Siddharth Seth created YARN-2264:

             Summary: Race in DrainDispatcher can cause random test failures
                 Key: YARN-2264
             Project: Hadoop YARN
          Issue Type: Bug
            Reporter: Siddharth Seth

This is what can happen.

This is the potential race.
DrainDispatcher is started via serviceStart() . As a last step, this starts the 
actual dispatcher thread (eventHandlingThread.start() - and returns immediately 
- which means the thread may or may not have started up by the time start 
Event sequence: 
UserThread: calls dispatcher.getEventHandler().handle()
This sets drained = false, and a context switch happens.
DispatcherThread: starts running
DispatcherThread drained = queue.isEmpty(); -> This sets drained to true, since 
Thread1 yielded before putting anything into the queue.
UserThread: actual.handle(event) - which puts the event in the queue for the 
dispatcher thread to process, and returns control.
UserThread: dispatcher.await() - Since drained is true, this returns 
immediately - even though there is a pending event to process.

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