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https://issues.apache.org/jira/browse/YARN-2264?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14063757#comment-14063757
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Jian He commented on YARN-2264:
-------------------------------

committing this.

> Race in DrainDispatcher can cause random test failures
> ------------------------------------------------------
>
>                 Key: YARN-2264
>                 URL: https://issues.apache.org/jira/browse/YARN-2264
>             Project: Hadoop YARN
>          Issue Type: Bug
>            Reporter: Siddharth Seth
>            Assignee: Li Lu
>         Attachments: YARN-2264-070814.patch
>
>
> 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 returns.
> 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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