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https://issues.apache.org/jira/browse/SLING-2896?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13672350#comment-13672350
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Carsten Ziegeler commented on SLING-2896:
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I've committed a potential fix in rev 1488653
> Job might be executed twice if a topology event occurs
> ------------------------------------------------------
>
> Key: SLING-2896
> URL: https://issues.apache.org/jira/browse/SLING-2896
> Project: Sling
> Issue Type: Bug
> Components: Extensions
> Affects Versions: Extensions Event 3.2.0
> Reporter: Carsten Ziegeler
> Assignee: Carsten Ziegeler
> Fix For: Extensions Event 3.2.0
>
>
> If a parallel queue is used (either parallel or round robing) with a limit of
> N parallel jobs and there are X > N jobs in the queue, the (N+1) job might
> get processed twice if a topology change occurs:
> Assume we have a parallel queue with max parallel processing set to 8 and 15
> jobs entering the queue.
> The AbstractParallelJobQueue.start is called with the first 8 jobs - all
> fine, limit is not yet hit, those jobs are started
> When AbstractParallelJobQueue.start is called with job #9 the acquireSlot()
> method waits, since the queue is full/busy: The waiting is done in
> acquireSlot(), in the syncLock.wait() (line 78 of AbstractParallelJobQueue).
> The calling method - start(JobHandler) - keeps a reference to job #9 !
> In the meantime a TopologyEvent occurs. AFAICS this triggers 'outdating' the
> existing and recreating a new queue.
> The BackgroundLoader.loadJobsInTheBackground starts filling the new queue.
> Unfortunatelly, this Backgroundloader schedules job #9 too - since it is
> not yet marked in the repository in any way (there's only the previous queue
> which has a reference to it in above mentioned start(JobHandler) method - but
> the job #9 is not yet marked as running in the repository).
> => Thus job #9 is executed the first time by the new queue.
> Eventually the outdated queue is finished with execution. The above
> acquireSlot() method returns, and:
> => job #9 is executed the second time (by the outdated queue).
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