[jira] [Created] (SLING-2896) Job might be executed twice if a topology event occurs

["Carsten Ziegeler (JIRA)" <ji...@apache.org>]

Carsten Ziegeler created SLING-2896:
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             Summary: 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Eventually the outdated queue is finished with execution.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 job #9 is executed the second time (by the outdated queue).


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