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From "Stefan Egli (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (SLING-7778) race-condition in scheduled job creation might keep scheduled job in-memory, and cause a duplicate scheduled job execution
Date Thu, 12 Jul 2018 15:39:00 GMT

     [ https://issues.apache.org/jira/browse/SLING-7778?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Stefan Egli updated SLING-7778:
-------------------------------
    Fix Version/s: Event 4.2.12

> race-condition in scheduled job creation might keep scheduled job in-memory, and cause
a duplicate scheduled job execution
> --------------------------------------------------------------------------------------------------------------------------
>
>                 Key: SLING-7778
>                 URL: https://issues.apache.org/jira/browse/SLING-7778
>             Project: Sling
>          Issue Type: Bug
>          Components: Event
>    Affects Versions: Event 4.2.0
>            Reporter: Stefan Egli
>            Assignee: Stefan Egli
>            Priority: Major
>             Fix For: Event 4.2.12
>
>
> The creation of a scheduled job is handled in [{{JobSchedulerImpl.addScheduledJob}}|https://github.com/apache/sling-org-apache-sling-event/blob/ca009499cbe30ae30df5069effe051e0bef9ef72/src/main/java/org/apache/sling/event/impl/jobs/scheduling/JobSchedulerImpl.java#L501]
which in turn [calls {{scheduledJobHandler.addOrUpdateJob}}|https://github.com/apache/sling-org-apache-sling-event/blob/ca009499cbe30ae30df5069effe051e0bef9ef72/src/main/java/org/apache/sling/event/impl/jobs/scheduling/JobSchedulerImpl.java#L523].
There, the job is first persisted to the repository via [calling {{writeScheduledJob}}|https://github.com/apache/sling-org-apache-sling-event/blob/ca009499cbe30ae30df5069effe051e0bef9ef72/src/main/java/org/apache/sling/event/impl/jobs/scheduling/ScheduledJobHandler.java#L213].
> Normally the sequence of events is then as follows:
> * [A] the [synchronized block in {{addOrUpdateJob}}|https://github.com/apache/sling-org-apache-sling-event/blob/ca009499cbe30ae30df5069effe051e0bef9ef72/src/main/java/org/apache/sling/event/impl/jobs/scheduling/ScheduledJobHandler.java#L216]
is entered and the job is scheduled with quartz.
> * [B] then shortly afterwards, observation is triggered and {{handleAddOrUpdate}} is
called. This will, as a *side-effect* [adds the scheduled job to the {{scheduledJobs}} map|https://github.com/apache/sling-org-apache-sling-event/blob/ca009499cbe30ae30df5069effe051e0bef9ef72/src/main/java/org/apache/sling/event/impl/jobs/scheduling/ScheduledJobHandler.java#L446]
> * the result of this being that the scheduled job is now in {{ScheduledJobHandler.scheduledJobs}}
> * later on, when the scheduled job is finished, {{handleRemove()}} (or similar) is called,
and it checks if it finds the scheduled job in {{ScheduledJobHandler.scheduledJobs}}. if it
does, all good. And here we're in the all good case, so all is fine
> However, there's the following race-condition:
> * first [B] happens, ie first the observation event is triggered, so {{handleAddOrUpdate}}
is called, and again the side-effect hits: the scheduled job is added to the {{ScheduledJobHandler.scheduledJobs}}.
all good so far
> * then [A] continues into the synchronized block. *But* now, the [remove of the scheduled
job|https://github.com/apache/sling-org-apache-sling-event/blob/ca009499cbe30ae30df5069effe051e0bef9ef72/src/main/java/org/apache/sling/event/impl/jobs/scheduling/ScheduledJobHandler.java#L217]
actually does damage: the scheduled job ends up *not* being in the {{ScheduledJobHandler.scheduledJobs}}
map.
> * so in this scenario, when a later {{handleRemove()}} (or similar) happens, it can't
find the scheduled job (see [here|https://github.com/apache/sling-org-apache-sling-event/blob/ca009499cbe30ae30df5069effe051e0bef9ef72/src/main/java/org/apache/sling/event/impl/jobs/scheduling/ScheduledJobHandler.java#L370]
h being null)
> * the result being that [{{JobScheduler.unscheduleJob}}|https://github.com/apache/sling-org-apache-sling-event/blob/ca009499cbe30ae30df5069effe051e0bef9ef72/src/main/java/org/apache/sling/event/impl/jobs/scheduling/ScheduledJobHandler.java#L371]
is *not invoked*, *leaving the scheduled job with the JobSchedulerImpl*.
> And the result of this race-condition is that {{JobSchedulerImpl.scheduledJobs}} map
has a zombie scheduled job, which it shouldn't actually have.
> So later on for example, a topology event happens, such as a PROPERTIES_EVENT. That in
turn will trigger [{{JobSchedulerImpl.configurationChanged()}}|https://github.com/apache/sling-org-apache-sling-event/blob/ca009499cbe30ae30df5069effe051e0bef9ef72/src/main/java/org/apache/sling/event/impl/jobs/scheduling/JobSchedulerImpl.java#L152]
which will now *re-schedule the previously scheduled job*. And we have a duplicate scheduled
job execution.



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