Retract workflow fails if run when a video is being played (with nfs storage)

Steps to reproduce:
1. Publish a recording (e.g. 30 to 60 min)
2. Start playing the recording with Theodul player
3. In the Admin UI, run the Retract workflow on the event

Actual Results:

Retract workflow operation fails.

org.opencastproject.serviceregistry.api.ServiceRegistryException: Error handling operation 'Retract'
at org.opencastproject.distribution.download.DownloadDistributionServiceImpl.process(DownloadDistributionServiceImpl.java:456)
at org.opencastproject.job.api.AbstractJobProducer$JobRunner.call(AbstractJobProducer.java:281)
at org.opencastproject.job.api.AbstractJobProducer$JobRunner.call(AbstractJobProducer.java:240)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: org.opencastproject.distribution.api.DistributionException: java.io.IOException: Unable to delete directory /data/opencast/distribution/downloads/mh_default_org/engage-player/df2858a9-51e7-4bce-a79d-dc40b9a8a8a0/cef65a14-f948-497f-8c7d-dbbbf8ac3791.
at org.opencastproject.distribution.download.DownloadDistributionServiceImpl.retractElement(DownloadDistributionServiceImpl.java:416)
at org.opencastproject.distribution.download.DownloadDistributionServiceImpl.retractElements(DownloadDistributionServiceImpl.java:359)
at org.opencastproject.distribution.download.DownloadDistributionServiceImpl.process(DownloadDistributionServiceImpl.java:445)
... 6 more
Caused by: java.io.IOException: Unable to delete directory /data/opencast/distribution/downloads/mh_default_org/engage-player/df2858a9-51e7-4bce-a79d-dc40b9a8a8a0/cef65a14-f948-497f-8c7d-dbbbf8ac3791.
at org.apache.commons.io.FileUtils.deleteDirectory(FileUtils.java:1337)
at org.apache.commons.io.FileUtils.forceDelete(FileUtils.java:1910)
at org.opencastproject.distribution.download.DownloadDistributionServiceImpl.retractElement(DownloadDistributionServiceImpl.java:403)
... 8 more

Expected Results:

Retract operation should succeed.

Workaround (if any):

Run the retract operation again later.