Comment 1 for bug 1256838

Reviewed: https://review.openstack.org/61075
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=2593469103aa7d9d2bcb759b78d5f8637911a1e0
Submitter: Jenkins
Branch: master

commit 2593469103aa7d9d2bcb759b78d5f8637911a1e0
Author: Arata Notsu <email address hidden>
Date: Tue Dec 3 18:27:26 2013 +0900

    Fix race conditions between imagebackend and imagecache

    The race may occur in the situation:
    * There is a base file that is not used for a long time enough
      to be removed by imagecache.
    * imagebackend is provisioning a virtual disk from the base file.
    * imagecache is removing the base file.

    Then, the base file is removed even though it is about to be used.

    To fix this, these changes are in this patch:

    * A new function imagecache.refresh_timestamp(base_file) updates
      the owner and mtime of the base file with the lock dedicated
      to the base file.
    * imagebacked calls refresh_timestamp(base_file) before provision
      a disk from the base file.
    * imagecache.ImageCacheManager._remove_base_file(base_file) uses
      the same lock as used by refresh_timestamp()

    Closes-Bug: #1256838
    Change-Id: I7c897cf6071d87a2c4532fb3a73863d649d02782