Comment 19 for bug 1809123

Reviewed: https://review.openstack.org/633493
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=2b1544390cd0afd4d2b0c6ea4636a49a73b50dcf
Submitter: Zuul
Branch: stable/queens

commit 2b1544390cd0afd4d2b0c6ea4636a49a73b50dcf
Author: Neil Jerram <email address hidden>
Date: Sat Jan 26 21:22:33 2019 +0000

    Allow utime call to fail on qcow2 image base file

    This utime call sometimes fails, with EACCES (Permission Denied), when
    the base file is on an NFS client filesystem. I don't understand why,
    but wonder if it's a similar problem as the one that motivated using
    touch instead of utime in ec9d5e375e2. In any case, IIUC, timing
    isn't the primary thing that the image cache manager uses to determine
    when the base file is in use. The primary mechanism for that is
    whether there is a matching disk file for a current instance. The
    timestamp on the base file is only used when deciding whether to
    delete a base file that is _not_ in use; so it is not a big deal if
    that deletion happens slightly earlier, for an unused base file,
    because of one of these preceding utime calls having failed.

    Conflicts:
          nova/tests/unit/virt/libvirt/test_imagebackend.py

    NOTE(mriedem): Conflict is due to not having change
    I154b1ef14dbace803841cb3eed3ff02dd0020588 in Queens.

    Closes-Bug: #1809123
    Co-Authored-By: Matthew Booth <email address hidden>
    Change-Id: Idc131ff426f1707150867030fa5a69b77a7fc832
    (cherry picked from commit 362bba9011f8445285c3f72a0b00630163bfbc2f)
    (cherry picked from commit 7d0dd625d35a08d159b50154809a53e8444ae25c)