Comment 11 for bug 1769131

Reviewed: https://review.openstack.org/567630
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=a16fa14ce47bd2d3a5189047e9bd330a607ef3cc
Submitter: Zuul
Branch: stable/ocata

commit a16fa14ce47bd2d3a5189047e9bd330a607ef3cc
Author: Matt Riedemann <email address hidden>
Date: Fri May 4 12:58:07 2018 -0400

    libvirt: check image type before removing snapshots in _cleanup_resize

    Change Ic683f83e428106df64be42287e2c5f3b40e73da4 added some disk
    cleanup logic to _cleanup_resize because some image backends (Qcow2,
    Flat and Ploop) will re-create the instance directory and disk.info
    file when initializing the image backend object.

    However, that change did not take into account volume-backed instances
    being resized will not have a root disk *and* if the local disk is
    shared storage, removing the instance directory effectively deletes
    the instance files, like the console.log, on the destination host
    as well. Change I29fac80d08baf64bf69e54cf673e55123174de2a was made
    to resolve that issue.

    However (see the pattern?), if you're doing a resize of a
    volume-backed instance that is not on shared storage, we won't remove
    the instance directory from the source host in _cleanup_resize. If the
    admin then later tries to live migrate the instance back to that host,
    it will fail with DestinationDiskExists in the pre_live_migration()
    method.

    This change is essentially a revert of
    I29fac80d08baf64bf69e54cf673e55123174de2a and alternate fix for
    Ic683f83e428106df64be42287e2c5f3b40e73da4. Since the root problem
    is that creating certain imagebackend objects will recreate the
    instance directory and disk.info on the source host, we simply need
    to avoid creating the imagebackend object. The only reason we are
    getting an imagebackend object in _cleanup_resize is to remove
    image snapshot clones, which is only implemented by the Rbd image
    backend. Therefore, we can check to see if the image type supports
    clones and if not, don't go through the imagebackend init routine
    that, for some, will recreate the disk.

    Change-Id: Ib10081150e125961cba19cfa821bddfac4614408
    Closes-Bug: #1769131
    Related-Bug: #1666831
    Related-Bug: #1728603
    (cherry picked from commit 8e3385707cb1ced55cd12b1314d8c0b68d354c38)
    (cherry picked from commit 174764340d3c965d31143b39af4ab2e8ecefe594)
    (cherry picked from commit c72a0a7665e96219f0301525edc513dda07b320b)