Comment 25 for bug 1732428

Reviewed: https://review.opendev.org/c/openstack/nova/+/785625
Committed: https://opendev.org/openstack/nova/commit/7003618884b5e033f822685541da6bc7955fe55a
Submitter: "Zuul (22348)"
Branch: stable/ussuri

commit 7003618884b5e033f822685541da6bc7955fe55a
Author: Alexandre Arents <email address hidden>
Date: Tue Nov 26 10:26:32 2019 +0000

    Rebase qcow2 images when unshelving an instance

    During unshelve, instance is spawn with image created by shelve
    and is deleted just after, instance.image_ref still point
    to the original instance build image.

    In qcow2 environment, this is an issue because instance backing file
    don't match anymore instance.image_ref and during live-migration/resize,
    target host will fetch image corresponding to instance.image_ref
    involving instance corruption.

    This change fetches original image and rebase instance disk on it.
    This avoid image_ref mismatch and bring back storage benefit to keep common
    image in cache.

    If original image is no more available in glance, backing file is merged into
    disk(flatten), ensuring instance integrity during next live-migration/resize
    operation.

    Change-Id: I1a33fadf0b7439cf06c06cba2bc06df6cef0945b
    Closes-Bug: #1732428
    (cherry picked from commit 8953a689467f8c3e996086392251de67953a45ba)