Comment 6 for bug 1746032

Reviewed: https://review.openstack.org/539003
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=834adeae9a3ff1bb87f22066131d48230ef96b69
Submitter: Zuul
Branch: stable/pike

commit 834adeae9a3ff1bb87f22066131d48230ef96b69
Author: Matt Riedemann <email address hidden>
Date: Mon Jan 29 10:50:36 2018 -0500

    Rollback instance.image_ref on failed rebuild

    When rebuilding and changing the image, we run the new image
    through the scheduler to see if it's valid for the instance
    on its current compute host. The API saves off the new image
    ref on the instance before casting to conductor to run through
    the scheduler. If the scheduler fails, the instance.image_ref was
    not being rolled back, which meant a user could attempt the rebuild
    with the same invalid image a second time and the API, seeing the
    instance.image_ref hasn't changed (even though it's not the actual
    backing image for the server), will bypass the scheduler and rebuild
    the instance with that invalid image.

    This fixes the issue by using the original image ref, passed from
    API to conductor during rebuild, to reset the instance.image_ref
    in the case of a failure.

    Note that there are other things changed on the instance in the API
    which this patch does not attempt to recover as that's a bigger
    work item which likely involves substantial refactoring of the code.

    Closes-Bug: #1746032

    Conflicts:
          nova/conductor/manager.py

    NOTE(mriedem): The conflict is due to not having change
    Ibc44e3b2261b314bb92062a88ca9ee6b81298dc3 in Pike. Also, six
    had to be imported in the functional test.

    Change-Id: I3399a66fe9b1297cd6b0dca440145393ceaef41f
    (cherry picked from commit 4a2c9a4887a219a6d4dfe83c430b040713fc4109)