Comment 2 for bug 1834349

Reviewed: https://review.opendev.org/667687
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=818419c9d313bd6151d1a05b3a087a15116f61b8
Submitter: Zuul
Branch: master

commit 818419c9d313bd6151d1a05b3a087a15116f61b8
Author: Matt Riedemann <email address hidden>
Date: Wed Jun 26 13:26:44 2019 -0400

    Fix AttributeError in RT._update_usage_from_migration

    Change Ieb539c9a0cfbac743c579a1633234537a8e3e3ee in Stein
    added some logging in _update_usage_from_migration to log
    the flavor for an inbound and outbound migration.

    If an instance is resized and then the resize is immediately
    confirmed, it's possible to race with ComputeManager._confirm_resize
    setting the instance.old_flavor to None before the migration
    status is changed to "confirmed" while the update_available_resource
    periodic is running which will result in _update_usage_from_migration
    hitting an AttributeError when trying to log instance.old_flavor.flavorid
    since instance.old_flavor is None.

    There are a few key points there:

    - We get into _update_usage_from_migration because the
      _update_available_resource method gets in-progress migrations
      related to the host (in this case the source compute) and the
      migration is consider in-progress until its status is "confirmed".

    - The instance is not in the tracked_instances dict when
      _update_usage_from_migration runs because RT only tracks instances
      where the instance.host matches the RT.host and in this case the
      instance has been resized to another compute and the instance.host
      is pointing at the dest compute.

    The fix here is to simply check if we got the instance.old_flavor and
    not log the message if we do not have it, which gets us back to the old
    behavior.

    This bug was found by noticing it in CI job logs - there is a link to
    hits in logstash in the bug report.

    As for the "incoming and not tracked" case in _update_usage_from_migration
    I have not modified that since I am not sure we have the same race nor
    have I seen it in CI logs.

    Change-Id: I43e34b3ff1424d42632a3e8f842c93508905aa1a
    Closes-Bug: #1834349