Potential IndexError if using the CachingScheduler and not getting alternates

[Matt Riedemann]

If we're using the CachingScheduler and we're not getting alternates, maybe because conductor is old, we'll get an IndexError because we're not returning a list of list of selected hosts, we're just returning a list of selected hosts here:

https://github.com/openstack/nova/blob/f974e3c3566f379211d7fdc790d07b5680925584/nova/scheduler/filter_scheduler.py#L342

And the IndexError would happen here:

https://github.com/openstack/nova/blob/f974e3c3566f379211d7fdc790d07b5680925584/nova/scheduler/filter_scheduler.py#L120

We obviously don't have a test covering this scenario.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/517134

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/517134
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=a9d92553b36e96daf6c8e6fca609f608e84ba336
Submitter: Zuul
Branch: master

commit a9d92553b36e96daf6c8e6fca609f608e84ba336
Author: Matt Riedemann <email address hidden>
Date: Wed Nov 1 19:48:31 2017 -0400

    Fix return type in FilterScheduler._legacy_find_hosts

    The FilterScheduler._schedule method should be returning
    a list of list of selected hosts. When include_alternatives
    is False in _legacy_find_hosts, it was only returning back
    a list of hosts, which would result in an IndexError when
    select_destinations() tries to take the first entry from each
    item in the list.

    Change-Id: Ia6c87900605d3604beb74b942b0e30575b814112
    Closes-Bug: #1729445

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova 17.0.0.0b2 development milestone.