Comment 0 for bug 1825537

This is purely based on code inspection right now, I need to write a functional test to recreate the issue.

While triaging bug 1821594 it got me thinking about handling placement allocations during resize when something fails, which got me thinking about an older fix:

https://review.openstack.org/#/c/543971/6/nova/compute/manager.py@4457

Looking back on that now, I think the revert during resize_instance is OK as long as the instance host/node has not changed, but I think doing it when finish_resize fails was probably a mistake because the instance.host in the nova db won't match where the allocations exist in placement. Before Pike this was fine since the ResourceTracker would heal the allocations in the update_available_resource periodic task, but we don't have that anymore.

So this could result in an instance reported as being on the dest host in the nova database with the new flavor, which is where it will get rebuilt/rebooted/etc, but placement will be tracking the instance resource allocations using the old flavor against the source host, which is not where the instance is.

Furthermore, if finish_resize fails, the instance should be in ERROR status and the user would likely try to hard reboot the instance to correct that status, which would happen on the dest host.