There is no interface for operators to migrate *all* the existing compute resource providers to be ready for nested providers

[Tetsuro Nakamura]

When nested resource provider feature was added in Rocky, root_provider_uuid column, which should be non-None value is created in the resource provider DB. For existing resource providers created before queens, we have an online data migration:

https://review.openstack.org/#/c/377138/62/nova/objects/resource_provider.py@917

But it's only done via listing/showing resource providers. We should have explicit migration script something like "placement-manage db online_data_migrations" to make sure all the resource providers are ready for the nested provider feature, that is all the root_provider_uuid column has non-None value.

This bug tracking can be closed when the following tasks are done
 - Provide something like "placement-manage db online_data_migrations" so that in Stein we are sure all the root_provider_uuid column is non-None value.
 - Clean placement/objects/resource_provider.py removing many TODOs like "Change this to an inner join when we are sure all root_provider_id values are NOT NULL"

NOTE: This report is created after fixing/closing https://bugs.launchpad.net/nova/+bug/1799892 in a temporary way without the explicit DB migration script.

[Chris Dent (cdent)]

related: https://review.openstack.org/#/c/619126/

[Tetsuro Nakamura (tetsuro0907)]

Abandoned https://review.openstack.org/#/c/619126/ in favor of https://review.openstack.org/#/c/624943/, which is now committed.