Comment 2 for bug 1727855

In fact, the ComputeManager.rebuild_instance method doesn't even take a request_spec parameter.

The request_spec is only used in the conductor rebuild_instance method if we're evacuating and need to ask the scheduler for another host:

https://github.com/openstack/nova/blob/d36dcd52c24c32418fd358d245688c86664025d5/nova/conductor/manager.py#L843

Which means the scheduler needs to pick a host that satisfies the original instance image metadata constraints for things like host aggregates.

So if you're evacuating and there is no request spec for the instance, we're just picking a host randomly without actually considering the image meta at all.