Comment 2 for bug 1781710

> I don't really see what could be an issue there, but we should pick a host for the first instance, "claim" resources on that host and update the group hosts, and then run the filters on the second instance and filter out the first host selected for the first instance.

Yes, it seems we need refresh host_state.instances[1] and spec_obj.instance_group.members[2] in claim for next server.
[1] https://review.openstack.org/#/c/571166/27/nova/scheduler/filters/affinity_filter.py@101
[2] https://review.openstack.org/#/c/571166/27/nova/scheduler/filters/affinity_filter.py@103

But my confusion is: Why compute didn't check the 2nd server as a overhead server in _validate_instance_group_policy of compute manager[3].

[3] https://review.openstack.org/#/c/571465/26/nova/compute/manager.py@1293