Comment 4 for bug 1752115

So looking into this the problem appears to be that Nova calls the brick initiator disconnect_volume method indiscriminately. Brick has no way currently to interrogate usage of a connection, and I'm not sure that something like that could be added in this case.

My first thought was that it would be logical to check for multiattach on the same host here:
  https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py#L1249
by using the objects.BlockDeviceMapping.get_by_volume() HOWEVER it turns out that's another special thing that isn't allowed when a volume is multiattach=True (I haven't figured out why that's there yet, but looking).