LXD VM hosts can't be refreshed if VLANs interfaces aren't named $parent.$vid

[Björn Tillenius]

This is with MAAS 3.0.0~rc1.

Register a LXD VM host that has a VLAN interface that isn't name like eth1.123 (parent name and vid), e.g. myvlan.

If you refresh the LXD VM host, it will stall.

If you look in the logs, you'll see that the metadata server return a 409 Conflict.

The reason for that is that we have code VLANInterface that ignores the given name, unless it's a controller.

update_vlan_interface() calls VLANInterface.objects.get_or_create(name="myvlan",....)
VLANInterface.get_or_create() first creates an interface with no parents, named "vlan123". After that it links the parent interface, and calls interface.save() which will rename the interface to eth1.123.

The first time that code runs it works, since there is no eth1.123. But the second time, eth1.123 already exists, so the unique (node, interface_name) is triggered.