rack controller fails to register because of failure updating interfaces

Running a local development maas from master, I got into a situation where the rackd fails to register to regiond because of a duplicated interface name.

The error is as follows:

2020-02-17 08:54:35 maasserver.rpc.regionservice: [info] Rack controller 'None' disconnected.
2020-02-17 08:54:35 RegionServer,51,::ffff:172.16.99.1: [info] RegionServer connection lost (HOST:IPv6Address(TCP, '::ffff:172.16.99.1', 5251) PEER:IPv6Address(TCP, '::ffff:172.16.99.1', 37082))
2020-02-17 08:54:35 maasserver.rpc.regionservice: [critical] Failed to register rack controller 'None' with the master. Connection will be dropped.
        Traceback (most recent call last):
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 653, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1442, in gotResult
            _inlineCallbacks(r, g, deferred)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/python/failure.py", line 408, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
        --- <exception caught here> ---
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/rpc/regionservice.py", line 743, in _register
            version=version,
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 250, in inContext
            result = inContext.theWork()
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 266, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/twisted/python/context.py", line 85, in callWithContext
            return func(*args,**kw)
          File "/snap/maas/x1/lib/python3.6/site-packages/provisioningserver/utils/twisted.py", line 885, in callInContext
            return func(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.6/site-packages/provisioningserver/utils/twisted.py", line 217, in wrapper
            result = func(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/utils/orm.py", line 692, in call_with_connection
            return func(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/utils/__init__.py", line 193, in call_with_lock
            return func(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/utils/orm.py", line 737, in call_within_transaction
            return func_outside_txn(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/utils/orm.py", line 540, in retrier
            return func(*args, **kwargs)
          File "/snap/maas/x1/usr/lib/python3.6/contextlib.py", line 52, in inner
            return func(*args, **kwds)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/rpc/rackcontrollers.py", line 168, in register
            rackcontroller.update_interfaces(interfaces, create_fabrics=create_fabrics)
          File "/snap/maas/x1/lib/python3.6/site-packages/provisioningserver/utils/twisted.py", line 217, in wrapper
            result = func(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/utils/orm.py", line 692, in call_with_connection
            return func(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/utils/__init__.py", line 193, in call_with_lock
            return func(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/utils/orm.py", line 721, in call_within_transaction
            return func_within_txn(*args, **kwargs)
          File "/snap/maas/x1/usr/lib/python3.6/contextlib.py", line 52, in inner
            return func(*args, **kwds)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/models/node.py", line 6827, in update_interfaces
            hints=topology_hints,
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/models/node.py", line 6130, in _update_interface
            name, config, create_fabrics=create_fabrics, hints=hints
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/models/node.py", line 6162, in _update_physical_interface
            defaults={"node": self, "name": name, "enabled": is_enabled},
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/models/interface.py", line 432, in get_or_create
            *args, **kwargs
          File "/snap/maas/x1/usr/lib/python3/dist-packages/django/db/models/manager.py", line 85, in manager_method
            return getattr(self.get_queryset(), name)(*args, **kwargs)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/django/db/models/query.py", line 466, in get_or_create
            return self._create_object_from_params(lookup, params)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/django/db/models/query.py", line 498, in _create_object_from_params
            obj = self.create(**params)
          File "/snap/maas/x1/usr/lib/python3/dist-packages/django/db/models/query.py", line 394, in create
            obj.save(force_insert=True, using=self.db)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/models/interface.py", line 1643, in save
            return super().save(*args, **kwargs)
          File "/snap/maas/x1/lib/python3.6/site-packages/maasserver/models/cleansave.py", line 187, in save
            self.validate_unique(exclude=[self._meta.pk.name])
          File "/snap/maas/x1/usr/lib/python3/dist-packages/django/db/models/base.py", line 1041, in validate_unique
            raise ValidationError(errors)
        django.core.exceptions.ValidationError: {'__all__': ['Interface with this Node and Name already exists.']}

The issue is the mpqemubr0-dummy interface (created by multipass), which somehow now got a different MAC address (but still has the previous name).

In MAAS' database an interface already existed for the node with that name, but a different MAC. The current logic looks up the MAC, doesn't find it, so tries to create a new interface, but the name is duplicated, so the constraint fails.

We should rework the Node.update_interfaces() logic so that it first figures out which interfaces need to be delete and which to be updated, delete first, then update and create new ones.