cannot delete already existing subnets

Hi Greg,

Have you tried deleting the subnet via the CLI ? If not, can you please do so and provide your results here? Also please provide:

- Exact way you are trying to delete a subnet (seem via libmaas)
- Logs (/var/snao/common/logs/*) IIRC.

Thanks.

2017-07-05 17:52:18 maasserver.websockets.protocol: [critical] Error on request (33) subnet.delete: Subnet matching query does not exist.

Traceback (most recent call last):
  File "/snap/maas/91/usr/lib/python3.5/threading.py", line 862, in run
    self._target(*self._args, **self._kwargs)
  File "/snap/maas/91/lib/python3.5/site-packages/provisioningserver/utils/twisted.py", line 842, in worker
    return target()
  File "/snap/maas/91/lib/python3.5/site-packages/twisted/_threads/_threadworker.py", line 46, in work
    task()
  File "/snap/maas/91/lib/python3.5/site-packages/twisted/_threads/_team.py", line 190, in doWork
    task()
--- <exception caught here> ---
  File "/snap/maas/91/lib/python3.5/site-packages/twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File "/snap/maas/91/lib/python3.5/site-packages/twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File "/snap/maas/91/lib/python3.5/site-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/snap/maas/91/lib/python3.5/site-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "/snap/maas/91/lib/python3.5/site-packages/provisioningserver/utils/twisted.py", line 875, in callInContext
    return func(*args, **kwargs)
  File "/snap/maas/91/lib/python3.5/site-packages/provisioningserver/utils/twisted.py", line 232, in wrapper
    result = func(*args, **kwargs)
  File "/snap/maas/91/lib/python3.5/site-packages/maasserver/utils/orm.py", line 686, in call_within_transaction
    return func_outside_txn(*args, **kwargs)
  File "/snap/maas/91/lib/python3.5/site-packages/maasserver/utils/orm.py", line 504, in retrier
    return func(*args, **kwargs)
  File "/snap/maas/91/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File "/snap/maas/91/lib/python3.5/site-packages/maasserver/websockets/handlers/subnet.py", line 97, in delete
    subnet.delete()
  File "/snap/maas/91/lib/python3.5/site-packages/maasserver/models/subnet.py", line 471, in delete
    super().delete(*args, **kwargs)
  File "/snap/maas/91/lib/python3.5/site-packages/django/db/models/base.py", line 872, in delete
    collector.delete()
  File "/snap/maas/91/lib/python3.5/site-packages/django/db/models/deletion.py", line 314, in delete
    sender=model, instance=obj, using=self.using
  File "/snap/maas/91/lib/python3.5/site-packages/django/dispatch/dispatcher.py", line 201, in send
    response = receiver(signal=self, sender=sender, **named)
  File "/snap/maas/91/lib/python3.5/site-packages/maasserver/models/signals/staticipaddress.py", line 151, in post_delete_check_range_utilization
    if instance.subnet is None:
  File "/snap/maas/91/lib/python3.5/site-packages/django/db/models/fields/related.py", line 602, in __get__
    rel_obj = qs.get()
  File "/snap/maas/91/lib/python3.5/site-packages/django/db/models/query.py", line 334, in get
    self.model._meta.object_name
maasserver.models.subnet.DoesNotExist: Subnet matching query does not exist.

Is ...

Or as a paste to be a bit more readable... https://paste.ubuntu.com/25026706/

From cli: https://paste.ubuntu.com/25026769/

I am using the code from here for a minimal reproduction: https://paste.ubuntu.com/25026933/

for some reason, you delete subnets by cidr, not id:

http://paste.ubuntu.com/25091374/

This DoesNotExist exception seems to be occurring due to Django's behavior of implicitly fetching a related object. In this case, we're trying to recalculate the usage of the subnet when the *IP address* that existed on the now-delete subnet is removed. Django believes there to be a subnet related to the IP address, since the subnet is not None, and tries to fetch it. But since it has been deleted, this race condition occurs.

I feel like this is arguably a bug in Django, but we'll have to handle it ourselves.

@jhobbs, you should be able to delete the subnet by its `id`, too, for the record.

Worked around it by making a temporary fabric and moving subnets onto that fabric's default vlan. Then updating in place rather than deleting/creating.

https://git.launchpad.net/~lutostag/cpe-foundation/commit/?id=72022fa1db17f3baa1ee8b6fee2b5947ca4210db

So not a blocker for us at the moment.

For the record, I'm seeing this as well but with a subnet we created ourselves. I'm getting

maasserver.models.subnet.DoesNotExist: Subnet matching query does not exist.

both when deleting via ID and CIDR

Details: http://paste.ubuntu.com/25904255/

For additional information, I have encountered the same behaviour in 2.3.0 (6434-gd354690-0ubuntu1~16.04.1)

https://paste.ubuntu.com/26135301/