[BUG] EventEngine alert export crashes on any alert from a device that is a member of a device group

[lunkwill42]

Describe the bug
Enabling an export script in eventengine.conf (as one would do to export alerts to Argus) causes the eventengine to sometimes fail to process alerts due to a serialization error.

Apparently, Django 2.2 changes the behavior of QuerySet.values_list(), so that it no longer returns a list, but a ValuesListQuerySet object. NAV's serializer definitions for alerts uses a call to values_list to generate a list of Device Groups that a device is a member of. If the result is an empty QuerySet, everything is just peachy. If, however, the alert comes from a device that is a member of any device group, json.dumps will be asked to dump a ValuesListQuerySet instead of a None value, which it can't, and a TypeError is raised.

To Reproduce
Steps to reproduce the behavior:

1. Set export=/bin/cat in eventengine.conf
2. nav restart eventengine
3. Use tools/eventgenerators/boxstate.py to inject a fake boxDown event for a device that is a member of any device group
4. See error in eventengine.log

Expected behavior

Serialization should be able to produce a list of device group names, as it did in NAV 5.0.

Traceback

Traceback (most recent call last):
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/eventengine/engine.py", line 189, in load_new_events
    self.handle_event(event)
  File "/usr/lib/python3.7/contextlib.py", line 74, in inner
    return func(*args, **kwds)
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/eventengine/engine.py", line 255, in handle_event
    self._post_generic_alert(event)
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/eventengine/engine.py", line 230, in _post_generic_alert
    alert.post()
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/eventengine/alerts.py", line 146, in post
    export.exporter.export(alert)
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/eventengine/export.py", line 83, in export
    data = json.dumps(serializer.data, cls=DjangoJSONEncoder)
  File "/usr/lib/python3.7/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "/usr/lib/python3.7/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3.7/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/opt/venvs/nav/lib/python3.7/site-packages/django/core/serializers/json.py", line 124, in default
    return super(DjangoJSONEncoder, self).default(o)
  File "/usr/lib/python3.7/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type QuerySet is not JSON serializable

Environment (please complete the following information):

• NAV version installed: 5.1.0

[hmpf]

I really liked step 1 in how to reproduce I must say.