[bug] Deleting VPN template does not delete VpnClient objects

[pandafy]

How to replicate

1. Open Django shell (in command line) and check the number of VpnClient objects using this query VpnClient.objects.count()
2. Create a VPN server object with OpenVPN backend
3. Create a VPN template for this VPN server
4. Add the VPN template to a device
5. Go to the VPN template page and delete the Template
6. Open Django shell (in command line) and check the number of VpnClient objects using this query VpnClient.objects.count()

Expected outcome
The number of VpnClients is different in Step 1 and Step 6 should be same.

Actual outcome

The number of VpnClients is different in Step 1 and Step 6 (increased by one). It is because the deletion of VPN template didn't trigger deletion of related VpnClient objects (there is no direct relation).

[pandafy]

I found that it is possible to have two different VPN templates for the same VPN server object. But, it is not possible to apply both the templates on the same device, as the JSONSchema generates the following error

There is a conflict with the specified templates. Invalid configuration triggered by "#/files", validator says:

I think we should discourage using different templates for the same VPN server object on the same device using validation (we can create a separate issue for this).

[pandafy]

We should double check the working of the following code. I think, it is not getting triggered when the template object is deleted.

openwisp-controller/openwisp_controller/config/base/config.py

Lines 229 to 275 in 226442c

	@classmethod
	def manage_vpn_clients(cls, action, instance, pk_set, **kwargs):
	"""
	automatically manages associated vpn clients if the
	instance is using templates which have type set to "VPN"
	and "auto_cert" set to True.
	This method is called from a django signal (m2m_changed)
	see config.apps.ConfigConfig.connect_signals
	"""
	if action not in ['post_add', 'post_remove']:
	return
	vpn_client_model = cls.vpn.through
	# coming from signal
	if isinstance(pk_set, set):
	template_model = cls.get_template_model()
	# Ordering the queryset here doesn't affect the functionality
	# since pk_set is a set. Though ordering the queryset is required
	# for tests.
	templates = template_model.objects.filter(pk__in=list(pk_set)).order_by(
	'created'
	)
	# coming from admin ModelForm
	else:
	templates = pk_set
	# delete VPN clients which have been cleared
	# by sortedm2m and have not been added back
	if action == 'post_add':
	vpn_list = instance.templates.filter(type='vpn').values_list('vpn')
	instance.vpnclient_set.exclude(vpn__in=vpn_list).delete()
	# when adding or removing specific templates
	for template in templates.filter(type='vpn'):
	if action == 'post_add':
	if vpn_client_model.objects.filter(
	config=instance, vpn=template.vpn
	).exists():
	continue
	client = vpn_client_model(
	config=instance,
	vpn=template.vpn,
	auto_cert=template.auto_cert,
	)
	client.full_clean()
	client.save()
	elif action == 'post_remove':
	for client in instance.vpnclient_set.filter(vpn=template.vpn):
	client.delete()