[bug] VPN templates conflict

[nemesifier]

Describe the bug
Assigning a variation of a OpenVPN client template while unassigning another one causes the resulting configuration to become bugged: the configuration variables of the certificates are not resolved.

Steps To Reproduce
Steps to reproduce the behavior:

1. Create a VPN-client template using OpenVPN
2. Create another VPN-client template using the same server, but with slightly different configurations
3. Assign the template created in step 1, save
4. Now unassign the template created in step 1 while at the same time assign the template created in step 2, save

Expected behavior
The resulting configuration should work normally.

Screenshots

System Informatioon:

• OS: Debian bookworm/sid
• Python Version: 3.10.12
• Django Version: 4.2

[shwetd19]

@nemesifier can I work on this ? would love to move forward with it!

[nemesifier]

@shwetd19 sure, you can try! Can you replicate the issue?

[pandafy]

@shwetd19 are you still working on this?

[shwetd19]

Yes I am, we'll raise PR in some time

[pandafy]

This bug occurs due to the following code:

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

Lines 279 to 298 in 129a42f

	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()

The m2m field sends the following signals after the operation in Step 4:

1. post_add: Triggered when adding template 2. This creates a new VpnClient object (referred to as VpnClient2) for this configuration, but it also deletes the existing VpnClient object created by template 1. Refer to this line:

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

Lines 279 to 281 in 129a42f

	if action == 'post_add':
	vpn_list = instance.templates.filter(type='vpn').values_list('vpn')
	instance.vpnclient_set.exclude(vpn__in=vpn_list).delete()

2. post_remove: Triggered when removing template 1. This deletes all VpnClients associated with the VPN related to template 1.

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

Lines 296 to 298 in 129a42f

	elif action == 'post_remove':
	for client in instance.vpnclient_set.filter(vpn=template.vpn):
	client.delete()

As a result, the configuration ends up with no VpnClient object, causing the VPN configuration variables to remain unresolved.

[shwetd19]

Hey @nemesifier @pandafy I've made a PR for this can you pls have a look, also I had a question as, Currently the fix focuses on preserving VPN clients when their VPN is still in use by other templates. Do we need additional validation to ensure this doesn't create any inconsistencies with other VPN-related features in OpenWISP?