privacy: Disable DNS fallback option if resolved is not installed

Tests:

- Ensure that systemd-resolved is not installed.

- Notice that form field for Fallback DNS servers is disabled. Form value is
unchecked (default value when initial value is not provided).

- Submitting the form works with and without changes.

- Ensure that systemd-resolved is installed.

- Notice that form field for Fallback DNS servers is not disabled. Form value
reflects the current state of fallback dns as shown in names app.

- Submitting the form works with and without changes.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/modules/privacy/forms.py

@@ -5,6 +5,7 @@ from django import forms
 from django.utils.translation import gettext_lazy as _
 
 from plinth import cfg
+from plinth.modules import names
 from plinth.utils import format_lazy
 
 
@@ -30,3 +31,9 @@ class PrivacyForm(forms.Form):
             'unusual circumstances where no DNS servers are known but '
             'internet connectivity is available. Can be disabled in most '
             'cases if network connectivity is stable and reliable.'))
+
+    def __init__(self, *args, **kwargs):
+        """Disable DNS fallback field if necessary."""
+        super().__init__(*args, **kwargs)
+        self.fields['dns_fallback'].disabled = (
+            not names.is_resolved_installed())

plinth/modules/privacy/views.py

@@ -5,6 +5,7 @@ from django.contrib import messages
 from django.utils.translation import gettext as _
 
 import plinth.modules.names.privileged as names_privileged
+from plinth.modules import names
 from plinth.modules.privacy.forms import PrivacyForm
 from plinth.views import AppView
 
@@ -21,7 +22,9 @@ class PrivacyAppView(AppView):
         """Return the values to fill in the form."""
         initial = super().get_initial()
         initial.update(privileged.get_configuration())
-        initial.update(names_privileged.get_resolved_configuration())
+        if names.is_resolved_installed():
+            initial.update(names_privileged.get_resolved_configuration())
+
         return initial
 
     def form_valid(self, form):
@@ -34,10 +37,11 @@ class PrivacyAppView(AppView):
         if old_config['enable_popcon'] != new_config['enable_popcon']:
             changes['enable_popcon'] = new_config['enable_popcon']
 
-        if old_config['dns_fallback'] != new_config['dns_fallback']:
-            names_privileged.set_resolved_configuration(
-                dns_fallback=new_config['dns_fallback'])
-            is_changed = True
+        if 'dns_fallback' in old_config:
+            if old_config['dns_fallback'] != new_config['dns_fallback']:
+                names_privileged.set_resolved_configuration(
+                    dns_fallback=new_config['dns_fallback'])
+                is_changed = True
 
         if changes:
             privileged.set_configuration(**changes)