names: Don't show resolver status if package is not installed

Tests:

- Ensure that systemd-resolved package is not installed.

- Resolver status table is now shown.

- Instead a message is shown with button to re-run setup. Clicking the button
re-runs setup of the names app.

- Configuration form is also now shown.

- If systemd-resolved package is installed during re-run of setup, then status
table is shown.

- Message to install systemd-resolved is not shown.

- Configuration form is shown.

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

plinth/modules/names/templates/names.html

@@ -55,7 +55,7 @@
 
   <h3>{% trans "Resolver Status" %}</h3>
 
-  {% if resolved_status %}
+  {% if resolved_installed and resolved_status %}
     <div class="table-responsive">
       <table class="table resolved-status-table">
         <tbody>
@@ -107,6 +107,20 @@
         </tbody>
       </table>
     </div>
+  {% elif not resolved_installed %}
+    <p>
+      {% blocktrans trimmed %}
+        systemd-resolved package is not installed. Install it for additional
+        functionality.
+      {% endblocktrans %}
+      <form class="form form-install-resolved" method="post"
+            action="{% url 'rerun-setup' app_id='names' %}">
+        {% csrf_token %}
+
+        <input type="submit" class="btn btn-primary" name="install_resolved"
+               value="{% trans 'Install' %}"/>
+      </form>
+    </p>
   {% else %}
     <div class="alert alert-danger">
       {% trans "Error retrieving status:" %} {{ resolved_status_error }}
@@ -114,3 +128,9 @@
   {% endif %}
 
 {% endblock %}
+
+{% block configuration %}
+  {% if resolved_installed %}
+    {{ block.super }}
+  {% endif %}
+{% endblock %}

plinth/modules/names/views.py

@@ -31,17 +31,21 @@ class NamesAppView(AppView):
     def get_initial(self):
         """Return the values to fill in the form."""
         initial = super().get_initial()
-        initial.update(privileged.get_resolved_configuration())
+        if names.is_resolved_installed():
+            initial.update(privileged.get_resolved_configuration())
+
         return initial
 
     def get_context_data(self, *args, **kwargs):
         """Add additional context data for template."""
         context = super().get_context_data(*args, **kwargs)
         context['status'] = get_status()
-        try:
-            context['resolved_status'] = resolved.get_status()
-        except Exception as exception:
-            context['resolved_status_error'] = exception
+        context['resolved_installed'] = names.is_resolved_installed()
+        if context['resolved_installed']:
+            try:
+                context['resolved_status'] = resolved.get_status()
+            except Exception as exception:
+                context['resolved_status_error'] = exception
 
         return context