wireguard: Show next available client IP in Add Client form

Display the next available IP address that will be
automatically assigned when adding a new client.

Helps admins know what client IP to provide when configuring client
connections back to this server.

Signed-off-by: Frederico Gomes <fredericojfgomes@gmail.com>
[sunil: Turn the IP address styling into a form element]
[sunil: Update the comment style for consistency]
[sunil: Update the label for clarity]
Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/wireguard/templates/wireguard_add_client.html

@@ -15,6 +15,18 @@
 
     {{ form|bootstrap }}
 
+    {% if next_ip %}
+      <div class="form-group">
+        <label for="id_next_ip" class="control-label">
+          {% trans "IP address that will be assigned to this client" %}
+        </label>
+        <div>
+          <input type="text" id="id_next_ip" class="form-control"
+                 value="{{ next_ip }}" disabled="disabled">
+        </div>
+      </div>
+    {% endif %}
+
     <input type="submit" class="btn btn-primary"
            value="{% trans "Add Client" %}"/>
   </form>

plinth/modules/wireguard/views.py

@@ -46,6 +46,16 @@ class AddClientView(SuccessMessageMixin, FormView):
         """Return additional context for rendering the template."""
         context = super().get_context_data(**kwargs)
         context['title'] = _('Add Allowed Client')
+
+        # Show next available IP.
+        try:
+            connection = utils._server_connection()
+            setting_name = utils.nm.SETTING_WIREGUARD_SETTING_NAME
+            settings = connection.get_setting_by_name(setting_name)
+            context['next_ip'] = utils._get_next_available_ip_address(settings)
+        except Exception:
+            context['next_ip'] = None
+
         return context
 
     def form_valid(self, form):