Merge remote-tracking branch 'origin/pr/633'

* origin/pr/633:
  Improve `qvm-prefs -D SOMEVM template` error message

Author: marmarek

qubes/tests/vm/appvm.py

@@ -147,6 +147,10 @@ def test_004_template_reset(self):
         vm = self.get_vm()
         with self.assertRaises(qubes.exc.QubesValueError):
             vm.template = qubes.property.DEFAULT
+        self.app.default_template = self.template
+        with self.assertRaises(qubes.exc.QubesValueError):
+            vm.template = qubes.property.DEFAULT
+        del self.app.default_template
 
     def test_500_property_migrate_template_for_dispvms(self):
         xml_template = '''

qubes/vm/appvm.py

@@ -120,6 +120,11 @@ def on_domain_loaded(self, event):
     def on_property_pre_reset_template(self, event, name, oldvalue=None):
         '''Forbid deleting template of running VM
         '''  # pylint: disable=unused-argument
+        default_template = getattr(self.app, 'default_template', None)
+        if default_template is not None:
+            raise qubes.exc.QubesValueError('Cannot unset template. ' \
+                'You can set it to the current default ' \
+                f'({default_template.name}) instead')
         raise qubes.exc.QubesValueError('Cannot unset template')
 
     @qubes.events.handler('property-pre-set:template')