Deprecate property-del:name events and introduce property-reset:name instead

And the same for -pre- events.

The property-del name is really confusing (it makes sense only for those
with deep knowledge of the implementation), because the property isn't
really deleted - it is only reverted to the "default" state (which most
properties have). So, name the event property-reset, intentionally
similar to property-set, as it is also kind of a value change.

Additionally the property-reset event is meant to be called when the
(dynamic) default value changes. Due to the current implementation, it
is a manual process so it can't be guaranteed to be called in all those
cases, but lets try to cover as much as possible.

Fixes QubesOS/qubes-issues#5834

qubes/__init__.py

@@ -284,6 +284,10 @@ class property:  # pylint: disable=redefined-builtin,invalid-name
             has_oldvalue = False
 
         if has_oldvalue:
+            instance.fire_event('property-pre-reset:' + self.__name__,
+                pre_event=True,
+                name=self.__name__, oldvalue=oldvalue)
+            # deprecated, to be removed in Qubes 5.0
             instance.fire_event('property-pre-del:' + self.__name__,
                 pre_event=True,
                 name=self.__name__, oldvalue=oldvalue)
@@ -291,13 +295,23 @@ class property:  # pylint: disable=redefined-builtin,invalid-name
                 delattr(instance, self._attr_name)
             except AttributeError:
                 pass
+            instance.fire_event('property-reset:' + self.__name__,
+                name=self.__name__, oldvalue=oldvalue)
+            # deprecated, to be removed in Qubes 5.0
             instance.fire_event('property-del:' + self.__name__,
                 name=self.__name__, oldvalue=oldvalue)
 
         else:
+            instance.fire_event('property-pre-reset:' + self.__name__,
+                pre_event=True,
+                name=self.__name__)
+            # deprecated, to be removed in Qubes 5.0
             instance.fire_event('property-pre-del:' + self.__name__,
                 pre_event=True,
                 name=self.__name__)
+            instance.fire_event('property-reset:' + self.__name__,
+                name=self.__name__)
+            # deprecated, to be removed in Qubes 5.0
             instance.fire_event('property-del:' + self.__name__,
                 name=self.__name__)
 
@@ -470,6 +484,9 @@ class PropertyHolder(qubes.events.Emitter):
             Fired when property gets deleted (is set to default). Signature is
             variable, *oldvalue* is present only if there was an old value.
 
+            This event is deprecated and will be removed in Qubes 5.0.
+            Use property-reset instead.
+
             :param name: Property name
             :param oldvalue: Old value of the property
 
@@ -479,6 +496,31 @@ class PropertyHolder(qubes.events.Emitter):
             Fired before property gets deleted (is set to default). Signature
             is variable, *oldvalue* is present only if there was an old value.
 
+            This event is deprecated and will be removed in Qubes 5.0.
+            Use property-pre-reset instead.
+
+            :param name: Property name
+            :param oldvalue: Old value of the property
+
+        .. event:: property-reset:<propname> \
+                (subject, event, name[, oldvalue])
+
+            Fired when property gets reset to the (possibly dynamic) default.
+            This even may be also fired when the property is already in
+            "default" state, but the calculated default value changes.
+            Signature is variable, *oldvalue* is present only if there was an
+            old value.
+
+            :param name: Property name
+            :param oldvalue: Old value of the property
+
+        .. event:: property-pre-reset:<propname> \
+                (subject, event, name[, oldvalue])
+
+            Fired before property gets reset to the (possibly dynamic) default.
+            Signature is variable, *oldvalue* is present only if there was an
+            old value.
+
             :param name: Property name
             :param oldvalue: Old value of the property
 

qubes/tests/init.py

@@ -170,8 +170,12 @@ class TC_10_property(qubes.tests.QubesTestCase):
 
         self.assertEventFired(self.holder, 'property-pre-del:testprop1',
             kwargs={'name': 'testprop1', 'oldvalue': 'testvalue'})
+        self.assertEventFired(self.holder, 'property-pre-reset:testprop1',
+            kwargs={'name': 'testprop1', 'oldvalue': 'testvalue'})
         self.assertEventFired(self.holder, 'property-del:testprop1',
             kwargs={'name': 'testprop1', 'oldvalue': 'testvalue'})
+        self.assertEventFired(self.holder, 'property-reset:testprop1',
+            kwargs={'name': 'testprop1', 'oldvalue': 'testvalue'})
 
     def test_081_delete_by_assign(self):
         self.holder.testprop1 = 'testvalue'
@@ -203,8 +207,12 @@ class TC_10_property(qubes.tests.QubesTestCase):
         self.assertEqual(holder.testprop1, 'defaultvalue')
         self.assertEventFired(holder, 'property-pre-del:testprop1', kwargs={
             'name': 'testprop1', 'oldvalue': 'testvalue'})
+        self.assertEventFired(holder, 'property-pre-reset:testprop1', kwargs={
+            'name': 'testprop1', 'oldvalue': 'testvalue'})
         self.assertEventFired(holder, 'property-del:testprop1', kwargs={
             'name': 'testprop1', 'oldvalue': 'testvalue'})
+        self.assertEventFired(holder, 'property-reset:testprop1', kwargs={
+            'name': 'testprop1', 'oldvalue': 'testvalue'})
 
     def test_090_write_once_set(self):
         class MyTestHolder(qubes.tests.TestEmitter, qubes.PropertyHolder):