Do not interrupt the whole dispatcher if a single handler fails

Catch exceptions from event handlers (and log them), instead of
interrupting the whole event dispatcher. This avoid misterious stopping
processing the events if a single thing goes wrong. Additionally,
logging event details may help debugging the issue.

qubesadmin/events/__init__.py

@@ -249,4 +249,9 @@ def handle(self, subject, event, **kwargs):
             for h_func in h_func_set
             if fnmatch.fnmatch(event, h_name)]
         for handler in handlers:
-            handler(subject, event, **kwargs)
+            try:
+                handler(subject, event, **kwargs)
+            except:  # pylint: disable=bare-except
+                self.app.log.exception(
+                    'Failed to handle event: %s, %s, %s',
+                    subject, event, kwargs)

qubesadmin/tests/events.py

@@ -90,6 +90,16 @@ def test_002_handler_glob_partial(self):
         self.dispatcher.handle('', 'some-event', arg1='value1')
         self.assertFalse(handler.called)
 
+    def test_003_handler_error(self):
+        handler = unittest.mock.Mock()
+        self.dispatcher.add_handler('some-event', handler)
+        handler2 = unittest.mock.Mock(side_effect=AssertionError)
+        self.dispatcher.add_handler('some-event', handler2)
+        # should catch the exception
+        self.dispatcher.handle('', 'some-event', arg1='value1')
+        handler.assert_called_once_with(None, 'some-event', arg1='value1')
+        handler2.assert_called_once_with(None, 'some-event', arg1='value1')
+
     async def mock_get_events_reader(self, stream, cleanup_func, expected_vm,
             vm=None):
         self.assertEqual(expected_vm, vm)