events: implicitly enable caching when creating EventsDispatcher

Application that runs EventsDispatcher can safely use also cache , which
greatly improve performance. This is because cache then is properly
updated/invalidated when needed.
Instead of modifying each application to explicitly enable cache based
on this simple rule, make it implicit when EventsDispatcher is created.

Do not enable caching when EventsDispatcher is created only temporarily
in wait_for_domain_shutdown.

QubesOS/qubes-issues#3293

qubesadmin/events/__init__.py

@@ -31,8 +31,19 @@
 class EventsDispatcher(object):
     ''' Events dispatcher, responsible for receiving events and calling
     appropriate handlers'''
-    def __init__(self, app, api_method='admin.Events'):
-        '''Initialize EventsDispatcher'''
+    def __init__(self, app, api_method='admin.Events', enable_cache=True):
+        """Initialize EventsDispatcher
+
+        :param app :py:class:`qubesadmin.Qubes` object
+        :param api_method Admin API method producing events
+        :param enable_cache Enable caching (see below)
+
+        Connecting :py:class:`EventsDispatcher` object to a
+        :py:class:`qubesadmin.Qubes` implicitly enables caching. It is important
+        to actually run the dispatcher (:py:meth:`listen_for_events`), otherwise
+        the cache won't be updated. Alternatively, disable caching by setting
+        :py:attr:`qubesadmin.Qubes.cache_enabled` property to `False`.
+        """
         #: Qubes() object
         self.app = app
 
@@ -41,6 +52,10 @@ def __init__(self, app, api_method='admin.Events'):
         #: event handlers - dict of event -> handlers
         self.handlers = {}
 
+        if enable_cache:
+            self.app.register_cache_handlers(self)
+            self.app.cache_enabled = True
+
     def add_handler(self, event, handler):
         '''Register handler for event
 

qubesadmin/events/utils.py

@@ -57,7 +57,7 @@ def wait_for_domain_shutdown(vms):
         return
     app = list(vms)[0].app
     vms = set(vms)
-    events = qubesadmin.events.EventsDispatcher(app)
+    events = qubesadmin.events.EventsDispatcher(app, enable_cache=False)
     events.add_handler('domain-shutdown',
         functools.partial(interrupt_on_vm_shutdown, vms))
     events.add_handler('connection-established',