qubes/ext: convert extensions to singletons

From now the extensions are instantiated once. They no longer have .app
attribute, but can access it from event handlers via vm.app.

qubes/__init__.py

@@ -1186,9 +1186,7 @@ class Qubes(PropertyHolder):
         #: logger instance for logging global messages
         self.log = logging.getLogger('app')
 
-        # pylint: disable=no-member
-        self.extensions = set(ext.load()(self)
-            for ext in pkg_resources.iter_entry_points('qubes.ext'))
+        self._extensions = qubes.ext.get_extensions()
 
         #: collection of all VMs managed by this Qubes instance
         self.domains = VMCollection(self)

qubes/ext/__init__.py

@@ -29,20 +29,20 @@ some systems. They may be OS- or architecture-dependent or custom-developed for
 particular customer.
 '''
 
+import pkg_resources
 import qubes.events
 
 
 class Extension(object):
     '''Base class for all extensions
-
-    :param qubes.Qubes app: application object
     '''  # pylint: disable=too-few-public-methods
 
-    def __init__(self, app):
-        self.app = app
+    def __new__(cls):
+        if '_instance' not in cls.__dict__:
+            cls._instance = super(Extension, cls).__new__(cls)
 
-        for name in dir(self):
-            attr = getattr(self, name)
+            for name in cls.__dict__:
+                attr = getattr(cls._instance, name)
                 if not qubes.events.ishandler(attr):
                     continue
 
@@ -52,7 +52,14 @@ class Extension(object):
                 else:
                     # global hook
                     for event in attr.ha_events:
-                    self.app.add_handler(event, attr)
+                        qubes.Qubes.add_handler(event, attr)
+
+        return cls._instance
+
+
+def get_extensions():
+    return set(ext.load()()
+        for ext in pkg_resources.iter_entry_points('qubes.ext'))
 
 
 def handler(*events, **kwargs):