Merge lp:~allenap/maas/events-register-unregister-during-fire into lp:~maas-committers/maas/trunk

[Gavin Panella]

Event.registerHandler and Event.unregisterHandler can now be called by handlers.

Previously calling these methods from a handler would cause a "size changed during iteration" exception in Event.fire.

[Blake Rouse (blake-rouse)]

Looks good.

review: Approve

=== modified file 'src/provisioningserver/utils/events.py'
--- src/provisioningserver/utils/events.py	2015-12-01 18:12:59 +0000
+++ src/provisioningserver/utils/events.py	2016-10-04 08:33:48 +0000
@@ -1,4 +1,4 @@
-# Copyright 2015 Canonical Ltd.  This software is licensed under the
+# Copyright 2015-2016 Canonical Ltd.  This software is licensed under the
 # GNU Affero General Public License version 3 (see the file LICENSE).
 
 """Event-related utilities."""
@@ -22,12 +22,14 @@
 
     def unregisterHandler(self, handler):
         """Unregister handler from event."""
-        if handler in self.handlers:
-            self.handlers.remove(handler)
+        self.handlers.discard(handler)
 
     def fire(self, *args, **kwargs):
         """Fire the event."""
-        for handler in self.handlers:
+        # Shallow-copy the set to avoid "size changed during iteration"
+        # errors. This is a fast operation: faster than switching to an
+        # immutable set or using thread-safe locks (and won't deadlock).
+        for handler in self.handlers.copy():
             handler(*args, **kwargs)
 
 

=== modified file 'src/provisioningserver/utils/tests/test_events.py'
--- src/provisioningserver/utils/tests/test_events.py	2016-06-22 17:03:02 +0000
+++ src/provisioningserver/utils/tests/test_events.py	2016-10-04 08:33:48 +0000
@@ -28,12 +28,26 @@
         event.registerHandler(sentinel.handler)
         self.assertItemsEqual([sentinel.handler], event.handlers)
 
+    def test_registerHandler_during_fire(self):
+        event = Event()
+        event.registerHandler(event.registerHandler)
+        event.fire(sentinel.otherHandler)
+        self.assertItemsEqual(
+            [event.registerHandler, sentinel.otherHandler],
+            event.handlers)
+
     def test_unregisterHandler(self):
         event = Event()
         event.registerHandler(sentinel.handler)
         event.unregisterHandler(sentinel.handler)
         self.assertItemsEqual([], event.handlers)
 
+    def test_unregisterHandler_during_fire(self):
+        event = Event()
+        event.registerHandler(event.unregisterHandler)
+        event.fire(event.unregisterHandler)
+        self.assertItemsEqual([], event.handlers)
+
     def test_fire_calls_all_handlers(self):
         event = Event()
         handler_one = MagicMock()