LocalDispatcher : Use WeakMethod for BackgroundTask

This was made necessary by changes in GafferHQ#5579
which meant that the functor wasn't destroyed immediately after running. Using the
WeakMethod is better though, because the old method wouldn't have broken the circular
reference if the background task was cancelled before it was started.

python/GafferDispatch/LocalDispatcher.py

@@ -190,7 +190,7 @@ def messagesChangedSignal( self ) :
 		def _execute( self ) :
 
 			if self.__executeInBackground :
-				self.__backgroundTask = Gaffer.BackgroundTask( None, self.__executeInternal )
+				self.__backgroundTask = Gaffer.BackgroundTask( None, Gaffer.WeakMethod( self.__executeInternal ) )
 			else :
 				self.__executeInternal()
 

python/GafferDispatchTest/LocalDispatcherTest.py

@@ -36,6 +36,7 @@
 
 import datetime
 import errno
+import gc
 import os
 import stat
 import shutil
@@ -60,6 +61,16 @@
 
 class LocalDispatcherTest( GafferTest.TestCase ) :
 
+	def tearDown( self ) :
+
+		GafferTest.TestCase.tearDown( self )
+
+		# Check for Jobs existing past their expected lifetime (most likely through circular references).
+		self.assertEqual(
+			[ o for o in gc.get_objects() if isinstance( o, GafferDispatch.LocalDispatcher.Job ) ],
+			[]
+		)
+
 	def __createLocalDispatcher( self, jobPool = None ) :
 
 		if jobPool is None :
@@ -528,7 +539,7 @@ def testFailure( self ) :
 		self.assertFalse( os.path.isfile( s.context().substitute( s["n2"]["fileName"].getValue() ) ) )
 		self.assertFalse( os.path.isfile( s.context().substitute( s["n1"]["fileName"].getValue() ) ) )
 
-		self.tearDown()
+		os.unlink( s.context().substitute( s["n3"]["fileName"].getValue() ) )
 
 		dispatcher["executeInBackground"].setValue( True )
 		dispatcher.dispatch( [ s["n1"] ] )