BackgroundTask : Remove deadlock mitigation

But keep detection, so in the unlikely event this happens we can prompt the user to provide a useful bug report.

python/GafferTest/BackgroundTaskTest.py

@@ -255,24 +255,5 @@ def f( canceller ) :
 		# check for cancellation, and we'll deadlock.
 		del task
 
-	def testSelfWait( self ) :
-
-		def f( canceller ) :
-			# It makes zero sense for a task to wait for itself. Rather than
-			# allow it to deadlock, we want to emit a diagnostic message and
-			# continue.
-			task.wait()
-
-		mh = IECore.CapturingMessageHandler()
-		IECore.MessageHandler.setDefaultHandler( mh )
-
-		task = Gaffer.BackgroundTask( None, f )
-		task.wait()
-
-		self.assertEqual( len( mh.messages ), 1 )
-		self.assertEqual( mh.messages[0].level, IECore.Msg.Level.Error )
-		self.assertEqual( mh.messages[0].context, "BackgroundTask::wait" )
-		self.assertEqual( mh.messages[0].message, "Task attempted to wait for itself" )
-
 if __name__ == "__main__":
 	unittest.main()

src/Gaffer/BackgroundTask.cpp

@@ -255,8 +255,7 @@ void BackgroundTask::wait()
 	std::unique_lock<std::mutex> lock( m_taskData->mutex );
 	if( m_taskData->threadId == std::this_thread::get_id() )
 	{
-		IECore::msg( IECore::Msg::Error, "BackgroundTask::wait", "Task attempted to wait for itself" );
-		return;
+		IECore::msg( IECore::Msg::Error, "BackgroundTask::wait", "Deadlock detected : Task is attempting to wait for itself. Please provide stack trace in bug report." );
 	}
 
 	m_taskData->conditionVariable.wait(