[PyKDE] Using thread and event filter crashes Python

[Phil Thompson]

On Wednesday 06 July 2005 9:40 am, Yann COINTEPAS wrote:
> Setting an event filter to the QApplication singleton and using threads
> lead to a Python crash due to a double deletion of a widget (at Python
> level). This happens with PyQt 3.14.1 and sip 4.2.1. I don't know if it is
> the same for other versions.
>
> For example, the following code:
>
> import sys, threading
> from qt import *
>
> class EventFilter( QObject ):
>   def eventFilter( self, o, e ):
>     return False
>
> class TestWidget( QWidget ):
>   def __init__( self ):
>     print '!init TestWidget!', threading.currentThread()
>     QWidget.__init__( self )
>
>   def __del__( self ):
>     print '!del TestWidget!', threading.currentThread()
>
> def createWidget():
>   qApp.lock()
>   try:
>     w = TestWidget()
>     w.show()
>     del w
>   finally:
>     qApp.unlock()
>
>
> app = QApplication( sys.argv )
>
> theEventFilter = EventFilter()
> qApp.installEventFilter( theEventFilter )
>
> main = QLabel( 'main', None )
> main.show()
> app.setMainWidget( main )
>
> t = threading.Thread( target=createWidget )
> t.start()
>
> app.exec_loop()
>
>
> Produces the following output:
>
> !init TestWidget! <Thread(Thread-1, started)>
> !del TestWidget! <Thread(Thread-1, started)>
> !del TestWidget! <Thread(Thread-1, started)>
> Segmentation fault
>
> I even try to create widgets only from the main thread (instead of using
> qApp.lock/qApp.unlock) by using a QTimer and a queue of functions and
> arguments. But the result is the same.

With current snapshots I don't get the double delete and get a SIGKILL rather 
than a seg fault.

The above code definately breaks Qt's rules - you must must create QWidgets in 
the main GUI thread. I need another example that demonstrate's the problem 
but conforms to Qt's rules.

Phil