[PyQt] Freezes and crashes with signal autoconnection

[Christian Roche]

Hi there,

I'm having some problems with a fairly simple multi-threaded application
under PyQt 4.5.4 and Windows XP. I have a number of worker threads that feed
one consumer in the main GUI thread. Initially I had the worker threads
directly call the "add" method of the consumer object; this piece of code
was protected by a mutex. So in short:

class Worker(QThread):
 def run(self):
   ...
   cache.add(link)

class Cache():
 def add(self, link):
   mutex.lock()
   # Add link to queue
   mutex.unlock()

cache = Cache()

This works pretty well but still I'm not completely satisfied. It's bad to
execute code that doesn't belong to your thread, right? So I tried to
replace that with a signal/slot mechanism:

class Worker(QThread):
 addLink = pyqtSignal(Link)

def init(self):
  self.addLink.connect(cache.add)

def run(self):
   ...
  link = Link(blah blah)
  self.addLink.emit(link)

class Cache(QObject):
 def add(self, link):
   # Add link to queue
   # No more mutex since access is now serial, right ?
   ...

However the application freezes after a while. No more GUI activity, window
not even repainting anymore and Windows tells me "this application is not
responding etc" when trying to close. This doesn't happen if I use
Qt.DirectConnection or Qt.BlockingQueuedConnection it seems, although I have
no idea why.

I also notice that with Qt.AutoConnection, the application crashes with no
error message unless I use 'PyQt_PyObject' types, with the quotes, in all
signal definitions (instead of Link for instance); I don't have this problem
either when using Qt.DirectConnection or Qt.BlockingQueuedConnection.

So does anyone understand what's going on here under the hood? Is the
signal/slot approach the correct way to garantee serial access to a shared
resource between threads (the link queue in this example)? Why is this whole
thing crashing and freezing like that?

Thanks for sharing your insight!
Chris

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