Twisted + qtreactor: How to cleanup after last window closed?

I have a Twisted/PyQt application that (among other things) connects to a bunch of remote resources. When the user closes the window I want to shut down all the connections, cleanly if possible, forcibly if not.

The problem is that by the time I go to close the connections, it appears that the reactor is no longer alive to let me do so.

Here's my app code:

# Create app and connect the Twisted/Qt reactors
app = QApplication(sys.argv)
qtreactor.qt4reactor.install()

# Shutdown Twisted when window is closed
@defer.inlineCallbacks
def stop():
    print "="*40, "Closing connections..."
    yield closeConnections()
    print "="*40, "closed."

    print "="*40, "Stopping reactor..."
    reactor.stop()
    print "="*40, "stopped."
app.connect(app, SIGNAL("lastWindowClosed()"), stop)

reactor.runReturn()
rc = app.exec_()
exit(rc)

And here's a stripped down version of my cleanup code:

@defer.inlineCallbacks
def closeConnections():
    for connection in connections:
        print "Closing connection #%s" % connection
        yield threads.deferToThread(popen("/foo/bar/cleanup %s" % con开发者_高级运维nection))
        print "Connection closed."

The first print statement is reached, and the command is executed, but I never get the second one, nor do I go through the for loop multiple times.

Is my analysis correct? Is the problem that the reactor is already down, so I never hear back from threads.deferToThread? Or is there some other problem? Furthermore, how do I fix it?

Thanks, Jonathan

I don't know exactly when that lastWindowClosed() signal fires. However, even if it fires early enough, before the reactor has shut down (preventing you from doing what you want to do), I'm sure that PyQt doesn't know what to do with the Deferred that is returned by your stop function. This means that the shutdown process will proceed merrily onward while your asynchronous cleanup code tries to run. Likely the GUI shutdown will finish before your network shutdown gets anywhere.

So, use reactor.addSystemEventTrigger('before', 'shutdown', stop) instead. I don't know if this will run slightly earlier or slightly later than lastWindowClosed(), but it will run early enough that the reactor will still be usable, and it will pay attention to the Deferred your function returns. Shutdown will be suspended, in fact, until that Deferred fires. This gives you all the time you need to do your cleanup.

Separately from all that, you shouldn't do threads.deferToThread(popen("/foo/bar/cleanup %s" % connection)):

• You need to pass a callable to deferToThread, not the result of calling the callable. As written, your code runs popen in the reactor thread and passes a file object to the thread to be called (which makes no sense, of course)
• Mixing threads and child processes is iffy. You might get away with it most of the time, I dunno.
• reactor.spawnProcess will let you run a child process without blocking, without threads, and without worrying about mixing threads and processes. See also twisted.internet.utils.getProcessOutput if you don't need all the features of spawnProcess (which you appear not to).