[PyQt] Bug Report: QThreadPool doesn't take the ownership of QRunnable

[Phil Thompson]

On 21-03-2014 9:43 am, Riccardo Ferrazzo wrote:
> Hi,
> In PyQt5 i'm having problems due to deleted objects inside the 
> QThreadPool.
> I've created a simple example to prove the existence of the problem:
>
> from PyQt5.QtCore import *
> from time import sleep
>
> class Test(QRunnable):
>     def __init__(self):
>         QRunnable.__init__(self)
>
>     def run(self):
>         sleep(10)
>         print(self, "Awake!")
>
>     def __del__(self):
>         print(self, "Deleted")
>
> def run_thread():
>     t = Test()
>     QThreadPool.globalInstance().start(t)
>
> if __name__ == "__main__":
>     for i in range(10):
>         run_thread()
>     sleep(20)
>
> It prints out:
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fcc8>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fc30>, 'Awake!')
> (<__main__.Test object at 0x7feb8499fc30>, 'Deleted')
> (<__main__.Test object at 0x7feb8499fcc8>, 'Awake!')
> (<__main__.Test object at 0x7feb8499fcc8>, 'Deleted')

The main problem should be fixed in tonight's snapshot. You can work 
around it by keeping an explicit reference to the global QThreadPool.

However there still seems to be a problem. My system hangs when the 
thread pool is exhausted - it seems unable to re-use a thread. However 
putting a sleep() (for 4 seconds in my case) after the call to start() 
makes everything work. Using a shorter delay allows about 7 runs before 
hanging.

Phil