In case this is a clue - the attached script "mp_hang2.py" adds a call to qsize() and uses only a single consumer. When I run it from the command line it does one of two things:


Option 1:

C:\TEMP\Py-3.7.2b-Venv\Scripts>.\python.exe "C:\Users\Tom.Wilson\Documents\Python-Bugs\mp_hang2.py"
Creating 1 consumers
Putting
Poisoning
Joining
Process Consumer-1:
Traceback (most recent call last):
  File "C:\Users\Tom.Wilson\AppData\Local\Programs\Python\Python37\lib\multiprocessing\process.py", line 297, in _bootstrap
    self.run()
  File "C:\Users\Tom.Wilson\Documents\Python-Bugs\mp_hang2.py", line 18, in run
    print(f'Queue size: {self.task_queue.qsize()}')
  File "C:\Users\Tom.Wilson\AppData\Local\Programs\Python\Python37\lib\multiprocessing\queues.py", line 117, in qsize
    return self._maxsize - self._sem._semlock._get_value()
PermissionError: [WinError 5] Access is denied


Option 2:

C:\TEMP\Py-3.7.2b-Venv\Scripts>.\python.exe "C:\Users\Tom.Wilson\Documents\Python-Bugs\mp_hang2.py"
Creating 1 consumers
Putting
Poisoning
Joining
Queue size: 2147483647
Getting task
 <<< Hangs here >>>


If I can provide anything else please let me know.