[MacPorts] #60274: multiprocessing broken in Python38

[MacPorts]

#60274: multiprocessing broken in Python38
------------------------+----------------------
 Reporter:  mouse07410  |      Owner:  (none)
     Type:  defect      |     Status:  new
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:
 Keywords:              |       Port:  python38
------------------------+----------------------
 This bug is from the upstream. Patch/solution to issue 33725 on
 https://bugs.python.org broke multiprocessing package. Now it crashes in
 {{{spawn.py}}} with
 {{{
 Traceback (most recent call last):
   File "<string>", line 1, in <module>
   File
 "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/spawn.py",
 line 116, in spawn_main
     exitcode = _main(fd, parent_sentinel)
   File
 "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/spawn.py",
 line 126, in _main
     self = reduction.pickle.load(from_parent)
   File
 "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/synchronize.py",
 line 110, in __setstate__
     self._semlock = _multiprocessing.SemLock._rebuild(*state)
 FileNotFoundError: [Errno 2] No such file or directory
 }}}


 Here's the program that triggers the bug:
 {{{
 #!/usr/bin/env python3
 #
 # Test "multiprocessing" package included with Python-3.6+
 #
 # Usage:
 #    ./mylti1.py [nElements [nProcesses [tSleep]]]
 #
 #        nElements  - total number of integers to put in the queue
 #                     default: 100
 #        nProcesses - total number of parallel processes/threads
 #                     default: number of physical cores available
 #        tSleep     - number of milliseconds for a thread to sleep
 #                     after it retrieved an element from the queue
 #                     default: 17
 #
 # Algorithm:
 #   1. Creates a queue and adds nElements integers to it,
 #   2. Creates nProcesses threads
 #   3. Each thread extracts an element from the queue and sleeps for
 tSleep milliseconds
 #

 import sys, queue, time
 import multiprocessing as mp


 def getElements(q, tSleep, idx):
     l = []  # list of pulled numbers
     while True:
         try:
             l.append(q.get(True, .001))
             time.sleep(tSleep)
         except queue.Empty:
             if q.empty():
                 print(f'worker {idx} done, got {len(l)} numbers')
                 return


 if __name__ == '__main__':
     nElements = int(sys.argv[1]) if len(sys.argv) > 1 else 100
     nProcesses = int(sys.argv[2]) if len(sys.argv) > 2 else mp.cpu_count()
     tSleep = float(sys.argv[3]) if len(sys.argv) > 3 else 17

     # Uncomment the following line to revert to old 'fork' and make it
 work on 3.8+
     #mp.set_start_method('fork')

     # Fill the queue with numbers from 0 to nElements
     q = mp.Queue()
     for k in range(nElements):
         q.put(k)

     # Start worker processes
     for m in range(nProcesses):
         p = mp.Process(target=getElements, args=(q, tSleep / 1000, m))
         p.start()
 }}}

 Proposed solution is reverting the default of {{{start}}} back to
 {{{'fork'}}}.

-- 
Ticket URL: <https://trac.macports.org/ticket/60274>
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