[New-bugs-announce] [issue43768] Possible race condition on multiprocessing.Manager().dict() on macOS

[Jerry Chen]

New submission from Jerry Chen <jerryc443 at gmail.com>:

I am not sure if this is a bug or an expected case.

Long story short, I tried to print the content of a `multiprocessing.Manager().dict()` in the main thread, but I got a strange error.

I encountered this error only when the number of pools is rather large (>20) and only on `macOS` (works perfectly on `Linux`).

Specs:
```
  CPU: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
macOS: 11.2.3
```

The minimum err code is attached:
```python3
#!/usr/bin/env python3

from contextlib import suppress
import multiprocessing as mp
import time


def run():
    D[mp.current_process().name] = 'some val'
    time.sleep(0.5)


if __name__ == '__main__':
    mp.set_start_method('fork')
    D, rets = mp.Manager().dict(), []
    with mp.Pool(25) as p:
        for _ in range(33):
            rets.append(p.apply_async(run))
        while rets:
            for ret in rets[:]:
                with suppress(mp.TimeoutError):
                    ret.get(timeout=0)
                    rets.remove(ret)
                    print(len(D))

```

Error: 
```
multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
  File "/usr/local/Cellar/python at 3.9/3.9.2_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/managers.py", line 801, in _callmethod
    conn = self._tls.connection
AttributeError: 'ForkAwareLocal' object has no attribute 'connection'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/Cellar/python at 3.9/3.9.2_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/pool.py", line 125, in worker
    result = (True, func(*args, **kwds))
  File "/Users/???", line 9, in run
    D[mp.current_process().name] = 'some val'
  File "<string>", line 2, in __setitem__
  File "/usr/local/Cellar/python at 3.9/3.9.2_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/managers.py", line 805, in _callmethod
    self._connect()
  File "/usr/local/Cellar/python at 3.9/3.9.2_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/managers.py", line 792, in _connect
    conn = self._Client(self._token.address, authkey=self._authkey)
  File "/usr/local/Cellar/python at 3.9/3.9.2_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/connection.py", line 507, in Client
    c = SocketClient(address)
  File "/usr/local/Cellar/python at 3.9/3.9.2_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/connection.py", line 635, in SocketClient
    s.connect(address)
ConnectionRefusedError: [Errno 61] Connection refused
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/???", line 22, in <module>
    ret.get(timeout=0)
  File "/usr/local/Cellar/python at 3.9/3.9.2_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/pool.py", line 771, in get
    raise self._value
ConnectionRefusedError: [Errno 61] Connection refused
```

----------
components: macOS
messages: 390468
nosy: jerryc443, ned.deily, ronaldoussoren
priority: normal
severity: normal
status: open
title: Possible race condition on multiprocessing.Manager().dict() on macOS
type: behavior
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43768>
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