[issue43944] Processes in Python 3.9 exiting with code 1 when It's created inside a ThreadPoolExecutor

[Genaro Camele]

New submission from Genaro Camele :

I've a piece of code that submits a task to a [ThreadPoolExecutor][1] which 
starts a [Process][2]. I've realised that in Python 3.8 that Process finished 
with exit code `0`. But I've updated Python to the 3.9 version and this started 
to finishing with exit code `1`! Even when the Process executes an empty task.

Here's a minimal example:

```python
from multiprocessing import Process
from concurrent.futures import ThreadPoolExecutor


def some_task():
pass


def execute_error():
p = Process(target=some_task)
p.start()
p.join()
print(p.exitcode)  # This is always 1 on a ThreadPoolExecutor!!!


executor = ThreadPoolExecutor(max_workers=4)
executor.submit(execute_error)
# execute_error()  # IMPORTANT: this works correctly (exit 0)
```

My versions:

```
Ubuntu 21.04
Python 3.9.4
```

**Note** that if `__execute_error` is called outside the ThreadPoolExecutor it 
works correctly.
Running on Python 3.8.6 exitcode = 0 too.


  [1]: 
https://docs.python.org/3/library/concurrent.futures.html#threadpoolexecutor
  [2]: 
https://docs.python.org/3.9/library/multiprocessing.html#multiprocessing.Process

--
messages: 391985
nosy: Genarito
priority: normal
severity: normal
status: open
title: Processes in Python 3.9 exiting with code 1 when It's created inside a 
ThreadPoolExecutor
type: behavior
versions: Python 3.9

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[issue43944] Processes in Python 3.9 exiting with code 1 when It's created inside a ThreadPoolExecutor

[Genaro Camele]

Genaro Camele  added the comment:

Hi @jack__d, thanks for your answer and time. Unfortunately, It's still a 
regression, as in Python < 3.9 my example works as expected

--

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