How to fix the DeprecationWarning in the following pytest example? Note this is minimalistic reproducer.

Use case is rather simple:

• I have an object (represented as Obj) that is tested;
• Obj interacts with an external server;
• For testing purposes, external server is mocked using multiprocessing.Process();
• Obj uses ProcessPoolExecutor to do some CPU-bound operations;

import asyncio
import concurrent.futures
import http.server
import multiprocessing

import pytest
import pytest_asyncio

# This does not work, as mock_server is not pickable...
# multiprocessing.set_start_method("spawn")

def hello_world():
    print("Hello, World!")

class Obj():
    def __init__(self):
        self.pool = concurrent.futures.ProcessPoolExecutor(max_workers=1)
        asyncio.get_running_loop().run_in_executor(self.pool, hello_world)

@pytest_asyncio.fixture
async def obj_to_test():
    return Obj()

@pytest.fixture
def mock_server(tmpdir):
    def server(path, host, port):
        os.chdir(path)
        httpd = http.server.HTTPServer(
            (host, port),
            http.server.SimpleHTTPRequestHandler
        )
        httpd.serve_forever()

    process = multiprocessing.Process(target=server, args=(".", "127.0.0.1", 9999))
    process.start()
    yield process
    process.terminate()
    process.join()

def test_something(obj_to_test, mock_server):
    # normally obj_to_test will interact here with mock_server
    pass

My environment is:

platform linux — Python 3.12.3, pytest-8.2.0, pluggy-1.5.0
plugins: asyncio-0.23.6

I am using Python installation from this Docker image: python:3.12-slim@sha256:2be8daddbb82756f7d1f2c7ece706aadcb284bf6ab6d769ea695cc3ed6016743

pip install pytest==8.2.0 pytest-asyncio==0.23.6

After above, I am getting the following warning, which I want to fix:

/usr/local/lib/python3.12/multiprocessing/popen_fork.py:66: DeprecationWarning: This process (pid=523) is multi-threaded, use of fork() may lead to deadlocks in the child.

Using multiprocessing.set_start_method("spawn") does not help.

I was able to reproduce DeprecationWarning outside of pytest environment as well:

import concurrent.futures
import multiprocessing
import time


pool = concurrent.futures.ProcessPoolExecutor(max_workers=1)
job = pool.submit(lambda: time.sleep(5))


process = multiprocessing.Process(target=lambda: None_)
process.start()
process.terminate()

and its run:

PYTHONWARNINGS=default python3 t5.py
/usr/local/lib/python3.12/multiprocessing/popen_fork.py:66: DeprecationWarning: This process (pid=1024) is multi-threaded, use of fork() may lead to deadlocks in the child.
  self.pid = os.fork()