multiprocessing.Pool starts to fail if command has a long list of arguments #144503
Description
Bug report
Bug description:
A multiprocessing pool can fail to start when the command that starts the pool was ran with a lot of arguments.
Minimal working example:
# mwe.py
import multiprocessing
if __name__ == "__main__":
with multiprocessing.Pool():
passRun this using the following script:
#!/bin/bash
PAGE_SIZE=$(getconf PAGE_SIZE)
MAX_ARG_STRLEN=$((32 * PAGE_SIZE))
COUNT=$((MAX_ARG_STRLEN / 100 ))
echo $COUNT
yes "$(printf 'a%.0s' {1..100})" | head -n $COUNT | xargs python3 mwe.py
What this will do is effectively run the following:
python3 mwe.py aaa..a aaa..a aaa..a ... aaa.a
The Python process itself launches correctly, as each argument is only 100 bytes long (way under the maximum argument length of MAX_ARG_STRLEN).
The creation of the multiprocessing pool however fails, because the sys.argv variable is taken, and used to build a single string in multiprocessing.forkserver.ForkServer.ensure_running here:
cpython/Lib/multiprocessing/forkserver.py
Line 175 in d5cb9f6
This is caused by a recent change: #143717, which was also backported to 3.13 and 3.14. (It works on 3.14.2, fails on 3.14.3).
CPython versions tested on:
3.15, 3.14
Operating systems tested on:
Linux