Incorrect exit status of an already waited process in subprocess.Popen

Bug report

If a process run by subprocess.Popen and its exit status is obtained by os.wait() or os.waitpid(), then calling the wait() method on the Popen object always returns zero.

Example:

import os
import subprocess

proc = subprocess.Popen(["false"]) # could be any process with non-zero exit status
pid, status = os.wait()
print("status from os.wait()", status)  # prints non-zero exit status correctly
print("status from proc.wait()", proc.wait()) # prints zero exit status incorrectly

The root cause of the problem is that the child has been already wait()ed, so the Popen object has no chance to obtain the exit code as the process no longer exists. It calls waitpid() here but it ignores ECHILD and defaults the status to zero.

Your environment

• CPython versions tested on: Python 3.10.4 (main, Jun 29 2022, 12:14:53) [GCC 11.2.0] on linux
• Operating system and architecture: Linux x86-64

Linked PRs

• gh-137804