File descriptor leak using multiprocessing.Queue

Bug report

Bug description:

"Too many open files" can occur with multiprocessing.Queue, even if the protocol used to work with the queue is correct.

The file descriptor leak can be demonstrated with two small programs:

File "ko.py"

import os
import multiprocessing

pid = os.getpid()
cmd = f'lsof -p {pid} | grep -i pipe | wc -l'
cmdfull = f'lsof -p {pid} | grep -i pipe'
queues = {}

for i in range(100):
    queues[i] = multiprocessing.Queue()
    os.system(cmd)
    queues[i].close()
    queues[i].join_thread()
    os.system(cmd)

os.system(cmdfull)

File "ok.py"

import os
import multiprocessing

pid = os.getpid()
cmd = f'lsof -p {pid} | grep -i pipe | wc -l'
cmdfull = f'lsof -p {pid} | grep -i pipe'
queues = {}

for i in range(100):
    queues[i] = multiprocessing.Queue()
    queues[i].put('Hello')
    os.system(cmd)
    queues[i].close()
    queues[i].join_thread()
    os.system(cmd)

os.system(cmdfull)

An empty queue that remains empty is not abnormal (e.g., an error queue when no errors occur).

The workaround is to create a class extending multiprocessing.Queue, adding a close method that inserts an extra message into the queue before performing the actual close.

This issue can be observed on Linux and macOS. However, it is not specific to a particular operating system.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Start by running the ko.py and ok.py reproductions on a supported Linux or macOS system and compare their file-descriptor counts. Then inspect the multiprocessing.Queue close and join_thread paths; done means repeatedly closing empty queues no longer leaves the descriptors shown by lsof while preserving the working behavior demonstrated by ok.py.