Python multiprocessing crashes docker container

Question

There is simple python multiprocessing code that works like a charm, when I run it in console:

# mp.py
import multiprocessing as mp


def do_smth():
    print('something')


if __name__ == '__main__':
    ctx = mp.get_context("spawn")
    p = ctx.Process(target=do_smth, args=tuple())
    p.start()
    p.join()

Result:

> $ python3 mp.py
something

Then I've created a simple Docker container with Dockerfile:

FROM python:3.6

ADD . /app
WORKDIR /app

And docker-compose.yml:

version: '3.6'

services:
  bug:
    build:
      context: .
    environment:
      - PYTHONUNBUFFERED=1
    command: su -c "python3.6 forever.py"

Where forever.py is:

from time import sleep

if __name__ == '__main__':
    i = 0
    while True:
        sleep(1.0)
        i += 1
        print(f'hello {i:3}')

Now I run forever.py with docker compose:

> $ docker-compose build && docker-compose up 
...
some output
...
Attaching to mpbug_bug_1
bug_1  | hello   1
bug_1  | hello   2
bug_1  | hello   3
bug_1  | hello   4

Up to this moment everything is good and understandable. But when I'm trying to run mp.py in the docker container it crashes without any message:

> $ docker exec -it mpbug_bug_1 /bin/bash
root@09779ec47f9d:/app# python mp.py 
something
root@09779ec47f9d:/app# % 

Gist with the code can be found here: https://gist.github.com/ilalex/83649bf21ef50cb74a2df5db01686f18

Can you explain why docker container is crashed and how to do it without crashing?

Thank you in advance!

Answer 1

for a quick fix, do not use spawn start method, and/or do not use su -c ... , both are unnecessary IMO. change to:

p = mp.Process(target=do_smth, args=tuple())

or you could start container with --init option.

with spawn start method, Python will also start a semaphore tracker process to prevent semaphore leaking, you could see this process by pausing mp.py in the middle, it looks like:

472   463 /usr/local/bin/python3 -c from multiprocessing.semaphore_tracker import main;main(3)

this process is started by mp.py but exited after mp.py , thus it will not be reaped by mp.py , but is supposed to be reaped by init by design.

the problem is there is no init in this container(namespace), instead of init , PID 1 is su -c , therefore the dead semaphore tracker process is adopted by su .

it appears that su consider the dead child process is the command process( forever.py ) mistakenly, without checking the relationship, so su exit blindly, as PID 1 exit, kernel kills all other processes in the container, including forever.py .

this behavior could be observed with strace :

docker run --security-opt seccomp:unconfined --rm -it ex_bug strace -e trace=process -f su -c 'python3 forever.py'

will output error message like:

strace: Exit of unknown pid 14 ignored

ref: Docker and the PID 1 zombie reaping problem (phusion.nl)

Answer 2

mp.py doesn't look like an equivalent of forever.py . mp.py will run new worker process, which will just print something and then it will exit => join() in the main process will exit immediately, when this worker process is done.

Better equivalent of forever.py : worker process prints hello message in the infinite loop and main process will be waiting for this worker process exit in join() - forever-mp.py :

import multiprocessing as mp
from time import sleep

def do_smth():
    i = 0
    while True:
        sleep(1.0)
        i += 1
        print(f'hello {i:3}')

if __name__ == '__main__':
    ctx = mp.get_context("spawn")
    p = ctx.Process(target=do_smth, args=tuple())
    p.start()
    p.join()

Updated docker-compose.yml :

version: '3.6'

services:
  bug:
    build:
      context: .
    environment:
      - PYTHONUNBUFFERED=1
    command: su -c "python3.6 forever-mp.py"

Test:

$ docker-compose build && docker-compose up 
...
some output
...
Attaching to multiprcs_bug_1_72681117a752
bug_1_72681117a752 | hello   1
bug_1_72681117a752 | hello   2
bug_1_72681117a752 | hello   3
bug_1_72681117a752 | hello   4

Check processes in the container:

$ docker top multiprcs_bug_1_72681117a752
UID                 PID                 PPID                C                   STIME               TTY                 TIME                CMD
root                38235               38217               0                   21:36               ?                   00:00:00            su -c python3.6 forever-mp.py
root                38297               38235               0                   21:36               ?                   00:00:00            python3.6 forever-mp.py
root                38300               38297               0                   21:36               ?                   00:00:00            /usr/local/bin/python3.6 -c from multiprocessing.semaphore_tracker import main;main(3)
root                38301               38297               0                   21:36               ?                   00:00:00            /usr/local/bin/python3.6 -c from multiprocessing.spawn import spawn_main; spawn_main(tracker_fd=4, pipe_handle=6) --multiprocessing-fork