Orphan Process Handling in Docker

child.py

"""A child process which receives a SIGTERM then waits 5 seconds to terminate"""
import sys, signal, time, os


def graceful_exit(_signum, _frame):
    print(f"Child {os.getpid()}[ppid={os.getppid()}] received SIGTERM")


def run():
    print(f"Hello from child {os.getpid()}")
    signal.signal(signal.SIGTERM, graceful_exit)

    while True:
        time.sleep(0.5)

Dockerfile

FROM python:3.9-alpine

COPY parent.py .
COPY child.py .

ENTRYPOINT ["python", "parent.py"]

parent.py

"""Misbehaving parent process which does not wait for its children, do not use anywhere in production!"""
import sys, signal, os
import time
from multiprocessing import Process

from child import run


processes = []


def graceful_exit(_signum, _frame):
    print("Gracefully shutting down")
    for p in processes:
        print(f"Shutting down process {p.pid}")
        p.terminate()
    sys.exit(0)


if __name__ == "__main__":
    for _ in range(1):
        proc = Process(target=run)
        proc.start()
        processes.append(proc)
    print(f"Parent process started: {os.getpid()}")
    signal.signal(signal.SIGTERM, graceful_exit)
    signal.signal(signal.SIGINT, graceful_exit)

    while True:
        print("Parent still alive!")
        time.sleep(0.5)

term1

# Run these commands in a terminal to build our misbehaving application

$ docker build -t bad-parent .
$ docker run --rm -ti bad-parent
Parent process started: 1
Hello from child 6

term2

# Run these commands in another terminal to inspect our misbehaving application

$ docker top a3cf7b6927c0
UID                 PID                 PPID                ...                 CMD
root                294147              294129              ...                 python parent.py
root                294200              294147              ...                 python parent.py