How to restart Python Docker Container from inside

Question

My Objective: I want to be able to restart a container based on the official Python Image using some command inside the container.

My system: I have a own Docker image based on the official python image which look like this:

FROM python:3.6.15-buster
WORKDIR /webserver
COPY requirements.txt /webserver
RUN /usr/local/bin/python -m pip install --upgrade pip
RUN pip3 install -r requirements.txt --no-binary :all:
COPY . /webserver
ENTRYPOINT ["./start.sh"]

As you can see, the image does not execute a single python file but it executes a script called start.sh, which looks like this:

#!/bin/bash

echo "Starting"
echo "Env: $ENTORNO"

exec python3 "$PATH_ENTORNO""Script1.py" &
exec python3 "$PATH_ENTORNO""Script2.py" &
exec python3 "$PATH_ENTORNO""Script3.py" &

All of this works perfectly, but, I want that if, for example, script 3 fails, the entire container based on this image get restarted.

My approach: I had two ideas about this problem. First, try to execute a reboot command in the python3 script, something like this:

from subprocess import call

[...]

call(["reboot"])

This does not work inside the Python Debian image, because of error:

reboot: command not found

The other approach was to mount the docker.sock inside the container, but the error this time is:

root@MachineName:/var/run# /var/run/docker.sock docker ps
bash: /var/run/docker.sock: Permission denied

I dont know if I'm doing right these two approach, or if anyone has any idea about this but any help will be very appreciated.

Answer 1

Well, in the end the solution was much simpler than I expected.

I started from the base where I mount the docker socket inside the container (I know that this practice is not recommended, but in my case, I know that it does not pose security problems), using the command in docker-compose:

volumes:
  - /var/run/docker.sock:/var/run/docker.sock

Then, it was as simple as using the Docker library for python, which gives a complete SDK through that socket that allowed me to restart the container inside the python script in an ultra-simple way.

import docker

[...]

docker_client = docker.DockerClient(base_url='unix://var/run/docker.sock')
docker_client.containers.get("container_name").restart()

Answer 2

Update

After thinking about it, I realised you could send some signal to the PID 1 (your entrypoint), trap it and use a handler to exit with an appropriate code so that docker will reschedule it.

Here's an MRE:

Dockerfile

FROM python:3.9
WORKDIR /app
COPY ./ /app
ENTRYPOINT ["./start.sh"]

start.sh

#!/usr/bin/env bash
python script.py &

# This traps user defined signal and kills the last command
# (`tail -f /dev/null`) before exiting with code 1.
trap 'kill ${!}; echo "Killed by backgrounded process"; exit 1' USR1

# Launches `tail` in the background and sets this program to wait
# for it to finish, so that it does not block execution
tail -f /dev/null & wait $!

script.py

import os
import signal

# Process 1 will be your entrypoint if you declared it in `exec-form`*
print("Sending signal to stop container")
os.kill(1, signal.SIGUSR1)

*exec form

Testing it

> docker build . -t test
> docker run test
Sending signal to stop container
Killed by backgrounded process
> docker inspect $(docker container ls -n 1 -q) --format='{{.State.ExitCode}}'
1

Original post

I think the safest bet would be to instruct docker to restart your container when there's some failure. Then you'd only have to exit your program with a non-zero code (i.e: run exit 1 from your start.sh) and docker will restart it from scratch.

Option 1: docker run --restart

Related documentation

docker run --restart on-failure <image>

Option 2: Using docker-compose

Version 3

In your docker-compose.yml you can set the restart_policy directive to the service you're interested on restarting. i.e:

version: "3"
services:
  app:
    ...
    restart_policy:
      condition: on-failure
    ...

Version 2

Before version 3, the same policy could be applied with the restart directive, which allows for less configuration.

version: "2"
services:
  app:
    ...
    restart: "on-failure"
    ...