Gracefully Stopping Docker Containers

Question

I'm having some trouble to understand how I can do some cleanup when the container is stopped.

To make it easier, I prepared a sample to reproduce the problem.

Here are the contents of my files:

Dockerfile

FROM opensuse:latest

# Install tcsh (non-interactive mode)
RUN zypper -n in tcsh

# Create user
RUN useradd -ms /bin/tcsh dummyuser

# Set the user
USER dummyuser

# Change Working Dir
WORKDIR /home/dummyuser

# Copy entrypoint script
COPY docker-entrypoint.sh $HOME

# Starter Script
ENTRYPOINT ["./docker-entrypoint.sh"]

docker-entrypoint.sh

#!/bin/tcsh

echo "Starting"

onintr cleanup

# Running something in foreground, otherwise the container will stop
while (1)
   sleep 1000
end
exit 0

cleanup:
   onintr -
   echo "cleanup on going"
   exit 0

Make docker-entrypoint.sh executable:

chmod 744 docker-entrypoint.sh

Build the image:

docker build -t my-dummy-img .

Notice that I'm using tcsh shell.

If you take a look at the docker-entrypoint.sh you can see that I'm waiting to cath the interrupt (onintr cleanup) and call a cleanup method.

Now, these are the commands I run:

mstack/dummy-project> docker run --name my-service -ti -d my-dummy-img ps -eaf
da1dc21281a58e384f2ff34aa49a82019214e204e6d7a77ff54e8c96e005f913
mstack/dummy-project> docker logs my-service
Starting
mstack/dummy-project> docker stop my-service
my-service
mstack/dummy-project> docker logs my-service
Starting
mstack/dummy-project>

Here is the problem, I would expect that after the second docker logs my-service the output would be:

Starting
cleanup on going

Instead of only

Starting

Because docker is supposed to send a signal when stopping...

On the other hand, if I run:

docker run --name my-service-attached -ti my-dummy-img ps -eaf

And hit CTRL+C, I can see the expected output.

What am I missing here? I hope the question is clear enough.

BTW, I used the following to articles as guideline:

Gracefully Stopping Docker Containers

Trapping signals in Docker containers

Answer 1

Finally solved the problem.

Tcsh shell doesn't receive most of the signals like SIGTERM which is the signal sent by docker when stopping the container.

So I changed the script to use bash shell and whenever I want to run a tcsh command I just do it like this:

/bin/tcsh ./my-command

So, my docker-entrypoint.sh is like this:

#!/bin/bash

# SIGTERM-handler this funciton will be executed when the container receives the SIGTERM signal (when stopping)
term_handler(){
   echo "***Stopping"
   /bin/tcsh ./my-cleanup-command
   exit 0
}

# Setup signal handlers
trap 'term_handler' SIGTERM

echo "***Starting"
/bin/tcsh ./my-command

# Running something in foreground, otherwise the container will stop
while true
do
   #sleep 1000 - Doesn't work with sleep. Not sure why.
   tail -f /dev/null & wait ${!}
done