perl-init

Solve Signal Handling in Perl Containers with tini

We should be able to containerize this simple Perl application with a minimal Docker file.

FROM perl

ADD app.pl /

ENTRYPOINT ["perl", "app.pl"]

Lets try it out:

$ docker build .
[...]
Successfully built e5b19c53b724

$ docker run --rm -it e5b19c53b724
Gonna sleep forever...
^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C^C

Looks good! But why doesn't CTRL-C kill my application?

Docker creates a new PID namespace for each container. The first process in the container has PID 1.

Note: A process running as PID 1 inside a container is treated specially by Linux: it ignores any signal with the default action. So, the process will not terminate on SIGINT or SIGTERM unless it is coded to do so.

• Docker Docs

A good cross-platform solution is running a tiny init system in front of the app. tini is a good candidate and even used by Docker itself. Let's use it:

 FROM perl
 
+RUN apt-get update -q && apt-get install tini
+
 ADD app.pl /
 
-ENTRYPOINT ["perl", "app.pl"]
+ENTRYPOINT ["tini" , "--", "perl", "app.pl"]

Lets try it out:

$ docker build .
[...]
Successfully built 93b6ca67c66a

$ docker run --rm -it 93b6ca67c66a
Gonna sleep forever...
^C

It works 🥳.