Infrastructure

h1. Container @init@ process

h2. Problem

To enable running multiple processes, containers require process/service management. This is normally provided by some kind of @init@ task (e.g. from @sysvinit@).

There are Docker-compatible replacements for full-fledged @init@'s. Unfortunately they require either custom init scripts or service configurations (https://wiki.gentoo.org/wiki/Comparison_of_init_systems). The process of migration from OS-provided OpenRC init scripts is time consuming and error prone.

Usage of system's default @sysvinit@ is hampered by following shortcomings:

* it mostly does not respond to Unix signals, which are used by Docker to manage containers (most importantly: signal termination),

* it does not stop properly on when Docker requests container to stop

** attempt to stop container with @init@ as PID 1 ends with error code 137:

<pre>

CONTAINER ID  IMAGE        COMMAND       CREATED             STATUS                      PORTS  NAMES

b755c0f1b1d8  gentoo-base  "/sbin/init"  About a minute ago  Exited (137) 9 seconds ago         gentoo-base

</pre>

** when invoking @shutdown@ from within container, @init@ process remains running afterwards, keeping container in running state:

<pre>

# docker-compose top

gentoo-base

UID    PID    PPID   C   STIME   TTY     TIME       CMD

----------------------------------------------------------

root   3510   3489   0   17:40   ?     00:00:00   init [0]

</pre>

h2. Solution

Nevertheless it is possible to use @sysvinit@ inside Docker container as an @init@ process. Required steps are following:

h3. 1. Change @sysvinit@ to exit @init@ process on hard shutdown (runlevel 0) with following patch

<pre><code class="diff">

--- sysvinit-3.01/src/init.c    2021-12-13 20:21:26.000000000 +0100

+++ sysvinit-3.01/src/init.c    2022-04-18 01:21:47.966751774 +0200

@@ -2367,6 +2367,11 @@

      read_inittab();

      fail_cancel();

      setproctitle("init [%c]", (int)runlevel);

+

+      /*

+       * Exit on halt - causes Docker container to stop.

+       */

+      if (runlevel == '0') exit(0);

    }

  }

        Write_Runlevel_Log(runlevel);

</code></pre>

On Gentoo it's enough to put this patch inside _/etc/portage/patches/sys-apps/sysvinit/exit-on-halt.patch_ and reemerge @sysvinit@.

h3. 2. Change Docker signal for container termination to SIGINT and set appropriate action in @inittab@

Container's _docker-compose.yml_:

<pre>

services:

  gentoo-base:

    ...

    stop_signal: SIGINT

</pre>

_/etc/inittab_ inside container - replace reboot action with shutdown:

<pre>

# What to do at the "Three Finger Salute".

ca:12345:ctrlaltdel:/sbin/shutdown -h now

</pre>