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 system 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 container 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>
@init@ process remains running afterwards:
<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>