Container init process¶

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:
    

    CONTAINER ID  IMAGE        COMMAND       CREATED             STATUS                      PORTS  NAMES
    b755c0f1b1d8  gentoo-base  "/sbin/init"  About a minute ago  Exited (137) 9 seconds ago         gentoo-base
    

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

    # docker-compose top
    gentoo-base
    UID    PID    PPID   C   STIME   TTY     TIME       CMD
    ----------------------------------------------------------
    root   3510   3489   0   17:40   ?     00:00:00   init [0]
    

Solution¶

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

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

--- 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);

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

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

Container's docker-compose.yml:

services:
  gentoo-base:
    ...
    stop_signal: SIGINT

/etc/inittab inside container - replace reboot action with shutdown:

# What to do at the "Three Finger Salute".
ca:12345:ctrlaltdel:/sbin/shutdown -h now