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cryptogopher, 2022-04-28 22:56
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.
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:CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES b755c0f1b1d8 gentoo-base "/sbin/init" About a minute ago Exited (137) 9 seconds ago gentoo-base
init
process remains running afterwards:# 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
Updated by cryptogopher over 2 years ago · 9 revisions