<div class="gmail_quote"><div dir="ltr">On Tue, Mar 20, 2018, 21:09 Colin Walters <<a href="mailto:walters@verbum.org">walters@verbum.org</a>> wrote: </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">I'm working on: <a href="https://github.com/ostreedev/ostree/issues/545" rel="noreferrer" target="_blank">https://github.com/ostreedev/ostree/issues/545</a> TL;DR: libostree is an image-like update system, and I want to take some actions at system shutdown time, specifically performing a "snapshot+merge" of /etc after most other services are shut down. The way ostree handles /etc (IMO a lot more correctly) is a major distinguishing point from a lot of other image-like systems out there. What I have so far is this (distilled into a simple easily tested unit not tied to ostree): [Unit] Description=cgwalters-shutdown DefaultDependencies=no After=shutdown.target Before=final.target # FIXME # Before=sysroot.mount [Service] Type=oneshot RemainAfterExit=yes ExecStart=/bin/sh -c 'sleep 5 && ls -ald /sysroot/ostree/repo' [Install] WantedBy=final.target I modeled this after `halt-local.service`. But as you might infer from the FIXME, the problem I'm hitting is that the /sysroot mount is unmounted before this service is started. I think the problem I'm hitting is this bit from `man systemd.unit`: > Given two units with any ordering dependency between them, if one unit is shut down and the other is started up, the shutdown is ordered before the start-up. Which does sort of make sense to me in general; but is problematic for this "start service on shutdown" case. I was looking a bit at the way plymouth hooks into shutdown, but it seemed complex, and as plymouth doesn't (AFAIK) have any real dependencies on the filesystem state I'm not sure it matches what I need. I can think of ways to hack around this, by e.g. having the service start up on shutdown, take a snapshot of a private mount namespace, have it be excluded from the Conflicts=shutdown.target, then have a unit that triggers before final.target (like the above) that sends an IPC to actually do what I want. But that adds a notable amount of complexity, and I'd like to have this process feel as obvious/transparent as possible. Another way I've thought about handling this is to basically invert things so that we have a "stub" unit that starts on bootup, and its ExecStop does the real work: [Unit] Description=cgwalters-startstop ConditionPathExists=/run/ostree-booted DefaultDependencies=no RequiresMountsFor=/sysroot After=basic.target Before=multi-user.target Conflicts=final.target Before=final.target [Service] Type=oneshot RemainAfterExit=yes ExecStart=/usr/bin/true ExecStop=/usr/bin/sh -c 'sleep 5 && ls -ald /ostree/repo' [Install] WantedBy=multi-user.target Which seems to work but feels...a bit ugly? </blockquote></div><div> </div><div>Currently this is the recommended method; it's a little ugly indeed but fits quite well into the shutdown process. Recent systemd versions should let you skip the dummy ExecStart.</div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> </blockquote></div>-- <div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr">Mantas Mikulėnas <<a href="mailto:grawity@gmail.com">grawity@gmail.com</a>> Sent from my phone </div></div>