[systemd-devel] Help required for configuring a blocking service during shutdown

[Mantas Mikulėnas]

On Mon, Aug 29, 2022 at 1:31 PM Henning Moll <newsScott at gmx.de> wrote:

> Hi,
>
> back in the rcX days I configured a backup system which blocks a system
> shutdown in a certain state (network and mounts still active) if a backup
> is still running. The init script looks like this:
>
> ...
> case "$1" in
> ...
>
> stop)
>
>     plymouth --ping
>     if [ $? -ne 0 ]; then
>       log_daemon_msg "waiting for running backup" "backup"
>     else
>       plymouth message --text="waiting for running backup"
>     fi
>     sleep 10
>     logger -t backup "rcS: trying to get lock..."
>     exec {FD}<>"$LOCKFILE"
>     {
>       logger -t backup "locked rsync_wrapper: waiting for lock"
>       flock ${FD}
>     }
>     logger -t backup "rcS: continue shutdown..."
>     plymouth message --text=""
>
> ...
> esac
> ...
>
> The strategy is to wait for a successful lock on a shared LOCKFILE.
>
> Now, I want to solve this with systemd. I've read tons of documentation
> but I don't get it, all my experiments failed. I am using Linux Mint 21
> "Vanessa", which is based on Ubunut 22.04.
>
> I am looking for a solution which
>
> * "pauses" a shutdown or reboot attempt as long as the LOCK cannot be
> obtained (which means the backup is still running)
> * while the network connection (ethernet) is still active
> * and all mounts (local, usb and cifs) are still active
> * openssh-server still running
> * The logging functionality (plymouth / log_daemon) would be nice. so the
> system should be allowed to shutdown to a stage where plymouth is already
> visible
>
> Can you please help me?
>
> Best regards
> Henning
>

Probably similar to what you already have – create a service that starts on
boot (doing nothing) and delays the *stop* action. (You'll probably need to
start with [Service] Type=oneshot, RemainAfterExit=yes, TimeoutStopSec=1d.)
If you list After=ssh.service as a dependency, then your service will be
started after OpenSSH and *stopped before OpenSSH*, same with
NetworkManager. (I'm not sure if there's a dependency that can be used to
target manually done SMB mounts, although remote-fs.target might work for
the ones in fstab?)

Not sure how it works with Plymouth, but in its default console output,
systemd itself will already show which services are waiting to be stopped.
Syslog (journald) should be available the entire time.

The backup system should also run under `systemd-inhibit --what=shutdown`
as an additional layer of precaution – that way the user will know that
something is still running *before* they initiate the actual shutdown.

-- 
Mantas Mikulėnas
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/systemd-devel/attachments/20220829/f2294c62/attachment.htm>