[systemd-devel] Service reactivation with device plugging

[Rob Spanton]

Hi,

I was recently working on an embedded system that had a daemon that
depended on the presence of two USB serial devices.  The daemon needed
to be run when both devices were plugged in.  If any of the devices was
unplugged, the daemon would exit, and would need to be restarted the
next time both devices were plugged in.

I created a service file pretty-much like the one at the end of this
email.  This kind-of did what I wanted for a short while.  However, if
the daemon was running, and then I unplugged the serial devices, systemd
seemed to attempt to continuously restart the daemon despite the serial
devices not being present.  Furthermore, it seemed to give up doing this
after some period of time (not sure exactly how long, but somewhere
around minute or two).  Once it had given up restarting the daemon, the
daemon was not restarted when I plugged the devices back in again.

In the end, due to looming deadlines, I added a cron job that just ran
`systemctl start mydaemon.service` every minute to ensure that systemd
didn't give up on reactivating the service.

Am I missing something here?  Is this a valid application of systemd?

Oh, and this is with systemd-44 (Fedora systemd-44-17.fc17.i686).

Cheers,

Rob

---

[Unit]
Description=...
Requires=dev-serial-a.device
Requires=dev-serial-b.device

After=dev-serial-a.device
After=dev-serial-b.device

[Service]
ExecStart=/usr/bin/my-magic-service
Type=notify
NotifyAccess=main

TimeoutSet=5s
Restart=always
RestartSec=1s
WatchdogSec=1500ms
StartLimitInterval=0

[Install]
WantedBy=multi-user.target
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