[systemd-devel] "semaphore" for .timer-triggered run

[Jérémy ROSEN]

Out of curiosity, and to check if I understand the systemd concepts
properly...

assuming the two units to lock are both systemd services and are both of
type oneshot, would a simple After= of one on the other be enough to
guarantee they never run together ?

Le mar. 27 août 2019 à 12:56, Lennart Poettering <lennart at poettering.net> a
écrit :

> On Fr, 02.08.19 13:24, Johannes Ernst (johannes.ernst at gmail.com) wrote:
>
> > I have a oneshot .service (certbot) that is run by its .timer with:
> >
> > OnCalendar=daily
> > RandomizedDelaySec=1day
> >
> > I also have a sometimes long-running script that may modify the same
> > data. So the script and the oneshot service should never run at the
> > same time.
> >
> > Is there a good systemd pattern for some kind of semaphore-like
> > thing that will delay the execution of the oneshot service and try
> > again when the script is done running?
> >
> > I could skip execution, but then I miss an entire day until the
> > service is run again -- I’d prefer it if it tried again a few
> > minutes later. Or I could poll for some flag before running the main
> > part of the service. Wondering whether there something more
> > systemd-native.
>
> Maybe change the service program invoked by the timer unit to use file
> locking of some kind early on. I mean, if you have shared data on disk
> and want to synchronize access to it then a file lock is the concept
> of choice really. If this is a proper program (i.e. not a shell
> script) maybe just use a BSD file lock (flock(2)) on the top-level
> directory you store your data in. If these are shell scripts you
> should be able to use util-linux' flock tool, you could even prefix
> your ExecStart= line with that if you want.
>
> But no, systemd does not provide any mechanism to do file locking for
> you.
>
> Lennart
>
> --
> Lennart Poettering, Berlin
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