[systemd-devel] org.freedesktop.timedate1.NTPSynchronized not signaled: rationale?

[Mantas Mikulėnas]

On Wed, Aug 17, 2022 at 1:59 PM Etienne Doms <etienne.doms at gmail.com> wrote:

> Hi,
>
> I'm developing an application for an embedded system that needs to
> wait for proper NTP synchronization. systemd-timesyncd is running and
> I can read NTPSynchronized from /org/freedesktop/timedate1 using
> D-Bus. I read in the manual that this property is not signaled, and
> that I need to do some weird magic with timerfd's
> TFD_TIMER_CANCEL_ON_SET flag.
>
> It works, but having the ECANCELLED on the read() means that something
> somewhere did clock_settime(CLOCK_REALTIME, <...>), not especially
> that I got a proper NTP synchronization. Then, I still need to query
> NTPSynchronized after, and retry the timerfd thing if it didn't switch
> to "true", which is still some kind of polling (but very unlikely,
> sure).
>
> As a result, I'm a bit curious, what was the rationale of not simply
> signaling NTPSynchronized?
>

timedated itself doesn't have knowledge of that event, because it isn't the
daemon that performs actual synchronization (that's timesyncd), so all that
the D-Bus property does is report you the status of adjtimex() –
specifically it returns whether ".maxerror < 16000000". Timedated would
still need to poll and/or do timerfd tricks in order to see that state
being reached. (Currently timedated is not a continuously running daemon –
it starts up only whenever properties are queried and exits when idle.)

A better question is why the timesyncd daemon does not have such a D-Bus
signal; looks like it *almost* does
(org.freedesktop.timesync1.Manager.NTPMessage) but it looks like it only
emits the raw messages and not whether they resulted in a successful sync.

For now, if you're using timesyncd you can use inotify to watch
/run/systemd/timesync/synchronized, which is touched after a sync.

-- 
Mantas Mikulėnas
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