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

[Etienne Doms]

Ok, that makes a lot of sense, I was confusing timedated and
timesyncd, and indeed the latter is what would be able to broadcast
the event, even if it does not yet.

I wasn't aware of /run/systemd/timesync/synchronized, looks fine to
me, will investigate that.

Thanks!

Le mer. 17 août 2022 à 15:17, Mantas Mikulėnas <grawity at gmail.com> a écrit :
>
> 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