Re: [chrony-dev] "leapsectz" and leapsecond announce (was:refclock: Add a new "tai" option)

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Thu, Oct 12, 2017 at 03:12:19PM +0200, FUSTE Emmanuel wrote:
> Le 12/10/2017 à 13:21, Miroslav Lichvar a écrit :
> > The TAI-UTC offset of the system clock will be wrong if the timezone
> > is missing a (genuine) leap second. If it is a fake leap second, the
> > clock will be twice (wrongly) corrected by one second.
> >
> So in the presence of a refclock with the "tai" option and after a leap 
> not present in the tz data but accepted because of a majority of other 
> sources /servers have announced it, the TAI-UTC offset will be wrong.
> So what will be done with the refclock ?
> - de-selected / ignored ?

The refclock will be off by one seconds. If there are at least two
good sources, it will be marked as a falseticker and ignored.

> - as it has a lower stratum, it could take precedence over the other ntp 
> sources and system time and served time will be re-offset-ed by 1s ?
> Or (after re-reading your answer and what I write) the TAI-UTC offset 
> was run-time adjusted and is not wrong.

The offset will be wrong. There is no adjustment of the TAI-UTC offset
which is used for correcting refclock offset. The local TAI-UTC offset
is adjusted after a leap second, but it will be overwritten on the
next clock update with the incorrect value from tzdata.

> - and after a restart of chrony (leap happened event lost) ?  => real 
> case of wrong TAI-UTC offset

Restart shouldn't make a difference.

> A complete solution could be to save the "not present in the leapsectz 
> zone" leap event in a journal and increment/decrement the TAI-UTC offset.
> At startup, or at next runtime check
> - clear the journal if the event was added in the leapsectz zone by an 
> update to the local TZ database and reset TAI-UTC offset with the one 
> given by leapsectz zone
> - or replay the event on top of the offset given by leapsectz zone to 
> get the right TAI-UTC offset

What if a fake leap second was accepted? This sounds too complicated
and fragile to me.

If using leapsectz and refclock with tai option, I think the
requirement should be to keep tzdata up to date.

-- 
Miroslav Lichvar

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