[TUHS] Leap Second

Warner Losh imp at bsdimp.com
Thu Jan 5 09:13:58 AEST 2017


On Wed, Jan 4, 2017 at 6:32 AM, Steffen Nurpmeso <steffen at sdaoden.eu> wrote:
> schily at schily.net (Joerg Schilling) wrote:
>  |Tony Finch <dot at dotat.at> wrote:
>  |> sds <stephen.strowes at gmail.com> wrote:
>  |>> Important question: did anybody have an "exciting" new year because \
>  |>> of a leap
>  |>> second bug?
>  |>
>  |> I've been collecting failure reports on the LEAPSECS list
>  |
>  |https://blog.cloudflare.com/how-and-why-the-leap-second-affected-cloudflare\
>  |-dns/
>  |
>  |"go" seems to have a related bug.
>  |
>  |BTW: The POSIX standard intentionally does not include leap seconds \
>  |in the UNIX
>  |time interface as it seems that this would cause more problems than \
>  |it claims
>  |to fix.
>
> I think it is a problem, or better a gap, a void, with the current
> standard that software has no option to become informed of the
> event of a leap second for one, but further more that CLOCK_TAI is
> not available.

And even if it was, nobody would use it. It's not used in legacy code,
and the subtle differences between the different CLOCK_xxx aren't well
enough documented enough for programmers to get it right. And even if
it were, the issue is a lot more subtle than that. If you use
CLOCK_TAI, then if the system has the proper TAI offset to UTC,
calling things like timegm will produce a time that's 40s different
than the current UTC time if you aren't also running the proper
"right" timezone files, and people will think your code is buggy. But
if you get a UTC time, then you have an ambiguous encoding of the leap
second (though CLOCK_UTC, where implemented, tries to cope with that
by having a denormalized ts_nsec field). It's a big can of warms since
most programmers expect time to be a uniform radix, and UTC transforms
time of day into a non-uniform radix on an unpredictable timetable.

But that's starting to get far afield for the historical unix group...

> I think it would make things easier if software
> which wants just that can get it, e.g., for periodic timer events
> etc.

CLOCK_MONOTONIC already exists for these things, and programmers still
screw it up :(

> This is surely not a healing given that most timestamps etc.
> are based on UTC, but i think the severity of the problems could
> possibly be lowered.  Especially now that multi-hour smears seem
> to become used by big companies it seems to be important to have
> a correct clock available.  This is in fact something i don't
> really understand, at _that_ level that is to say.  If, e.g.,
> Google and Bloomberg both would have stated instead that they
> slew the leap second, then only a single second would have been
> affected, instead of multiple hours.

You can't just slew the one second. It introduces too large of a
frequency error in the time base. ntpd will view it as a large error
and freak out. Programs that want to sleep for 100ms will wind up
sleeping for 200ms instead, which could be a big problem. With the
slew over several hours, programs wind up sleeping for 100.01ms
instead, which is down in the noise of the error you get from a sleep.
Google is trading a small phase error and frequency error against the
real UTC timestamp to maintain a well-defined monotonically increasing
time series with no repeating seconds as its method of coping with
POSIX's deliberate decision to not define what happens over a leap
second, provide no encoding for a leap second and generally specifies
an interface in which it is nearly impossible to get the leap second
pedantically correct. Makes one question whether leap seconds are a
good idea or not, but that's a political discussion for another group.

Warner


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