[chrony-dev] Chrony and leap-second table expiration

[Paul Eggert <eggert@xxxxxxxxxxx>]

The TZDB mailing list has recently been wrestling with the topic of 
leap-second table expiration (see [1] and [2] for some of this).

Here's the issue. The leap-seconds.list file, maintained by Judah Levine 
of NIST and redistributed by TZDB, contains a line like this:

#@	3928521600

which in this example specifies an expiration date of 2024-06-28 00:00 
UTC. The idea is that this particular leap-seconds.list file should not 
be used after the expiration date, after which the file will be obsolete 
and pehaps incorrect.

As I understand it, NTPsec parses this comment and rejects obsolete 
leap-seconds.list files; this is the traditional ntpd behavior.

Currently Chrony does not look at the leap-seconds.list file. However, 
yesterday Patrick Oppenlander proposed[3] a patch to do that.

So, three questions:

* If Chrony reads leap-seconds.list should it also look at the leap 
second expiration and reject old files?

* If Chrony does not read leap-seconds.list, but instead uses the 
right/UTC file, should it also look at any leap-second expiration 
information in that file and reject old files? This would involve Chrony 
looking at the file directly instead of relying on localtime/mktime, and 
building right/UTC with TZDB's EXPIRES_LINE=1 option.

* If leap seconds are discontinued, which is a distinct possibility, 
what sort of "leap second expiration" should Chrony use?

[1]: https://mm.icann.org/pipermail/tz/2023-November/033268.html
[2]: https://mm.icann.org/pipermail/tz/2023-November/033275.html
[3]: 
https://listengine.tuxfamily.org/chrony.tuxfamily.org/chrony-dev/2023/11/msg00013.html

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