Before, we would add/subtract the leap second when the system had been
up for an even multiple of days, rather than at the end of the day, as
a leap second is defined (at least wrt ntp). We do this by
calculating the notion of UTC earlier in the loop, and passing that to
get it adjusted. Any adjustments that ntp_update_second makes to this
time are then transferred to boot time. We can't pass it either the
boot time or the uptime because their sum is what determines when a
leap second is needed. This code adds an extra assignment and two
extra compare in the typical case, which is as cheap as I could made
it.
I have confirmed with this code the kernel time does the correct thing
for both positive and negative leap seconds. Since the ntp interface
doesn't allow for +2 or -2, those cases can't be tested (and the folks
in the know here say there will never be a +2s or -2s leap event, but
rather two +1s or -1s leap events).
There will very likely be no leap seconds for a while, given how the
earth is speeding up and slowing down, so there will be plenty of time
for this fix to propigate. UT1-UTC is currently at "about -0.4s" and
decrementing by .1s every 8 months or so. 6 * 8 is 48 months, or 4
years.
-stable has different code, but a similar bug that was introduced
about the time of the last leap second, which is why nobody has
noticed until now.
MFC After: 3 weeks
Reviewed by: phk
"Furthermore, leap seconds must die." -- Cato the Elder
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