[tz] Problem with WORK_AROUND_QTBUG_53071

[Bradley White]

Happy leap second.

In 2016i Pacific/Tongatapu changed to:

 (1) use numeric time zone abbreviations, and
 (2) observe DST from the first Sunday in November through the third Sunday
in January.

That is, its POSIX future rule became "<+13>-13<+14>,M11.1.0,M1.3.0/3".

(1) results, under WORK_AROUND_QTBUG_53071, in zic adding a "y2038_boundary
- 1" (i.e., 2038-01-19 03:14:07 +00:00) no-op transition to the end of the
generated transitions.  So, the Pacific/Tongatapu zoneinfo file looks like
...

      :
  2115810000 gmtoff=46800 is_dst=F abbr="+13"
  2140606800 gmtoff=50400 is_dst=T abbr="+14"
  2147483647 gmtoff=50400 is_dst=T abbr="+14"

When loading Pacific/Tongatapu at runtime, (2) results in tzloadbody()
generating additional transitions ...

      :
  2140606800 gmtoff=50400 is_dst=T abbr="+14"
  2147259600 gmtoff=46800 is_dst=F abbr="+13"
  2172661200 gmtoff=50400 is_dst=T abbr="+14"
  2178709200 gmtoff=46800 is_dst=F abbr="+13"
      :

When those generated transitions are patched onto the end of the zic
transitions, we end up with ...

  2115810000 gmtoff=46800 is_dst=F abbr="+13"
  2140606800 gmtoff=50400 is_dst=T abbr="+14"
  2147483647 gmtoff=50400 is_dst=T abbr="+14"
  2172661200 gmtoff=50400 is_dst=T abbr="+14"
  2178709200 gmtoff=46800 is_dst=F abbr="+13"

This is wrong.  The presence of the 2147483647 transition caused us to drop
the 2147259600 transition.  That is, we lost the shift out of DST at
2038-01-17 02:00:00 +13:00 (2038-01-16 13:00:00 +00:00).

One solution would be to trim trailing, no-op transitions from the end of
the zic transitions before adding the rule-generated ones.  The reference
implementation doesn't need them.

Bradley
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