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mktime: improve heuristic for ca-1986 Indiana DST 

This patch syncs mktime.c from Gnulib, fixing a
problem reported by Mark Krenz <https://bugs.gnu.org/48085>,
and it should fix BZ#29035 too.
* time/mktime.c (__mktime_internal): Be more generous about
accepting arguments with the wrong value of tm_isdst, by falling
back to a one-hour DST difference if we find no nearby DST that is
unusual.  This fixes a problem where "1986-04-28 00:00 EDT" was
rejected when TZ="America/Indianapolis" because the nearest DST
timestamp occurred in 1970, a temporal distance too great for the
old heuristic.  This also also narrows the search a bit, which
is a minor performance win.
This commit is contained in:
Paul Eggert 2022-09-08 20:08:32 -05:00
parent 82a1ec8510
commit 83859e1115
1 changed files with 20 additions and 8 deletions
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28
time/mktime.c
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@ -429,8 +429,13 @@ __mktime_internal (struct tm *tp,
time with the right value, and use its UTC offset.
Heuristic: probe the adjacent timestamps in both directions,
looking for the desired isdst. This should work for all real
time zone histories in the tz database. */
looking for the desired isdst. If none is found within a
reasonable duration bound, assume a one-hour DST difference.
This should work for all real time zone histories in the tz
database. */
/* +1 if we wanted standard time but got DST, -1 if the reverse. */
int dst_difference = (isdst == 0) - (tm.tm_isdst == 0);
/* Distance between probes when looking for a DST boundary. In
tzdata2003a, the shortest period of DST is 601200 seconds
@ -441,12 +446,14 @@ __mktime_internal (struct tm *tp,
periods when probing. */
int stride = 601200;
/* The longest period of DST in tzdata2003a is 536454000 seconds
(e.g., America/Jujuy starting 1946-10-01 01:00). The longest
period of non-DST is much longer, but it makes no real sense
to search for more than a year of non-DST, so use the DST
max. */
int duration_max = 536454000;
/* In TZDB 2021e, the longest period of DST (or of non-DST), in
which the DST (or adjacent DST) difference is not one hour,
is 457243209 seconds: e.g., America/Cambridge_Bay with leap
seconds, starting 1965-10-31 00:00 in a switch from
double-daylight time (-05) to standard time (-07), and
continuing to 1980-04-27 02:00 in a switch from standard time
(-07) to daylight time (-06). */
int duration_max = 457243209;
/* Search in both directions, so the maximum distance is half
the duration; add the stride to avoid off-by-1 problems. */
@ -483,6 +490,11 @@ __mktime_internal (struct tm *tp,
}
}
/* No unusual DST offset was found nearby. Assume one-hour DST. */
t += 60 * 60 * dst_difference;
if (mktime_min <= t && t <= mktime_max && convert_time (convert, t, &tm))
goto offset_found;
__set_errno (EOVERFLOW);
return -1;
}