* lib/mktime.c (__mktime_internal): When tm_isdst disagrees with what was requested, search at most a year (+ stride) from the requested time for a matching tm_isdst, and ignore the request if the search fails. This is more likely to match user expectations in timezones like Asia/Kolkata. Problem reported by Florian Weimer in: https://sourceware.org/pipermail/libc-alpha/2025-January/163342.html --- ChangeLog | 9 +++++++++ lib/mktime.c | 38 ++++++++++++-------------------------- 2 files changed, 21 insertions(+), 26 deletions(-)
diff --git a/ChangeLog b/ChangeLog index b3c6e48e6d..629f1b8d3f 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,5 +1,14 @@ 2025-01-04 Paul Eggert <egg...@cs.ucla.edu> + mktime: improve tm_isdst heuristic + * lib/mktime.c (__mktime_internal): When tm_isdst disagrees with + what was requested, search at most a year (+ stride) from the + requested time for a matching tm_isdst, and ignore the request if + the search fails. This is more likely to match user expectations + in timezones like Asia/Kolkata. + Problem reported by Florian Weimer in: + https://sourceware.org/pipermail/libc-alpha/2025-January/163342.html + mktime: support glibc locking This is part of my attempt to merge glibc and gnulib mktime.c. It should not affect Gnulib-using code. diff --git a/lib/mktime.c b/lib/mktime.c index 99f014c6bd..4448a92c06 100644 --- a/lib/mktime.c +++ b/lib/mktime.c @@ -355,9 +355,7 @@ __mktime_internal (struct tm *tp, bool local, mktime_offset_t *offset) int mday = tp->tm_mday; int mon = tp->tm_mon; int year_requested = tp->tm_year; - - /* Ignore any tm_isdst request for timegm. */ - int isdst = local ? tp->tm_isdst : 0; + int isdst = tp->tm_isdst; /* True if the previous probe was DST. */ bool dst2 = false; @@ -449,13 +447,10 @@ __mktime_internal (struct tm *tp, bool local, mktime_offset_t *offset) Heuristic: probe the adjacent timestamps in both directions, looking for the desired isdst. If none is found within a - reasonable duration bound, assume a one-hour DST difference. + reasonable duration bound, ignore the disagreement. 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 (e.g., America/Recife starting 2000-10-08 01:00), and the @@ -465,21 +460,17 @@ __mktime_internal (struct tm *tp, bool local, mktime_offset_t *offset) periods when probing. */ int stride = 601200; - /* 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. */ - int delta_bound = duration_max / 2 + stride; + /* Do not probe too far away from the requested time, + by striding until at least a year has passed, but then giving up. + This helps avoid unexpected results in (for example) Asia/Kolkata, + for which today's users expect to see no DST even though it + did observe DST long ago. */ + int year_seconds_bound = 366 * 24 * 60 * 60 + 1; + int delta_bound = year_seconds_bound + stride; int delta, direction; + /* Search in both directions, closest first. */ for (delta = stride; delta < delta_bound; delta += stride) for (direction = -1; direction <= 1; direction += 2) { @@ -509,13 +500,8 @@ __mktime_internal (struct tm *tp, bool local, mktime_offset_t *offset) } } - /* No unusual DST offset was found nearby. Assume one-hour DST. */ - t += 60 * 60 * dst_difference; - if (mktime_min <= t && t <= mktime_max && __tz_convert (t, local, &tm)) - goto offset_found; - - __set_errno (EOVERFLOW); - return -1; + /* No probe with the requested tm_isdst was found nearby. + Ignore the requested tm_isdst. */ } offset_found: -- 2.45.2
