From 10f93018f44f1f5a10319657a90e8b9645a6ba79 Mon Sep 17 00:00:00 2001 From: Paul Eggert Date: Tue, 16 Dec 2025 20:09:29 -0800 Subject: [PROPOSED] Omit no-op transitions when Rule+Zone cancel MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Fix zic so that it does not generate a no-op transition for TZDB 2025c’s Asia/Tbilisi at POSIX time 859662000, i.e., at 1997-03-29 19:00 UTC or 1997-03-30 00:00 local time. The no-op transition was generated because there is a Zone line "4:00 1:00 %z 1997 Mar lastSun" followed by "4:00 E-EurAsia %z ..." and the relevant Rule as of 1997 Mar lastSun is "Rule E-EurAsia 1981 max - Mar lastSun 0:00 1:00 -". Because the first Zone line’s UNTIL and the Rule’s time are the same, zic coalesced the two into a no-op transition where the timestamps before and after are both +05 with DST. This change also fixes a bug with the following test: Rule NOS 1970 2037 - Jan 27 23:43:01 2:0 SDST Rule NOS 1970 2037 - Jan 28 0:51:01 0 S Zone OS/NOS +7:0:0 NOS NO%s In this unrealistic test, the fall-back occurs during the spring-forward gap so there should be no TZif transitions. Without the fix zic incorrectly outputs a 2-hour spring-forward transition from 1970-01-27 22:51:01 +07 to 1970-01-28 00:51:01 +09, and then outputs 66 no-op transitions every year until 2037-01-28 00:51:01 +09, and then the non-matching TZ string "NOS-7". This test case was reported by Renchunhui in: https://lists.iana.org/hyperkitty/list/tz@iana.org/message/TEUFHPYKORVAXWQV5FTNAOAQOXEMQDWZ/ * NEWS: Mention this. * zic.c (writezone): When removing an unwanted transition due to a Rule line applying at the same time as a Zone’s UNTIL time, do not turn the preceding transition into a no-op. --- NEWS | 10 ++++++++++ zic.c | 16 ++++++++++------ 2 files changed, 20 insertions(+), 6 deletions(-) diff --git a/NEWS b/NEWS index 70116304..2ae02d59 100644 --- a/NEWS +++ b/NEWS @@ -1,5 +1,15 @@ News for the tz database +Unreleased, experimental changes + + Changes to code + + zic no longer generates a no-op transition when + simultaneous Rule and Zone changes cancel each other out. + This occurs in tzdata only in Asia/Tbilisi on 1997-03-30. + (Thanks to Renchunhui for a test case showing the bug.) + + Release 2025c - 2025-12-10 14:42:37 -0800 Briefly: diff --git a/zic.c b/zic.c index 81a1c5a8..163eefa8 100644 --- a/zic.c +++ b/zic.c @@ -2674,15 +2674,19 @@ writezone(const char *const name, const char *const string, char version, toi = 0; fromi = 0; for ( ; fromi < timecnt; ++fromi) { - if (toi != 0 - && ((attypes[fromi].at + if (toi != 0) { + unsigned char type_2 = + toi == 1 ? 0 : attypes[toi - 2].type; + if ((attypes[fromi].at + utoffs[attypes[toi - 1].type]) - <= (attypes[toi - 1].at - + utoffs[toi == 1 ? 0 - : attypes[toi - 2].type]))) { + <= attypes[toi - 1].at + utoffs[type_2]) { + if (attypes[fromi].type == type_2) + toi--; + else attypes[toi - 1].type = attypes[fromi].type; - continue; + continue; + } } if (toi == 0 || attypes[fromi].dontmerge -- 2.51.0