From: DJ Delorie <dj@redhat.com>
To: libc-alpha@sourceware.org
Subject: [swbz 29035] mktime vs non-DST
Date: Wed, 17 Aug 2022 17:18:51 -0400 [thread overview]
Message-ID: <xnv8qq601w.fsf@greed.delorie.com> (raw)
https://sourceware.org/bugzilla/show_bug.cgi?id=29035
TL;DR - requesting a partial reversion of 86aece3 to become
bug-compatible with older releases.
Long version:
In investigating this, I did a deep-dive on how tm_isdst works in
mktime(). It seems to be less of a hint and more of an override, in
that, if you set tm_isdst=1 you're going to get a result that seems an
hour off if you're in the middle of a standard time period. Same for
tm_isdst=0. Setting tm_isdst=-1 is the only way to let mktime use the
dst-in-effect for the time period specified. Note: I'm not
considering the time duplication that happens at period boundaries
(i.e. the "fall back" that causes an hour of clock time to repeat each
fall).
So if you set tm_isdst=1 in a call to mktime(), it figures out the
local DST offset and applies it regardless of timezone rules.
In the BZ case, however, the zoneinfo in effect does not have a DST
defined (or, as we'll see later, hasn't had DST in a long time). If
there's no DST, and you set tm_isdst=1, what happens?
Well, prior to 2.29, mktime just overrode tm_isdst and returned a
suitable time according to the current zoneinfo, as if you had passed
tm_isdst=0 or -1 instead.
As of 2.29, we have commit 86aece3bfbd44538ba4fdc947872c81d4c5e6e61
by Paul which includes:
(__mktime_internal): Set errno to EOVERFLOW if the spring-forward
gap code fails.
/* We have a match. Check whether tm.tm_isdst has the requested
value, if any. */
if (isdst_differ (isdst, tm.tm_isdst))
{
. . .
+ __set_errno (EOVERFLOW);
+ return -1;
}
With this change, tm_isdst becomes a hard requirement, and if the
current zone doesn't have a DST defined, you get a failure, where we
used to succeed (but with a non-DST result).
The relevent standards are pretty quiet on this topic; what little
they say can be interpreted either way - tm_isdst is a requirement, or
tm_isdst is a hint to be corrected by mktime() like other fields.
This breaks the logic down into three categories:
1. You're in a transition period where clock time repeats, and you
need tm_isdst to decide which to return.
2. You're not in a transition period, and you might as well set
tm_isdst=-1 unless you want an off-by-an-hour result.
3. Your zone doesn't have dst and setting tm_isdst=1 is meaningless.
I can't see an obvious way to detect case 1 from 2, so this seems to
be a useless set of categories. A better breakdown would be:
1. You set tm_isdst=-1 by default. Most of the time, this works.
2. If the time is ambiguous due to a transition, case 1 returns EAGAIN
and you try again with tm_isdst=0 or 1.
3. If you set tm_isdst=0 or 1 outside of a transition, it returns
EINVAL if it's incorrect for that time.
But that would be a BIG world-breaking change. One can dream :-)
Meanwhile, I would like us to consider reverting the commit mentioned
above (not the whole commit, just the two lines I included). This
will have the effect of making the current code bug-compatible with
older code, in that, setting tm_isdst=1 in a no-dst zone returns a
non-dst (but otherwise valid) time, and updates tm_idst to 0.
Returning EOVERFLOW in these new cases is not useful.
next reply other threads:[~2022-08-17 21:18 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-17 21:18 DJ Delorie [this message]
2022-08-17 21:50 ` Carlos O'Donell
2022-08-17 23:10 ` Paul Eggert
2022-08-18 1:39 ` DJ Delorie
2022-08-18 2:37 ` Carlos O'Donell
2022-08-18 3:16 ` Paul Eggert
2022-08-18 4:05 ` Carlos O'Donell
2022-08-18 21:17 ` DJ Delorie
2022-08-18 21:57 ` Paul Eggert
2022-08-18 22:40 ` DJ Delorie
2022-08-18 22:58 ` Paul Eggert
2022-08-19 18:15 ` DJ Delorie
2022-08-19 22:04 ` Paul Eggert
2022-08-18 3:02 ` Paul Eggert
2022-09-08 20:25 ` DJ Delorie
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