Re: [musl] Broken mktime calculations when crossing DST boundary

[Thorsten Glaser <tg@mirbsd.de>]

Alexander Weps dixit:
>On Monday, March 25th, 2024 at 13:21, Rich Felker <dalias@libc.org> wrote:
>> On Mon, Mar 25, 2024 at 11:52:00AM +0000, Alexander Weps wrote:

>> > Musl cannot reliably increment date by a day. Incrementing struct tm
>> > representing 2011-12-29 01:00:00 -10 by one day leads to the same
>> > date.

No, that’s correct.

Your chosen timezone has a discontinuity:

$ TZ=Pacific/Apia date -d @1325239199
Thu Dec 29 23:59:59 -10 2011
$ TZ=Pacific/Apia date -d @1325239200
Sat Dec 31 00:00:00 +14 2011

This means any time between this simply does not exist
in broken-down time.

>> > Causing a program to loop or stack overflow.

That’s because your application violates the constraints
that bind both, not just the libc, to the spec.

>Output from musl:
>
>2011-12-29 01:00:00 -10
>
>    tm.tm_mday += 1;
>    t = mktime(&tm);
>
>2011-12-29 01:00:00 -10 <-- date is the same after incrementing

This is… not as incorrect as you state.

The steps here are:

• 2011-12-30 01:00:00 ← input
• 2011-12-30 01:00:00 ← input after normalisation (!)
• conversion to time_t (1325206800), application of timezone offset
• detection of the discontinuity between 2011-12-29 23:59:59 and
      2011-12-31 00:00:00
• arbitrary choice of selecting either endpoint

tbh I’d expect this to end up in 1325239199=2011-12-29 23:59:59
instead of 2011-12-29 01:00:00 though, at least from reading the
latest Issue 8 proofreading draft. WDYT dalias?

>    tm.tm_mday -= 1;
>    t = mktime(&tm);
>
>2011-12-28 01:00:00 -10 <-- going below the original date while decrementing

This is entirely correct, again. You’re starting from
2011-12-28 01:00:00, you’re getting it back.

bye,
//mirabilos
-- 
“It is inappropriate to require that a time represented as
 seconds since the Epoch precisely represent the number of
 seconds between the referenced time and the Epoch.”
	-- IEEE Std 1003.1b-1993 (POSIX) Section B.2.2.2