Re: [swbz 29035] mktime vs non-DST

[DJ Delorie <dj@redhat.com>]

> We have also seen a somewhat odd use of mktime() to detect DST transitions that
> DJ and I need to audit and verify against the changes you have implemented in
> gnulib.

FYI we've finished this, with no problems :-)

Paul Eggert <eggert@cs.ucla.edu> writes:

> From 674d4588995d24f8bc5167f73d8661ef5c82c2de Mon Sep 17 00:00:00 2001

> -# elif HAVE_TZSET
> +# else

Ok.

> From bfe919ce3bba22178429c5b39db98c26fd83272e Mon Sep 17 00:00:00 2001

>  	 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);

Ok.

> -      /* 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;

Ok.

> +      /* 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;
>      }

Ok.

LGTM
Reviewed-by: DJ Delorie <dj@redhat.com>