[Bug time/31144] New: mktime: returns clock for UTC with isdst=1

["aurelien at aurel32 dot net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31144

            Bug ID: 31144
           Summary: mktime: returns clock for UTC with isdst=1
           Product: glibc
           Version: unspecified
               URL: http://bugs.debian.org/1057856
            Status: NEW
          Severity: normal
          Priority: P2
         Component: time
          Assignee: unassigned at sourceware dot org
          Reporter: aurelien at aurel32 dot net
  Target Milestone: ---

Created attachment 15253
  --> https://sourceware.org/bugzilla/attachment.cgi?id=15253&action=edit
Reproducer

A Debian user reported a behaviour change of the mktime function affecting
Erlang/OTP. When passing isdst=1 with an UTC timezone, a time is now returned
(with isdst=0), while in the past it returned an error.

The issue has been tracked to the following commit
83859e1115269cf56d21669361d4ddbe2687831c which went into 2.37, and got
backported to the 2.34, 2.35 and 2.36 branches.

The C standard is not fully clear about the behaviour in that case:

"A positive or 0 value for tm_isdst shall cause mktime() to presume initially
that Daylight Savings Time, respectively, is or is not in effect for the
specified time. A negative value for tm_isdst shall cause mktime() to attempt
to determine whether Daylight Savings Time is in effect for the specified
time."

However it appears that the original GNU libc behaviour before that commit
matches the FreeBSD libc behaviour.

The user provided a small test program to exercise the issue (see attachement):

$ gcc -o check_dst check_dst.c
$ TZ=UTC ./check_dst 0 
tzname[0]: UTC, tzname[1]: UTC
        dst before: 0
        dst after: 0
        clock: 1217548800
$ TZ=UTC ./check_dst 1
tzname[0]: UTC, tzname[1]: UTC
        dst before: 1
        dst after: 0
        clock: 1217545200

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