Test fix for GH #23878

Author: khwilliamson

ext/POSIX/POSIX.xs

@@ -3589,7 +3589,7 @@ difftime(time1, time2)
 #     sv_setpv(TARG, ...) could be used rather than
 #     ST(0) = sv_2mortal(newSVpv(...))
 void
-strftime(fmt, sec, min, hour, mday, mon, year, wday = -1, yday = -1, isdst = 0)
+strftime(fmt, sec, min, hour, mday, mon, year, wday = -1, yday = -1, isdst = -1)
 	SV *		fmt
 	int		sec
 	int		min
@@ -3605,10 +3605,8 @@ strftime(fmt, sec, min, hour, mday, mon, year, wday = -1, yday = -1, isdst = 0)
             PERL_UNUSED_ARG(wday);
             PERL_UNUSED_ARG(yday);
 
-            /* -isdst triggers backwards compatibility mode for non-zero
-             * 'isdst' */
             SV *sv = sv_strftime_ints(fmt, sec, min, hour, mday, mon, year,
-                                           -abs(isdst));
+                                      isdst);
 	    if (sv) {
                 sv = sv_2mortal(sv);
             }

ext/POSIX/lib/POSIX.pod

@@ -1872,7 +1872,7 @@ Returns the string.
 Synopsis:
 
 	strftime(fmt, sec, min, hour, mday, mon, year,
-		 wday = -1, yday = -1, isdst = 0)
+		 wday = -1, yday = -1, isdst = -1)
 
 The month (C<mon>) begins at zero,
 I<e.g.>, January is 0, not 1.  The
@@ -1883,8 +1883,16 @@ about these and the other arguments.
 The C<wday> and C<yday> parameters are both ignored.  Their values are
 always determinable from the other parameters.
 
-C<isdst> should be C<1> or C<0>, depending on whether or not daylight
-savings time is in effect for the given time or not.
+C<isdst> should be 
+
+=over
+
+=item positive to indicate that daylight savings time is in effect.
+
+=item zero to indicate that daylight savings time is not in effect.
+
+=item negative to ask the function to determine if daylight savings time
+is in effect.
 
 If you want your code to be portable, your format (C<fmt>) argument
 should use only the conversion specifiers defined by the ANSI C
@@ -1899,17 +1907,19 @@ The C<Z> specifier is notoriously unportable since the names of
 timezones are non-standard. Sticking to the numeric specifiers is the
 safest route.
 
-The arguments, except for C<isdst>, are made consistent as though by
-calling C<mktime()> before calling your system's C<strftime()> function.
-To get correct results, you must set C<isdst> to be the proper value.
-When omitted, the function assumes daylight savings is not in effect.
+The arguments are made consistent as though by calling C<mktime()>
+before calling your system's C<strftime()> function.
 
 The string for Tuesday, December 12, 1995 in the C<C> locale.
 
 	$str = POSIX::strftime( "%A, %B %d, %Y",
 				 0, 0, 0, 12, 11, 95, 2 );
 	print "$str\n";
 
+Note that older libc versions of this function had a hard time
+determining if daylight savings time is in effect.  That should no
+longer be an issue.
+
 =item C<strlen>
 
 Not implemented.  C<strlen()> is C-specific, use C<length()> instead, see L<perlfunc/length>.

ext/POSIX/t/time.t

@@ -9,7 +9,7 @@ use strict;
 
 use Config;
 use POSIX;
-use Test::More tests => 31;
+use Test::More tests => 40;
 
 # For the first go to UTC to avoid DST issues around the world when testing.  SUS3 says that
 # null should get you UTC, but some environments want the explicit names.
@@ -226,3 +226,24 @@ SKIP: {
     is(strftime(undef, CORE::localtime), '', "strftime() works if format is undef");
     like($warnings, qr/^Use of uninitialized value in subroutine entry /, "strftime(undef, ...) produces expected warning");
 }
+
+{   # GH #23878
+    skip "No mktime()", 8 if $Config{d_mktime} ne 'define';
+    $ENV{TZ}="Europe/Paris"; 
+    my $b = 1761436800;     # an hour before time should have changed
+    my @data = (
+                 [   -1, "2025-10-26 01:59:59+0200", "-1 second" ],
+                 [    0, "2025-10-26 02:00:00+0200", "+0 seconds" ],
+                 [    1, "2025-10-26 02:00:01+0200", "+1 second" ],
+                 [ 3599, "2025-10-26 02:59:59+0200", "+59 minutes 59 seconds" ],
+                 [ 3600, "2025-10-26 02:00:00+0100", "+1 hour" ],
+                 [ 3601, "2025-10-26 02:00:01+0100", "+1 hour 1 second" ],
+                 [ 7199, "2025-10-26 02:59:59+0100", "+2 hours minus 1 sec" ],
+                 [ 7200, "2025-10-26 03:00:00+0100", "+2 hours" ],
+                 [ 7201, "2025-10-26 03:00:01+0100", "+2 hours 1 second" ],
+               );
+    for (my $i = 0; $i < @data; $i++) {
+        is(POSIX::strftime("%F %T%z", localtime $b + $data[$i][0]),
+           $data[$i][1], $data[$i][2]);
+    }
+}

locale.c

@@ -8190,17 +8190,16 @@ The value of C<isdst> is used as follows:
 
 =item 0
 
-No daylight savings time is in effect
+The caller knows that daylight savings time is not in effect
 
 =item E<gt>0
 
-Check if daylight savings time is in effect, and adjust the results
-accordingly.
+The caller knows that daylight savings time is in effect, though the
+underlying libc function may choose to override this.
 
 =item E<lt>0
 
-This value is reserved for internal use by the L<POSIX> module for backwards
-compatibility purposes.
+The function is to calculate itself if daylight savings time is in effect.
 
 =back
 
@@ -8269,15 +8268,8 @@ Perl_sv_strftime_ints(pTHX_ SV * fmt, int sec, int min, int hour,
     const char * locale = "C";
 #endif
 
-    /* A negative 'isdst' triggers backwards compatibility mode for
-     * POSIX::strftime(), in which 0 is always passed to ints_to_tm() so that
-     * the possibility of daylight savings time is never considered,  But, a 1
-     * is eventually passed to libc strftime() so that it returns the results
-     * it always has for a non-zero 'isdst'.  See GH #22351 */
     struct tm  mytm;
-    ints_to_tm(&mytm, locale, sec, min, hour, mday, mon, year,
-                              MAX(0, isdst));
-    mytm.tm_isdst = MIN(1, abs(isdst));
+    ints_to_tm(&mytm, locale, sec, min, hour, mday, mon, year, isdst);
     return sv_strftime_common(fmt, locale, &mytm);
 }
 
@@ -8383,30 +8375,34 @@ S_ints_to_tm(pTHX_ struct tm * mytm,
     if (isdst == 0) {
         mini_mktime(mytm);
 
-#  ifndef ALWAYS_RUN_MKTIME
-
-    /* When we don't always need libc mktime(), we call it only when we didn't
-     * call mini_mktime() */
-    } else {
+#  ifdef ALWAYS_RUN_MKTIME
 
-#  else
         /* Here will have to run libc mktime() in order to get the values of
          * some fields that mini_mktime doesn't populate.  We don't want
-         * mktime's side effect of looking for dst, so we have to have a
-         * separate tm structure from which we copy just those fields into the
-         * returned structure.  Initialize its values.  mytm should now be a
-         * normalized version of the input. */
+         * mktime's side effect of looking for dst (because isdst==0), so we
+         * have to have a separate tm structure from which we copy just those
+         * fields into the structure we return.  Initialize its values, which
+         * have now been normalized by mini_mktime. */
         aux_tm = *mytm;
-        aux_tm.tm_isdst = isdst;
         which_tm = &aux_tm;
 
 #  endif
 
+    }
+
+#  ifndef ALWAYS_RUN_MKTIME
+
+    else    /* Don't run libc mktime if both: we ran mini_mktime above, and we
+               don't have to always run libc mktime */
+
+#  endif
+
+    {
         /* Here, we need to run libc mktime(), either because we want to take
          * dst into consideration, or because it calculates one or two fields
-         * that we need that mini_mktime() doesn't handle.
-         *
-         * Unlike mini_mktime(), it does consider the locale, so have to switch
+         * that we need that mini_mktime() doesn't handle. */
+
+        /* Unlike mini_mktime(), it does consider the locale, so have to switch
          * to the correct one. */
         const char * orig_TIME_locale = toggle_locale_c(LC_TIME, locale);
         MKTIME_LOCK;