Fix POSIX::strftime()

ext/POSIX/POSIX.xs

@@ -3588,7 +3588,7 @@ 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 = -1)
+strftime(fmt, sec, min, hour, mday, mon, year, wday = -1, yday = -1, isdst = 0)
 	SV *		fmt
 	int		sec
 	int		min
@@ -3603,9 +3603,11 @@ strftime(fmt, sec, min, hour, mday, mon, year, wday = -1, yday = -1, isdst = -1)
 	{
             PERL_UNUSED_ARG(wday);
             PERL_UNUSED_ARG(yday);
-            PERL_UNUSED_ARG(isdst);
 
-            SV *sv = sv_strftime_ints(fmt, sec, min, hour, mday, mon, year, 0);
+            /* -isdst triggers backwards compatibility mode for non-zero
+             * 'isdst' */
+            SV *sv = sv_strftime_ints(fmt, sec, min, hour, mday, mon, year,
+                                           -abs(isdst));
 	    if (sv) {
                 sv = sv_2mortal(sv);
             }

ext/POSIX/lib/POSIX.pod

@@ -1872,15 +1872,19 @@ Returns the string.
 Synopsis:
 
 	strftime(fmt, sec, min, hour, mday, mon, year,
-		 wday = -1, yday = -1, isdst = -1)
+		 wday = -1, yday = -1, isdst = 0)
 
 The month (C<mon>) begins at zero,
 I<e.g.>, January is 0, not 1.  The
 year (C<year>) is given in years since 1900, I<e.g.>, the year 1995 is 95; the
 year 2001 is 101.  Consult your system's C<strftime()> manpage for details
 about these and the other arguments.
 
-The C<wday>, C<yday>, and C<isdst> parameters are all ignored.
+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.
 
 If you want your code to be portable, your format (C<fmt>) argument
 should use only the conversion specifiers defined by the ANSI C
@@ -1895,11 +1899,10 @@ 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 given arguments are made consistent as though by calling
-C<mktime()> before calling your system's C<strftime()> function,
-except that the C<isdst> value is not affected, so that the returned
-value will always be as if the locale doesn't have daylight savings
-time.
+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 string for Tuesday, December 12, 1995 in the C<C> locale.
 

ext/POSIX/t/time.t

@@ -9,7 +9,7 @@ use strict;
 
 use Config;
 use POSIX;
-use Test::More tests => 26;
+use Test::More tests => 27;
 
 # 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.
@@ -205,3 +205,12 @@ SKIP: {
     is(mktime(CORE::localtime($time)), $time, "mktime()");
     is(mktime(POSIX::localtime($time)), $time, "mktime()");
 }
+
+SKIP: {
+    skip "'%s' not implemented in strftime", 1 if $^O eq "VMS"
+                                               || $^O eq "MSWin32";
+    # Somewhat arbitrarily, put in 60 seconds of slack;  if this fails, it
+    # will likely be off by 1 hour
+    ok(abs(POSIX::strftime('%s', localtime) - time) < 60,
+       'GH #22351; pr: GH #22369');
+}

locale.c

@@ -8095,11 +8095,27 @@ The caller assumes ownership of the returned SV with a reference count of 1.
 
 C<sv_strftime_ints> takes a bunch of integer parameters that together
 completely define a given time.  It calculates the S<C<struct tm>> to pass to
-libc strftime(), and calls that function.  Setting C<isdst> to 0 causes the
-date to be calculated as if there is no daylight savings time in effect; any
-other value causes the possibility of daylight savings time to be considered.
-A positive value is a hint to the function that daylight savings is likely to
-be in effect for the passed-in time.
+libc strftime(), and calls that function.
+
+The value of C<isdst> is used as follows:
+
+=over
+
+=item 0
+
+No daylight savings time is in effect
+
+=item E<gt>0
+
+Check if daylight savings time is in effect, and adjust the results
+accordingly.
+
+=item E<lt>0
+
+This value is reserved for internal use by the L<POSIX> module for backwards
+compatibility purposes.
+
+=back
 
 The caller assumes ownership of the returned SV with a reference count of 1.
 
@@ -8166,8 +8182,15 @@ 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, isdst);
+    ints_to_tm(&mytm, locale, sec, min, hour, mday, mon, year,
+                              MAX(0, isdst));
+    mytm.tm_isdst = MIN(1, abs(isdst));
     return sv_strftime_common(fmt, locale, &mytm);
 }