time: Set daylight to 1 for matching DST/offset change (bug 29951)

The daylight variable is supposed to be set to 1 if DST is ever in
use for the current time zone.  But __tzfile_read used to do this:

  __daylight = rule_stdoff != rule_dstoff;

This check can fail to set __daylight to 1 if the DST and non-DST
offsets happen to be the same.

time/tzfile.c

@@ -61,6 +61,10 @@ static size_t num_leaps;
 static struct leap *leaps;
 static char *tzspec;
 
+/* Used to restore the daylight variable during time conversion, as if
+   tzset had been called.  */
+static int daylight_saved;
+
 #include <endian.h>
 #include <byteswap.h>
 
@@ -438,36 +442,35 @@ __tzfile_read (const char *file, size_t extra, char **extrap)
   if (__tzname[1] == NULL)
     __tzname[1] = __tzname[0];
 
+  daylight_saved = 0;
   if (num_transitions == 0)
     /* Use the first rule (which should also be the only one).  */
     rule_stdoff = rule_dstoff = types[0].offset;
   else
     {
-      int stdoff_set = 0, dstoff_set = 0;
+      rule_stdoff = 0;
-      rule_stdoff = rule_dstoff = 0;
+
+      /* Search for the last rule with a standard time offset.  This
+	 will be used for the global timezone variable.  */
       i = num_transitions - 1;
       do
+	if (!types[type_idxs[i]].isdst)
 	  {
-	  if (!stdoff_set && !types[type_idxs[i]].isdst)
-	    {
-	      stdoff_set = 1;
 	    rule_stdoff = types[type_idxs[i]].offset;
-	    }
-	  else if (!dstoff_set && types[type_idxs[i]].isdst)
-	    {
-	      dstoff_set = 1;
-	      rule_dstoff = types[type_idxs[i]].offset;
-	    }
-	  if (stdoff_set && dstoff_set)
 	    break;
 	  }
+	else
+	  daylight_saved = 1;
       while (i-- > 0);
 
-      if (!dstoff_set)
+      /* Keep searching to see if there is a DST rule.  This
-	rule_dstoff = rule_stdoff;
+	 information will be used to set the global daylight
+	 variable.  */
+      while (i-- > 0 && !daylight_saved)
+	daylight_saved = types[type_idxs[i]].isdst;
     }
 
-  __daylight = rule_stdoff != rule_dstoff;
+  __daylight = daylight_saved;
   __timezone = -rule_stdoff;
 
  done:
@@ -731,7 +734,7 @@ __tzfile_compute (__time64_t timer, int use_localtime,
 	}
 
       struct ttinfo *info = &types[i];
-      __daylight = rule_stdoff != rule_dstoff;
+      __daylight = daylight_saved;
       __timezone = -rule_stdoff;
 
       if (__tzname[0] == NULL)

timezone/Makefile

@@ -23,7 +23,7 @@ subdir	:= timezone
 include ../Makeconfig
 
 others	:= zdump zic
-tests	:= test-tz tst-timezone tst-tzset tst-bz28707
+tests	:= test-tz tst-timezone tst-tzset tst-bz28707 tst-bz29951
 
 generated-dirs += testdata
 
@@ -86,11 +86,13 @@ $(objpfx)tst-timezone.out: $(addprefix $(testdata)/, \
 				       Europe/London)
 $(objpfx)tst-tzset.out: $(addprefix $(testdata)/XT, 1 2 3 4)
 $(objpfx)tst-bz28707.out: $(testdata)/XT5
+$(objpfx)tst-bz29951.out: $(testdata)/XT6
 
 test-tz-ENV = TZDIR=$(testdata)
 tst-timezone-ENV = TZDIR=$(testdata)
 tst-tzset-ENV = TZDIR=$(testdata)
 tst-bz28707-ENV = TZDIR=$(testdata)
+tst-bz29951-ENV = TZDIR=$(testdata)
 
 # Note this must come second in the deps list for $(built-program-cmd) to work.
 zic-deps = $(objpfx)zic $(leapseconds) yearistype

timezone/tst-bz29951.c

@@ -0,0 +1,68 @@
+/* Check that daylight is set if the last DST transition did not change offset.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <time.h>
+
+/* Set the specified time zone with error checking.  */
+static void
+set_timezone (const char *name)
+{
+  TEST_VERIFY (setenv ("TZ", name, 1) == 0);
+  errno = 0;
+  tzset ();
+  TEST_COMPARE (errno, 0);
+}
+
+static int
+do_test (void)
+{
+  /* Test zone based on tz-2022g version of Africa/Tripoli.  The last
+     DST transition coincided with a change in the standard time
+     offset, effectively making it a no-op.
+
+     Africa/Tripoli  Thu Oct 24 23:59:59 2013 UT
+       = Fri Oct 25 01:59:59 2013 CEST isdst=1 gmtoff=7200
+     Africa/Tripoli  Fri Oct 25 00:00:00 2013 UT
+       = Fri Oct 25 02:00:00 2013 EET isdst=0 gmtoff=7200
+   */
+  set_timezone ("XT6");
+  TEST_VERIFY (daylight != 0);
+  TEST_COMPARE (timezone, -7200);
+
+  /* Check that localtime re-initializes the two variables.  */
+  daylight = timezone = 17;
+  time_t t = 844034401;
+  struct tm *tm = localtime (&t);
+  TEST_VERIFY (daylight != 0);
+  TEST_COMPARE (timezone, -7200);
+  TEST_COMPARE (tm->tm_year, 96);
+  TEST_COMPARE (tm->tm_mon, 8);
+  TEST_COMPARE (tm->tm_mday, 29);
+  TEST_COMPARE (tm->tm_hour, 23);
+  TEST_COMPARE (tm->tm_min, 0);
+  TEST_COMPARE (tm->tm_sec, 1);
+  TEST_COMPARE (tm->tm_gmtoff, 3600);
+  TEST_COMPARE (tm->tm_isdst, 0);
+
+  return 0;
+}
+
+#include <support/test-driver.c>