Add support for reading leap second data from tz database

leapsectz directive is used to set the name of the timezone in the
system tz database which chronyd can use to find out when will the next
leap second occur.  It will periodically check if dates Jun 30 23:59:60
and Dec 31 23:59:60 are valid in that timezone. This is mainly useful
with reference clocks which don't provide the leap second information.
It is not necessary to restart chronyd if the tz database is updated
with a new leap second at least 12 hours before the event.

reference.c

@@ -89,6 +89,11 @@ static double drift_file_age;
 
 static void update_drift_file(double, double);
 
+/* Name of a system timezone containing leap seconds occuring at midnight */
+static char *leap_tzname;
+static time_t last_tz_leap_check;
+static NTP_Leap tz_leap;
+
 /* ================================================== */
 
 static LOG_FileID logfileid;
@@ -196,6 +201,8 @@ REF_Initialise(void)
 
   enable_local_stratum = CNF_AllowLocalReference(&local_stratum);
 
+  leap_tzname = CNF_GetLeapSecTimezone();
+
   CNF_GetMakeStep(&make_step_limit, &make_step_threshold);
   CNF_GetMaxChange(&max_offset_delay, &max_offset_ignore, &max_offset);
   CNF_GetLogChange(&do_log_change, &log_change_threshold);
@@ -526,20 +533,82 @@ is_offset_ok(double offset)
 
 /* ================================================== */
 
-static void
-update_leap_status(NTP_Leap leap)
+static NTP_Leap
+get_tz_leap(time_t when)
+{
+  struct tm stm;
+  time_t t;
+  char *tz_env, tz_orig[128];
+
+  /* Do this check at most twice a day */
+  when = when / (12 * 3600) * (12 * 3600);
+  if (last_tz_leap_check == when)
+      return tz_leap;
+
+  last_tz_leap_check = when;
+  tz_leap = LEAP_Normal;
+
+  stm = *gmtime(&when);
+
+  /* Check for leap second only in the latter half of June and December */
+  if (stm.tm_mon == 5 && stm.tm_mday > 14)
+    stm.tm_mday = 30;
+  else if (stm.tm_mon == 11 && stm.tm_mday > 14)
+    stm.tm_mday = 31;
+  else
+    return tz_leap;
+
+  /* Temporarily switch to the timezone containing leap seconds */
+  tz_env = getenv("TZ");
+  if (tz_env) {
+    if (strlen(tz_env) >= sizeof (tz_orig))
+      return tz_leap;
+    strcpy(tz_orig, tz_env);
+  }
+  setenv("TZ", leap_tzname, 1);
+  tzset();
+
+  /* Set the time to 23:59:60 and see how it overflows in mktime() */
+  stm.tm_sec = 60;
+  stm.tm_min = 59;
+  stm.tm_hour = 23;
+
+  t = mktime(&stm);
+
+  if (tz_env)
+    setenv("TZ", tz_orig, 1);
+  else
+    unsetenv("TZ");
+  tzset();
+
+  if (t == -1)
+    return tz_leap;
+
+  if (stm.tm_sec == 60)
+    tz_leap = LEAP_InsertSecond;
+  else if (stm.tm_sec == 1)
+    tz_leap = LEAP_DeleteSecond;
+
+  return tz_leap;
+}
+
+/* ================================================== */
+
+static void
+update_leap_status(NTP_Leap leap, time_t now)
 {
-  time_t now;
   struct tm stm;
   int leap_sec;
 
   leap_sec = 0;
 
+  if (leap_tzname && now && leap == LEAP_Normal)
+    leap = get_tz_leap(now);
+
   if (leap == LEAP_InsertSecond || leap == LEAP_DeleteSecond) {
     /* Insert/delete leap second only on June 30 or December 31
        and in other months ignore the leap status completely */
 
-    now = time(NULL);
     stm = *gmtime(&now);
 
     if (stm.tm_mon != 5 && stm.tm_mon != 11) {
@@ -600,7 +669,7 @@ REF_SetReference(int stratum,
   double update_interval;
   double elapsed;
   double correction_rate;
-  struct timeval now;
+  struct timeval now, raw_now;
 
   assert(initialised);
 
@@ -628,7 +697,9 @@ REF_SetReference(int stratum,
     }
   }
     
-  LCL_ReadCookedTime(&now, NULL);
+  LCL_ReadRawTime(&raw_now);
+  LCL_CookTime(&raw_now, &now, NULL);
+
   UTI_DiffTimevalsToDouble(&elapsed, &now, ref_time);
   our_offset = offset + elapsed * frequency;
 
@@ -646,7 +717,7 @@ REF_SetReference(int stratum,
   our_root_delay = root_delay;
   our_root_dispersion = root_dispersion;
 
-  update_leap_status(leap);
+  update_leap_status(leap, raw_now.tv_sec);
 
   if (last_ref_update.tv_sec) {
     UTI_DiffTimevalsToDouble(&update_interval, &now, &last_ref_update);
@@ -827,7 +898,7 @@ REF_SetUnsynchronised(void)
 
   are_we_synchronised = 0;
 
-  update_leap_status(LEAP_Unsynchronised);
+  update_leap_status(LEAP_Unsynchronised, 0);
 }
 
 /* ================================================== */