Ignore measurements around leap second

When current time is within 5 seconds of a leap second, don't accumulate
new samples or update the leap second status to increase the chances of
getting through safely.

reference.c

@@ -680,7 +680,7 @@ update_leap_status(NTP_Leap leap, time_t now)
     }
   }
   
-  if (leap_sec != our_leap_sec) {
+  if (leap_sec != our_leap_sec && !REF_IsLeapSecondClose()) {
     LCL_SetLeap(leap_sec);
     our_leap_sec = leap_sec;
   }
@@ -1150,6 +1150,31 @@ REF_IsLocalActive(void)
 
 /* ================================================== */
 
+#define LEAP_SECOND_CLOSE 5
+
+int REF_IsLeapSecondClose(void)
+{
+  struct timeval now, now_raw;
+  time_t t;
+
+  if (!our_leap_sec)
+    return 0;
+
+  SCH_GetLastEventTime(&now, NULL, &now_raw);
+
+  t = now.tv_sec > 0 ? now.tv_sec : -now.tv_sec;
+  if ((t + LEAP_SECOND_CLOSE) % (24 * 3600) < 2 * LEAP_SECOND_CLOSE)
+    return 1;
+
+  t = now_raw.tv_sec > 0 ? now_raw.tv_sec : -now_raw.tv_sec;
+  if ((t + LEAP_SECOND_CLOSE) % (24 * 3600) < 2 * LEAP_SECOND_CLOSE)
+    return 1;
+
+  return 0;
+}
+
+/* ================================================== */
+
 void
 REF_GetTrackingReport(RPT_TrackingReport *rep)
 {

reference.h

@@ -161,6 +161,10 @@ extern void REF_EnableLocal(int stratum);
 extern void REF_DisableLocal(void);
 extern int REF_IsLocalActive(void);
 
+/* Check if current raw or cooked time is close to a leap second
+   and is better to discard any measurements */
+extern int REF_IsLeapSecondClose(void);
+
 extern void REF_GetTrackingReport(RPT_TrackingReport *rep);
 
 #endif /* GOT_REFERENCE_H */

sources.c

@@ -312,6 +312,11 @@ void SRC_AccumulateSample
   DEBUG_LOG(LOGF_Sources, "ip=[%s] t=%s ofs=%f del=%f disp=%f str=%d",
       source_to_string(inst), UTI_TimevalToString(sample_time), -offset, root_delay, root_dispersion, stratum);
 
+  if (REF_IsLeapSecondClose()) {
+    LOG(LOGS_INFO, LOGF_Sources, "Dropping sample around leap second");
+    return;
+  }
+
   /* WE HAVE TO NEGATE OFFSET IN THIS CALL, IT IS HERE THAT THE SENSE OF OFFSET
      IS FLIPPED */
   SST_AccumulateSample(inst->stats, sample_time, -offset, peer_delay, peer_dispersion, root_delay, root_dispersion, stratum);