sources: add support for orphan sources

When the local reference is configured with the orphan option, NTP
sources that have stratum equal to the configured local stratum are
considered to be orphans (i.e. serving local time while not being
synchronised with real time) and are excluded from the normal source
selection. Sources with stratum larger than the local stratum are
considered to be directly on indirectly synchronised to an orphan and
are always ignored.

If no selectable source is available and all orphan sources have
reference IDs larger than the local ID, no source will be selected and
the local reference mode will be activated at some point, i.e. this host
will become an orphan. Otherwise, the orphan source with the smallest
reference ID will be selected. This ensures a group of servers polling
each other (with the same orphan configuration) which have no external
source can settle down to a state where only one server is serving its
local unsychronised time and others are synchronised to it.

sources.c

@@ -73,6 +73,7 @@ typedef enum {
   SRC_BAD_DISTANCE,     /* Has root distance longer than allowed maximum */
   SRC_WAITS_STATS,      /* Others have bad stats, selection postponed */
   SRC_STALE,            /* Has older samples than others */
+  SRC_ORPHAN,           /* Has stratum equal or larger than orphan stratum */
   SRC_FALSETICKER,      /* Doesn't agree with others */
   SRC_JITTERY,          /* Scatter worse than other's dispersion (not used) */
   SRC_WAITS_SOURCES,    /* Not enough sources, selection postponed */
@@ -607,6 +608,7 @@ SRC_SelectSource(SRC_Instance updated_inst)
   int n_badstats_sources, max_sel_reach, max_badstat_reach, sel_req_source;
   int depth, best_depth, trust_depth, best_trust_depth;
   int combined, stratum, min_stratum, max_score_index;
+  int orphan_stratum, orphan_source;
   double src_offset, src_offset_sd, src_frequency, src_skew;
   double src_root_delay, src_root_dispersion;
   double best_lo, best_hi, distance, sel_src_distance, max_score;
@@ -680,6 +682,9 @@ SRC_SelectSource(SRC_Instance updated_inst)
       max_sel_reach = sources[i]->reachability;
   }
 
+  orphan_stratum = REF_GetOrphanStratum();
+  orphan_source = INVALID_SOURCE;
+
   for (i = 0; i < n_sources; i++) {
     if (sources[i]->status != SRC_OK)
       continue;
@@ -694,7 +699,52 @@ SRC_SelectSource(SRC_Instance updated_inst)
       continue;
     }
 
+    /* When the local reference is configured with the orphan option, NTP
+       sources that have stratum equal to the configured local stratum are
+       considered to be orphans (i.e. serving local time while not being
+       synchronised with real time) and are excluded from the normal source
+       selection.  Sources with stratum larger than the local stratum are
+       considered to be directly on indirectly synchronised to an orphan and
+       are always ignored.
+
+       If no selectable source is available and all orphan sources have
+       reference IDs larger than the local ID, no source will be selected and
+       the local reference mode will be activated at some point, i.e. this host
+       will become an orphan.  Otherwise, the orphan source with the smallest
+       reference ID will be selected.  This ensures a group of servers polling
+       each other (with the same orphan configuration) which have no external
+       source can settle down to a state where only one server is serving its
+       local unsychronised time and others are synchronised to it. */
+
+    if (si->stratum >= orphan_stratum && sources[i]->type == SRC_NTP) {
+      sources[i]->status = SRC_ORPHAN;
+
+      if (si->stratum == orphan_stratum &&
+          (orphan_source == INVALID_SOURCE ||
+           sources[i]->ref_id < sources[orphan_source]->ref_id))
+        orphan_source = i;
+
+      continue;
+    }
+
     ++n_sel_sources;
+  }
+
+  /* If no selectable source is available, consider the orphan source */
+  if (!n_sel_sources && orphan_source != INVALID_SOURCE &&
+      sources[orphan_source]->ref_id <
+        NSR_GetLocalRefid(sources[orphan_source]->ip_addr)) {
+    sources[orphan_source]->status = SRC_OK;
+    n_sel_sources = 1;
+    DEBUG_LOG(LOGF_Sources, "selecting orphan refid=%"PRIx32,
+              sources[orphan_source]->ref_id);
+  }
+
+  for (i = 0; i < n_sources; i++) {
+    if (sources[i]->status != SRC_OK)
+      continue;
+
+    si = &sources[i]->sel_info;
 
     j1 = n_endpoints;
     j2 = j1 + 1;
@@ -1254,6 +1304,7 @@ SRC_ReportSource(int index, RPT_SourceReport *report, struct timeval *now)
       case SRC_BAD_STATS:
       case SRC_BAD_DISTANCE:
       case SRC_STALE:
+      case SRC_ORPHAN:
       case SRC_WAITS_STATS:
         report->state = RPT_UNREACH;
         break;