ntp: don't stop online burst when sending fails

Don't stop online burst for unreachable sources until sending succeeds.
This is mainly useful with iburst when chronyd is started before the
network is configured.

ntp_core.c

@@ -891,6 +891,10 @@ transmit_timeout(void *arg)
 
   switch (inst->opmode) {
     case MD_BURST_WAS_ONLINE:
+      /* When not reachable, don't stop online burst until sending succeeds */
+      if (!sent && !SRC_IsReachable(inst->source))
+        break;
+      /* Fall through */
     case MD_BURST_WAS_OFFLINE:
       --inst->burst_total_samples_to_go;
       break;

sources.c

@@ -1167,6 +1167,14 @@ SRC_IsSyncPeer(SRC_Instance inst)
 
 /* ================================================== */
 
+int
+SRC_IsReachable(SRC_Instance inst)
+{
+  return inst->reachability != 0;
+}
+
+/* ================================================== */
+
 int
 SRC_ReadNumberOfSources(void)
 {

sources.h

@@ -171,6 +171,7 @@ extern void SRC_DumpSources(void);
 extern void SRC_ReloadSources(void);
 
 extern int SRC_IsSyncPeer(SRC_Instance inst);
+extern int SRC_IsReachable(SRC_Instance inst);
 extern int SRC_ReadNumberOfSources(void);
 extern int SRC_ActiveSources(void);
 extern int SRC_ReportSource(int index, RPT_SourceReport *report, struct timeval *now);