Add delayed name resolving for servers and peers

Resolving is retried in increasing intervals (maximum is one hour)
until it succeeds or fails with a non-temporary error.

Unresolved sources are included in the activity report as offline
sources and the online command can be used to retry it immediately.

This could be improved by resolving in a separate thread/process
to avoid blocking.

ntp_sources.c

@@ -37,6 +37,9 @@
 #include "util.h"
 #include "logging.h"
 #include "local.h"
+#include "memory.h"
+#include "nameserv.h"
+#include "sched.h"
 
 /* ================================================== */
 
@@ -60,6 +63,19 @@ static int n_sources;
 /* The largest number of sources we want to have stored in the hash table */
 #define MAX_SOURCES 64
 
+/* Source with unknown address (which may be resolved later) */
+struct UnresolvedSource {
+  char *name;
+  int port;
+  NTP_Source_Type type;
+  SourceParameters params;
+  struct UnresolvedSource *next;
+};
+
+static struct UnresolvedSource *unresolved_sources = NULL;
+static int resolving_interval = 0;
+static SCH_TimeoutID resolving_id;
+
 /* ================================================== */
 /* Forward prototypes */
 static void
@@ -203,6 +219,75 @@ NSR_AddSource(NTP_Remote_Address *remote_addr, NTP_Source_Type type, SourceParam
 
 /* ================================================== */
 
+static void
+resolve_sources(void *arg)
+{
+  NTP_Remote_Address address;
+  struct UnresolvedSource *us, **i;
+  DNS_Status s;
+
+  memset(&address.local_ip_addr, 0, sizeof (address.local_ip_addr));
+
+  DNS_Reload();
+
+  for (i = &unresolved_sources; *i; ) {
+    us = *i;
+    s = DNS_Name2IPAddress(us->name, &address.ip_addr);
+    if (s == DNS_TryAgain) {
+      i = &(*i)->next;
+      continue;
+    } else if (s == DNS_Success) {
+      address.port = us->port;
+      NSR_AddSource(&address, us->type, &us->params);
+    } else {
+      LOG(LOGS_WARN, LOGF_NtpSources, "Invalid host %s", us->name);
+    }
+    
+    *i = us->next;
+
+    Free(us->name);
+    Free(us);
+  }
+
+  if (unresolved_sources) {
+    /* Try again later */
+    if (resolving_interval < 9)
+      resolving_interval++;
+    resolving_id = SCH_AddTimeoutByDelay(7 * (1 << resolving_interval), resolve_sources, NULL);
+  } else {
+    resolving_interval = 0;
+  }
+}
+
+/* ================================================== */
+
+/* Procedure to add a new server or peer source, but instead of an IP address
+   only a name is provided */
+void
+NSR_AddUnresolvedSource(char *name, int port, NTP_Source_Type type, SourceParameters *params)
+{
+  struct UnresolvedSource *us, **i;
+
+  us = MallocNew(struct UnresolvedSource);
+
+  us->name = name;
+  us->port = port;
+  us->type = type;
+  us->params = *params;
+  us->next = NULL;
+
+  for (i = &unresolved_sources; *i; i = &(*i)->next)
+    ;
+  *i = us;
+
+  if (!resolving_interval) {
+    resolving_interval = 2;
+    resolving_id = SCH_AddTimeoutByDelay(7 * (1 << resolving_interval), resolve_sources, NULL);
+  }
+}
+
+/* ================================================== */
+
 /* Procedure to remove a source.  We don't bother whether the port
    address is matched - we're only interested in removing a record for
    the right IP address.  Thus the caller can specify the port number
@@ -311,6 +396,13 @@ NSR_TakeSourcesOnline(IPAddr *mask, IPAddr *address)
     }
   }
 
+  if (resolving_interval) {
+    /* Try to resolve any unresolved sources now */
+    SCH_RemoveTimeout(resolving_id);
+    resolving_interval--;
+    resolve_sources(NULL);
+  }
+
   return any;
 }
 
@@ -472,6 +564,7 @@ void
 NSR_GetActivityReport(RPT_ActivityReport *report)
 {
   int i;
+  struct UnresolvedSource *us;
 
   report->online = 0;
   report->offline = 0;
@@ -484,6 +577,12 @@ NSR_GetActivityReport(RPT_ActivityReport *report)
                                     &report->burst_online, &report->burst_offline);
     }
   }
+
+  /* Add unresolved sources to offline count */
+  for (us = unresolved_sources; us; us = us->next) {
+    report->offline++;
+  }
+
   return;
 }