Set the recursion available flag

go mod tidy

internal/dns/dns.go

@@ -172,40 +172,43 @@ func (m measurement) String() string {
 }
 
 func (s *Server) probeUpstreamLatency() {
-	upstreams := s.upstreams()
+	if !s.once {
-	results := make([]measurement, len(upstreams))
+		s.once = true
-	var wg sync.WaitGroup
+		upstreams := s.upstreams()
-	for idx, u := range upstreams {
+		results := make([]measurement, len(upstreams))
-		wg.Add(1)
+		var wg sync.WaitGroup
-		go func(idx int, u string) {
+		for idx, u := range upstreams {
-			defer wg.Done()
+			wg.Add(1)
-			// resolve a most-definitely cached record
+			go func(idx int, u string) {
-			m := new(dns.Msg)
+				defer wg.Done()
-			m.SetQuestion("google.ch.", dns.TypeA)
+				// resolve a most-definitely cached record
-			start := time.Now()
+				m := new(dns.Msg)
-			_, _, err := s.client.Exchange(m, u)
+				m.SetQuestion("google.ch.", dns.TypeA)
-			rtt := time.Since(start)
+				start := time.Now()
-			if err != nil {
+				_, _, err := s.client.Exchange(m, u)
-				// including unresponsive upstreams in results makes the update
+				rtt := time.Since(start)
-				// code simpler:
+				if err != nil {
-				results[idx] = measurement{u, time.Duration(math.MaxInt64)}
+					// including unresponsive upstreams in results makes the update
-				return
+					// code simpler:
-			}
+					results[idx] = measurement{u, time.Duration(math.MaxInt64)}
-			results[idx] = measurement{u, rtt}
+					return
-		}(idx, u)
+				}
+				results[idx] = measurement{u, rtt}
+			}(idx, u)
+		}
+		wg.Wait()
+		// Re-order by resolving latency:
+		sort.Slice(results, func(i, j int) bool {
+			return results[i].rtt < results[j].rtt
+		})
+		log.Printf("probe results: %v %v", s.once, results)
+		for idx, result := range results {
+			upstreams[idx] = result.upstream
+		}
+		s.upstreamMu.Lock()
+		defer s.upstreamMu.Unlock()
+		s.upstream = upstreams
 	}
-	wg.Wait()
-	// Re-order by resolving latency:
-	sort.Slice(results, func(i, j int) bool {
-		return results[i].rtt < results[j].rtt
-	})
-	log.Printf("probe results: %v", results)
-	for idx, result := range results {
-		upstreams[idx] = result.upstream
-	}
-	s.upstreamMu.Lock()
-	defer s.upstreamMu.Unlock()
-	s.upstream = upstreams
 }
 
 func (s *Server) hostByName(n lcHostname) (string, bool) {