Set the recursion available flag

go mod tidy

internal/dns/dns.go

@@ -172,43 +172,40 @@ func (m measurement) String() string {
 }
 
 func (s *Server) probeUpstreamLatency() {
-	if !s.once {
+	upstreams := s.upstreams()
-		s.once = true
+	results := make([]measurement, len(upstreams))
-		upstreams := s.upstreams()
+	var wg sync.WaitGroup
-		results := make([]measurement, len(upstreams))
+	for idx, u := range upstreams {
-		var wg sync.WaitGroup
+		wg.Add(1)
-		for idx, u := range upstreams {
+		go func(idx int, u string) {
-			wg.Add(1)
+			defer wg.Done()
-			go func(idx int, u string) {
+			// resolve a most-definitely cached record
-				defer wg.Done()
+			m := new(dns.Msg)
-				// resolve a most-definitely cached record
+			m.SetQuestion("google.ch.", dns.TypeA)
-				m := new(dns.Msg)
+			start := time.Now()
-				m.SetQuestion("google.ch.", dns.TypeA)
+			_, _, err := s.client.Exchange(m, u)
-				start := time.Now()
+			rtt := time.Since(start)
-				_, _, err := s.client.Exchange(m, u)
+			if err != nil {
-				rtt := time.Since(start)
+				// including unresponsive upstreams in results makes the update
-				if err != nil {
+				// code simpler:
-					// including unresponsive upstreams in results makes the update
+				results[idx] = measurement{u, time.Duration(math.MaxInt64)}
-					// code simpler:
+				return
-					results[idx] = measurement{u, time.Duration(math.MaxInt64)}
+			}
-					return
+			results[idx] = measurement{u, rtt}
-				}
+		}(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) {