Optimize stopping CaptivePortalHandler - 2 (#2155)

1. Fix early return that triggers port rebinding error by 8e46f447.
2. Reduce waiting time while there are multiple listen_addresses.

dnscrypt-proxy/coldstart.go

@@ -15,13 +15,17 @@ type CaptivePortalEntryIPs []net.IP
 type CaptivePortalMap map[string]CaptivePortalEntryIPs
 
 type CaptivePortalHandler struct {
-	cancelChannels []chan struct{}
+	cancelChannel chan struct{}
+	countChannel  chan struct{}
+	channelCount  int
 }
 
 func (captivePortalHandler *CaptivePortalHandler) Stop() {
-	for _, cancelChannel := range captivePortalHandler.cancelChannels {
-		close(cancelChannel)
+	close(captivePortalHandler.cancelChannel)
+	for len(captivePortalHandler.countChannel) < captivePortalHandler.channelCount {
+		time.Sleep(time.Millisecond)
 	}
+	close(captivePortalHandler.countChannel)
 }
 
 func (ipsMap *CaptivePortalMap) GetEntry(msg *dns.Msg) (*dns.Question, *CaptivePortalEntryIPs) {
@@ -119,7 +123,7 @@ func addColdStartListener(
 	proxy *Proxy,
 	ipsMap *CaptivePortalMap,
 	listenAddrStr string,
-	cancelChannel chan struct{},
+	captivePortalHandler *CaptivePortalHandler,
 ) error {
 	listenUDPAddr, err := net.ResolveUDPAddr("udp", listenAddrStr)
 	if err != nil {
@@ -130,9 +134,10 @@ func addColdStartListener(
 		return err
 	}
 	go func() {
-		for !handleColdStartClient(clientPc, cancelChannel, ipsMap) {
+		for !handleColdStartClient(clientPc, captivePortalHandler.cancelChannel, ipsMap) {
 		}
 		clientPc.Close()
+		captivePortalHandler.countChannel <- struct{}{}
 	}()
 	return nil
 }
@@ -178,15 +183,15 @@ func ColdStart(proxy *Proxy) (*CaptivePortalHandler, error) {
 		ipsMap[name] = ips
 	}
 	listenAddrStrs := proxy.listenAddresses
-	cancelChannels := make([]chan struct{}, 0)
-	for _, listenAddrStr := range listenAddrStrs {
-		cancelChannel := make(chan struct{})
-		if err := addColdStartListener(proxy, &ipsMap, listenAddrStr, cancelChannel); err == nil {
-			cancelChannels = append(cancelChannels, cancelChannel)
-		}
-	}
 	captivePortalHandler := CaptivePortalHandler{
-		cancelChannels: cancelChannels,
+		cancelChannel: make(chan struct{}),
+		countChannel:  make(chan struct{}, len(listenAddrStrs)),
+		channelCount:  0,
+	}
+	for _, listenAddrStr := range listenAddrStrs {
+		if err := addColdStartListener(proxy, &ipsMap, listenAddrStr, &captivePortalHandler); err == nil {
+			captivePortalHandler.channelCount++
+		}
 	}
 	proxy.captivePortalMap = &ipsMap
 	return &captivePortalHandler, nil