Fix goroutines memory leak by unbuffered channel blocking (#2136)

* Use buffered channel to avoid goroutine hanging on

A send on an unbuffered channel can proceed if a receiver is ready.

* Balance captivePortalHandler.cancelChannels for Stop

dnscrypt-proxy/coldstart.go

@@ -84,6 +84,7 @@ func handleColdStartClient(clientPc *net.UDPConn, cancelChannel chan struct{}, i
 	exit := false
 	select {
 	case <-cancelChannel:
+		cancelChannel <- struct{}{}
 		exit = true
 	default:
 	}
@@ -95,6 +96,12 @@ func handleColdStartClient(clientPc *net.UDPConn, cancelChannel chan struct{}, i
 	}
 	if err != nil {
 		dlog.Warn(err)
+		select {
+		case <-cancelChannel:
+			cancelChannel <- struct{}{}
+		default:
+			cancelChannel = make(chan struct{}, 1)
+		}
 		return true
 	}
 	packet := buffer[:length]
@@ -134,7 +141,6 @@ func addColdStartListener(
 		for !handleColdStartClient(clientPc, cancelChannel, ipsMap) {
 		}
 		clientPc.Close()
-		cancelChannel <- struct{}{}
 	}()
 	return nil
 }

dnscrypt-proxy/dnsutils.go

@@ -316,7 +316,7 @@ func DNSExchange(
 ) (*dns.Msg, time.Duration, bool, error) {
 	for {
 		cancelChannel := make(chan struct{})
-		channel := make(chan DNSExchangeResponse)
+		channel := make(chan DNSExchangeResponse, 6)
 		var err error
 		options := 0
 
@@ -325,32 +325,32 @@ func DNSExchange(
 				queryCopy := query.Copy()
 				queryCopy.Id += uint16(options)
 				go func(query *dns.Msg, delay time.Duration) {
-					option := _dnsExchange(proxy, proto, query, serverAddress, relay, 1500)
+					time.Sleep(delay)
+					option := DNSExchangeResponse{err: errors.New("Canceled")}
+					select {
+					case <-cancelChannel:
+					default:
+						option = _dnsExchange(proxy, proto, query, serverAddress, relay, 1500)
+					}
 					option.fragmentsBlocked = false
 					option.priority = 0
 					channel <- option
-					time.Sleep(delay)
-					select {
-					case <-cancelChannel:
-						return
-					default:
-					}
 				}(queryCopy, time.Duration(200*tries)*time.Millisecond)
 				options++
 			}
 			queryCopy := query.Copy()
 			queryCopy.Id += uint16(options)
 			go func(query *dns.Msg, delay time.Duration) {
-				option := _dnsExchange(proxy, proto, query, serverAddress, relay, 480)
+				time.Sleep(delay)
+				option := DNSExchangeResponse{err: errors.New("Canceled")}
+				select {
+				case <-cancelChannel:
+				default:
+					option = _dnsExchange(proxy, proto, query, serverAddress, relay, 480)
+				}
 				option.fragmentsBlocked = true
 				option.priority = 1
 				channel <- option
-				time.Sleep(delay)
-				select {
-				case <-cancelChannel:
-					return
-				default:
-				}
 			}(queryCopy, time.Duration(250*tries)*time.Millisecond)
 			options++
 		}