From 9542109d66370e25c0af90042423c950e6f2ba4f Mon Sep 17 00:00:00 2001
From: Frank Denis <github@pureftpd.org>
Date: Thu, 26 Mar 2020 12:53:22 +0100
Subject: [PATCH] Cancel dnsExchange goroutines as soon as we have a best
 response

---
 dnscrypt-proxy/dnscrypt_certs.go | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/dnscrypt-proxy/dnscrypt_certs.go b/dnscrypt-proxy/dnscrypt_certs.go
index 505dd6db..ed9aecc4 100644
--- a/dnscrypt-proxy/dnscrypt_certs.go
+++ b/dnscrypt-proxy/dnscrypt_certs.go
@@ -193,31 +193,42 @@ type dnsExchangeResponse struct {
 
 func dnsExchange(proxy *Proxy, proto string, query *dns.Msg, serverAddress string, relayUDPAddr *net.UDPAddr, relayTCPAddr *net.TCPAddr, serverName *string, tryFragmentsSupport bool) (*dns.Msg, time.Duration, bool, error) {
 	for {
+		cancelChannel := make(chan struct{})
 		channel := make(chan dnsExchangeResponse)
 		var err error
 		options := 0
 
-		for tries := 2; tries >= 0; tries-- {
+		for tries := 0; tries < 3; tries++ {
 			if tryFragmentsSupport {
 				queryCopy := query.Copy()
 				queryCopy.Id += uint16(options)
 				go func(query *dns.Msg, delay time.Duration) {
-					time.Sleep(delay)
 					option := _dnsExchange(proxy, proto, query, serverAddress, relayUDPAddr, relayTCPAddr, 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) {
-				time.Sleep(delay)
 				option := _dnsExchange(proxy, proto, query, serverAddress, relayUDPAddr, relayTCPAddr, 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++
 		}
@@ -228,6 +239,7 @@ func dnsExchange(proxy *Proxy, proto string, query *dns.Msg, serverAddress strin
 					(dnsExchangeResponse.priority == bestOption.priority && dnsExchangeResponse.rtt < bestOption.rtt) {
 					bestOption = &dnsExchangeResponse
 					if bestOption.priority == 0 {
+						close(cancelChannel)
 						break
 					}
 				}