Serialize resolver Start and Stop

Signed-off-by: Santhosh Manohar <santhosh@docker.com>

resolver.go

@@ -74,6 +74,7 @@ type extDNSEntry struct {
 
 // resolver implements the Resolver interface
 type resolver struct {
+	sync.Mutex
 	backend       DNSBackend
 	extDNSList    [maxExtDNS]extDNSEntry
 	server        *dns.Server
@@ -86,6 +87,7 @@ type resolver struct {
 	queryLock     sync.Mutex
 	listenAddress string
 	proxyDNS      bool
+	startCh       chan struct{}
 	resolverKey   string
 }
 
@@ -140,38 +142,67 @@ func (r *resolver) Start() error {
 	if r.err != nil {
 		return r.err
 	}
+	// startCh is to seralize resolver Start and Stop
+	r.Lock()
+	r.startCh = make(chan struct{})
+	r.Unlock()
 
 	if err := r.setupIPTable(); err != nil {
 		return fmt.Errorf("setting up IP table rules failed: %v", err)
 	}
 
 	s := &dns.Server{Handler: r, PacketConn: r.conn}
-	r.server = s
 	go func() {
 		s.ActivateAndServe()
 	}()
 
 	tcpServer := &dns.Server{Handler: r, Listener: r.tcpListen}
-	r.tcpServer = tcpServer
 	go func() {
 		tcpServer.ActivateAndServe()
 	}()
+
+	r.Lock()
+	r.server = s
+	r.tcpServer = tcpServer
+	close(r.startCh)
+	r.startCh = nil
+	r.Unlock()
+
 	return nil
 }
 
-func (r *resolver) Stop() {
-	if r.server != nil {
-		r.server.Shutdown()
-	}
-	if r.tcpServer != nil {
-		r.tcpServer.Shutdown()
+func (r *resolver) waitForStart() {
+	r.Lock()
+	startCh := r.startCh
+	r.Unlock()
+
+	if startCh != nil {
+		<-startCh
 	}
+}
+
+func (r *resolver) Stop() {
+	r.waitForStart()
+
+	r.Lock()
+	udpServer := r.server
+	tcpServer := r.tcpServer
 	r.conn = nil
+	r.tcpListen = nil
+	r.server = nil
 	r.tcpServer = nil
 	r.err = fmt.Errorf("setup not done yet")
 	r.tStamp = time.Time{}
 	r.count = 0
 	r.queryLock = sync.Mutex{}
+	r.Unlock()
+
+	if udpServer != nil {
+		udpServer.Shutdown()
+	}
+	if tcpServer != nil {
+		tcpServer.Shutdown()
+	}
 }
 
 func (r *resolver) SetExtServers(dns []string) {