Reduce UDP server memory usage (#735)

* Clear the response.msg field after unpacking

The allocated buffer cannot be freed by the garbage collector while the
response is alive, by clearing msg here, the GC can collect the buffer
sooner.

* Use a sync.Pool for UDP message buffers

* Return UDP message buffer to pool in all paths

* Move udpPool.New closure out of (*Server).init

The closure used to capture the *Server which would cause a reference
loop and prevent it from ever being released by the garbage collector.

This also gives the closure a more obvious name in memory profiles:
  github.com/miekg/dns.makeUDPBuffer.func1 rather than
  github.com/miekg/dns.(*Server).init.func1.

server.go

@@ -320,6 +320,9 @@ type Server struct {
 	// Shutdown handling
 	lock    sync.RWMutex
 	started bool
+
+	// A pool for UDP message buffers.
+	udpPool sync.Pool
 }
 
 func (srv *Server) isStarted() bool {
@@ -374,6 +377,22 @@ func (srv *Server) spawnWorker(w *response) {
 	}
 }
 
+func makeUDPBuffer(size int) func() interface{} {
+	return func() interface{} {
+		return make([]byte, size)
+	}
+}
+
+func (srv *Server) init() {
+	srv.queue = make(chan *response)
+
+	if srv.UDPSize == 0 {
+		srv.UDPSize = MinMsgSize
+	}
+
+	srv.udpPool.New = makeUDPBuffer(srv.UDPSize)
+}
+
 func unlockOnce(l sync.Locker) func() {
 	var once sync.Once
 	return func() { once.Do(l.Unlock) }
@@ -393,11 +412,10 @@ func (srv *Server) ListenAndServe() error {
 	if addr == "" {
 		addr = ":domain"
 	}
-	if srv.UDPSize == 0 {
-		srv.UDPSize = MinMsgSize
-	}
-	srv.queue = make(chan *response)
+
+	srv.init()
 	defer close(srv.queue)
+
 	switch srv.Net {
 	case "tcp", "tcp4", "tcp6":
 		a, err := net.ResolveTCPAddr(srv.Net, addr)
@@ -459,14 +477,12 @@ func (srv *Server) ActivateAndServe() error {
 		return &Error{err: "server already started"}
 	}
 
+	srv.init()
+	defer close(srv.queue)
+
 	pConn := srv.PacketConn
 	l := srv.Listener
-	srv.queue = make(chan *response)
-	defer close(srv.queue)
 	if pConn != nil {
-		if srv.UDPSize == 0 {
-			srv.UDPSize = MinMsgSize
-		}
 		// Check PacketConn interface's type is valid and value
 		// is not nil
 		if t, ok := pConn.(*net.UDPConn); ok && t != nil {
@@ -565,6 +581,9 @@ func (srv *Server) serveUDP(l *net.UDPConn) error {
 			return err
 		}
 		if len(m) < headerSize {
+			if cap(m) == srv.UDPSize {
+				srv.udpPool.Put(m[:srv.UDPSize])
+			}
 			continue
 		}
 		srv.spawnWorker(&response{msg: m, tsigSecret: srv.TsigSecret, udp: l, udpSession: s})
@@ -630,6 +649,10 @@ func (srv *Server) serve(w *response) {
 func (srv *Server) serveDNS(w *response) {
 	req := new(Msg)
 	err := req.Unpack(w.msg)
+	if w.udp != nil && cap(w.msg) == srv.UDPSize {
+		srv.udpPool.Put(w.msg[:srv.UDPSize])
+	}
+	w.msg = nil
 	if err != nil { // Send a FormatError back
 		x := new(Msg)
 		x.SetRcodeFormatError(req)
@@ -698,9 +721,10 @@ func (srv *Server) readTCP(conn net.Conn, timeout time.Duration) ([]byte, error)
 
 func (srv *Server) readUDP(conn *net.UDPConn, timeout time.Duration) ([]byte, *SessionUDP, error) {
 	conn.SetReadDeadline(time.Now().Add(timeout))
-	m := make([]byte, srv.UDPSize)
+	m := srv.udpPool.Get().([]byte)
 	n, s, err := ReadFromSessionUDP(conn, m)
 	if err != nil {
+		srv.udpPool.Put(m)
 		return nil, nil, err
 	}
 	m = m[:n]