wgengine/magicsock: add a reSTUN method

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -34,10 +34,12 @@ type Conn struct {
 	stunServers   []string
 	derpServer    string
 	startEpUpdate chan struct{} // send to trigger endpoint update
-	epUpdateCancel func()
 	epFunc        func(endpoints []string)
 	logf          func(format string, args ...interface{})
 
+	epUpdateCtx    context.Context // endpoint updater context
+	epUpdateCancel func()          // the func to cancel epUpdateCtx
+
 	// indexedAddrs is a map of every remote ip:port to a priority
 	// list of endpoint addresses for a peer.
 	// The priority list is provided by wgengine configuration.
@@ -137,6 +139,7 @@ func Listen(opts Options) (*Conn, error) {
 		stunServers:    append([]string{}, opts.STUN...),
 		derpServer:     opts.DERP,
 		startEpUpdate:  make(chan struct{}, 1),
+		epUpdateCtx:    epUpdateCtx,
 		epUpdateCancel: epUpdateCancel,
 		epFunc:         opts.endpointsFunc(),
 		logf:           log.Printf,
@@ -144,7 +147,7 @@ func Listen(opts Options) (*Conn, error) {
 	}
 	c.ignoreSTUNPackets()
 	c.pconn.Reset(packetConn.(*net.UDPConn))
-	c.startEpUpdate <- struct{}{} // STUN immediately on start
+	c.reSTUN()
 	go c.epUpdate(epUpdateCtx)
 	return c, nil
 }
@@ -472,8 +475,7 @@ func (c *Conn) SetPrivateKey(privateKey wgcfg.PrivateKey) error {
 				time.Sleep(250 * time.Millisecond)
 			}
 
-			// Trigger re-STUN.
-			c.startEpUpdate <- struct{}{}
+			c.reSTUN()
 
 			addr := c.pconn.LocalAddr()
 			if _, err := c.pconn.WriteToUDP(b[:n], addr); err != nil {
@@ -501,10 +503,15 @@ func (c *Conn) Close() error {
 	return c.pconn.Close()
 }
 
+func (c *Conn) reSTUN() {
+	select {
+	case c.startEpUpdate <- struct{}{}:
+	case <-c.epUpdateCtx.Done():
+	}
+}
+
 func (c *Conn) LinkChange() {
-	defer func() {
-		c.startEpUpdate <- struct{}{} // re-STUN
-	}()
+	defer c.reSTUN()
 
 	if c.pconnPort != 0 {
 		c.pconn.mu.Lock()