tsnet: fix deadlock in Server.Close during shutdown

Server.Close held s.mu for the entire shutdown duration, including
netstack.Close (which waits for gVisor goroutines to exit) and
lb.Shutdown. gVisor callbacks like getTCPHandlerForFlow acquire s.mu via
listenerForDstAddr, so any in-flight gVisor goroutine attempting that
callback during stack shutdown would deadlock with Close.

Replace the mu-guarded closed bool with a sync.Once, and release s.mu
after closing listeners but before the heavy shutdown operations. Also
cancel shutdownCtx before netstack.Close so pending handlers observe
cancellation rather than contending on the lock.

Updates #18423

Signed-off-by: James Tucker <james@tailscale.com>

tsnet/tsnet.go

@@ -198,7 +198,7 @@ type Server struct {
 	listeners           map[listenKey]*listener
 	fallbackTCPHandlers set.HandleSet[FallbackTCPHandler]
 	dialer              *tsdial.Dialer
-	closed              bool
+	closeOnce           sync.Once
 }
 
 // FallbackTCPHandler describes the callback which
@@ -439,11 +439,29 @@ func (s *Server) Up(ctx context.Context) (*ipnstate.Status, error) {
 //
 // It must not be called before or concurrently with Start.
 func (s *Server) Close() error {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-	if s.closed {
+	didClose := false
+	s.closeOnce.Do(func() {
+		didClose = true
+		s.close()
+	})
+	if !didClose {
 		return fmt.Errorf("tsnet: %w", net.ErrClosed)
 	}
+	return nil
+}
+
+func (s *Server) close() {
+	// Close listeners under s.mu, then release before the heavy shutdown
+	// operations. We must not hold s.mu during netstack.Close, lb.Shutdown,
+	// etc. because callbacks from gVisor (e.g. getTCPHandlerForFlow)
+	// acquire s.mu, and waiting for those goroutines while holding the lock
+	// would deadlock.
+	s.mu.Lock()
+	for _, ln := range s.listeners {
+		ln.closeLocked()
+	}
+	s.mu.Unlock()
+
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 	var wg sync.WaitGroup
@@ -466,13 +484,12 @@ func (s *Server) Close() error {
 		}
 	}()
 
-	if s.netstack != nil {
-		s.netstack.Close()
-		s.netstack = nil
-	}
 	if s.shutdownCancel != nil {
 		s.shutdownCancel()
 	}
+	if s.netstack != nil {
+		s.netstack.Close()
+	}
 	if s.lb != nil {
 		s.lb.Shutdown()
 	}
@@ -489,13 +506,8 @@ func (s *Server) Close() error {
 		s.loopbackListener.Close()
 	}
 
-	for _, ln := range s.listeners {
-		ln.closeLocked()
-	}
 	wg.Wait()
 	s.sys.Bus.Get().Close()
-	s.closed = true
-	return nil
 }
 
 func (s *Server) doInit() {