tsnet: avoid deadlock on close

tsnet.Server.Close was calling listener.Close with the server mutex
held, but the listener close method tries to grab that mutex, resulting
in a deadlock.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

tsnet/tsnet.go

@@ -349,12 +349,17 @@ func (s *Server) Close() error {
 		s.loopbackListener.Close()
 	}
 
+	var lns []*listener
+
 	s.mu.Lock()
-	defer s.mu.Unlock()
 	for _, ln := range s.listeners {
+		lns = append(lns, ln)
+	}
+	s.mu.Unlock()
+
+	for _, ln := range lns {
 		ln.Close()
 	}
-	s.listeners = nil
 
 	wg.Wait()
 	return nil
@@ -997,10 +1002,11 @@ type listenKey struct {
 }
 
 type listener struct {
-	s    *Server
-	keys []listenKey
-	addr string
-	conn chan net.Conn
+	s      *Server
+	keys   []listenKey
+	addr   string
+	conn   chan net.Conn
+	closed bool // guarded by s.mu
 }
 
 func (ln *listener) Accept() (net.Conn, error) {
@@ -1015,12 +1021,16 @@ func (ln *listener) Addr() net.Addr { return addr{ln} }
 func (ln *listener) Close() error {
 	ln.s.mu.Lock()
 	defer ln.s.mu.Unlock()
+	if ln.closed {
+		return fmt.Errorf("tsnet: %w", net.ErrClosed)
+	}
 	for _, key := range ln.keys {
 		if v, ok := ln.s.listeners[key]; ok && v == ln {
 			delete(ln.s.listeners, key)
 		}
 	}
 	close(ln.conn)
+	ln.closed = true
 	return nil
 }
 

tsnet/tsnet_test.go

@@ -9,6 +9,7 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"net/netip"
@@ -344,3 +345,24 @@ func TestTailscaleIPs(t *testing.T) {
 			sIp4, upIp4, sIp6, upIp6)
 	}
 }
+
+func TestListenerCleanup(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	controlURL := startControl(t)
+	s1, _ := startServer(t, ctx, controlURL, "s1")
+
+	ln, err := s1.Listen("tcp", ":8081")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := s1.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := ln.Close(); !errors.Is(err, net.ErrClosed) {
+		t.Fatalf("second ln.Close error: %v, want net.ErrClosed", err)
+	}
+}