Use listen to force a connection timeout

network/transports_test.go

@@ -111,8 +111,20 @@ func testDialWithBackoffConnectionRefused(tlsConf *tls.Config) func(t *testing.T
 
 func testDialWithBackoffTimeout(tlsConf *tls.Config) func(t *testing.T) {
 	return func(t *testing.T) {
-		// Timeout. Use non-routable IP. See: https://stackoverflow.com/a/31581323/844449
-		c, err := dialer(context.TODO(), tlsConf)("10.0.0.0:81")
+		// Create a listening socket with backlog 1, then occupy the backlog to force a timeout.
+		closer, addr, err := listenOne()
+		if err != nil {
+			t.Fatal("Unable to create listener:", err)
+		}
+		defer closer()
+		c1, err := net.Dial("tcp4", addr.String())
+		if err != nil {
+			t.Fatalf("Unable to connect to server on %s: %s", addr, err)
+		}
+		defer c1.Close()
+
+		// Since the backlog is full, the next request must time out.
+		c, err := dialer(context.TODO(), tlsConf)(addr.String())
 		if err == nil {
 			closeOrFail(t, c)
 			t.Error("Unexpected success dialing")
@@ -193,3 +205,34 @@ func findUnusedPortOrFail(tb testing.TB) int {
 	defer closeOrFail(tb, l)
 	return l.Addr().(*net.TCPAddr).Port
 }
+
+// Golang doesn't allow us to set the backlog argument on syscall.Listen from
+// net.ListenTCP, so we need to get directly into syscall land.
+func listenOne() (func(), *net.TCPAddr, error) {
+	fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_STREAM, 0)
+	if err != nil {
+		return nil, nil, fmt.Errorf("Couldn't get socket: %w", err)
+	}
+	sa := &syscall.SockaddrInet4{
+		Port: 0,
+		Addr: [4]byte{127, 0, 0, 1},
+	}
+	if err = syscall.Bind(fd, sa); err != nil {
+		return nil, nil, fmt.Errorf("Unable to bind: %w", err)
+	}
+	if err = syscall.Listen(fd, 0); err != nil {
+		return nil, nil, fmt.Errorf("Unable to Listen: %w", err)
+	}
+	closer := func() { syscall.Close(fd) }
+	listenaddr, err := syscall.Getsockname(fd)
+	if err != nil {
+		closer()
+		return nil, nil, fmt.Errorf("Could not get sockname: %w", err)
+	}
+	sa = listenaddr.(*syscall.SockaddrInet4)
+	addr := &net.TCPAddr{
+		IP:   sa.Addr[:],
+		Port: sa.Port,
+	}
+	return closer, addr, nil
+}