webdev/lib/src/serve/utils.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// @dart = 2.9

import 'dart:async';
import 'dart:io';

/// Returns a port that is probably, but not definitely, not in use.
///
/// This has a built-in race condition: another process may bind this port at
/// any time after this call has returned.
Future<int> findUnusedPort() async {
  int port;
  ServerSocket socket;
  try {
    socket = await ServerSocket.bind(InternetAddress.loopbackIPv6, 0);
  } on SocketException {
    socket = await ServerSocket.bind(InternetAddress.loopbackIPv4, 0);
  }
  port = socket.port;
  await socket.close();
  return port;
}