// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

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

import 'package:process/process.dart';

import 'common/logging.dart';
import 'common/network.dart';
import 'dart/dart_vm.dart';
import 'runners/ssh_command_runner.dart';

final String _ipv4Loopback = InternetAddress.loopbackIPv4.address;

final String _ipv6Loopback = InternetAddress.loopbackIPv6.address;

const ProcessManager _processManager = LocalProcessManager();

const Duration _kIsolateFindTimeout = Duration(minutes: 1);

const Duration _kDartVmConnectionTimeout = Duration(seconds: 9);

const Duration _kVmPollInterval = Duration(milliseconds: 1500);

final Logger _log = Logger('FuchsiaRemoteConnection');

/// A function for forwarding ports on the local machine to a remote device.
///
/// Takes a remote `address`, the target device's port, and an optional
/// `interface` and `configFile`. The config file is used primarily for the
/// default SSH port forwarding configuration.
typedef PortForwardingFunction =
    Future<PortForwarder> Function(
      String address,
      int remotePort, [
      String? interface,
      String? configFile,
    ]);

/// The function for forwarding the local machine's ports to a remote Fuchsia
/// device.
///
/// Can be overwritten in the event that a different method is required.
/// Defaults to using SSH port forwarding.
PortForwardingFunction fuchsiaPortForwardingFunction = _SshPortForwarder.start;

/// Sets [fuchsiaPortForwardingFunction] back to the default SSH port forwarding
/// implementation.
void restoreFuchsiaPortForwardingFunction() {
  fuchsiaPortForwardingFunction = _SshPortForwarder.start;
}

/// A general error raised when something fails within a
/// [FuchsiaRemoteConnection].
class FuchsiaRemoteConnectionError extends Error {
  /// Basic constructor outlining the reason for the failure in `message`.
  FuchsiaRemoteConnectionError(this.message);

  /// The reason for the failure.
  final String message;

  @override
  String toString() {
    return '$FuchsiaRemoteConnectionError: $message';
  }
}

/// An enum specifying a Dart VM's state.
enum DartVmEventType {
  /// The Dart VM has started.
  started,

  /// The Dart VM has stopped.
  ///
  /// This can mean either the host machine cannot be connect to, the VM service
  /// has shut down cleanly, or the VM service has crashed.
  stopped,
}

/// An event regarding the Dart VM.
///
/// Specifies the type of the event (whether the VM has started or has stopped),
/// and contains the service port of the VM as well as a URL to connect to it.
class DartVmEvent {
  DartVmEvent._({required this.eventType, required this.servicePort, required this.uri});

  /// The URL used to connect to the Dart VM.
  final Uri uri;

  /// The type of event regarding this instance of the Dart VM.
  final DartVmEventType eventType;

  /// The port on the host machine that the Dart VM service is/was running on.
  final int servicePort;
}

/// Manages a remote connection to a Fuchsia Device.
///
/// Provides affordances to observe and connect to Flutter views, isolates, and
/// perform actions on the Fuchsia device's various VM services.
///
/// This class can be connected to several instances of the Fuchsia device's
/// Dart VM at any given time.
class FuchsiaRemoteConnection {
  FuchsiaRemoteConnection._(this._useIpV6, this._sshCommandRunner) : _pollDartVms = false;

  bool _pollDartVms;
  final List<PortForwarder> _forwardedVmServicePorts = <PortForwarder>[];
  final SshCommandRunner _sshCommandRunner;
  final bool _useIpV6;

  /// A mapping of Dart VM ports (as seen on the target machine), to
  /// [PortForwarder] instances mapping from the local machine to the target
  /// machine.
  final Map<int, PortForwarder> _dartVmPortMap = <int, PortForwarder>{};

  /// Tracks stale ports so as not to reconnect while polling.
  final Set<int> _stalePorts = <int>{};

  /// A broadcast stream that emits events relating to Dart VM's as they update.
  Stream<DartVmEvent> get onDartVmEvent => _onDartVmEvent;
  late Stream<DartVmEvent> _onDartVmEvent;
  final StreamController<DartVmEvent> _dartVmEventController = StreamController<DartVmEvent>();

  /// VM service cache to avoid repeating handshakes across function
  /// calls. Keys a URI to a DartVm connection instance.
  final Map<Uri, DartVm?> _dartVmCache = <Uri, DartVm?>{};

  /// Same as [FuchsiaRemoteConnection.connect] albeit with a provided
  /// [SshCommandRunner] instance.
  static Future<FuchsiaRemoteConnection> connectWithSshCommandRunner(
    SshCommandRunner commandRunner,
  ) async {
    final connection = FuchsiaRemoteConnection._(
      isIpV6Address(commandRunner.address),
      commandRunner,
    );
    await connection._forwardOpenPortsToDeviceServicePorts();

    Stream<DartVmEvent> dartVmStream() {
      Future<void> listen() async {
        while (connection._pollDartVms) {
          await connection._pollVms();
          await Future<void>.delayed(_kVmPollInterval);
        }
        connection._dartVmEventController.close();
      }

      connection._dartVmEventController.onListen = listen;
      return connection._dartVmEventController.stream.asBroadcastStream();
    }

    connection._onDartVmEvent = dartVmStream();
    return connection;
  }

  /// Opens a connection to a Fuchsia device.
  ///
  /// Accepts an `address` to a Fuchsia device, and optionally a `sshConfigPath`
  /// in order to open the associated ssh_config for port forwarding.
  ///
  /// Will throw an [ArgumentError] if `address` is malformed.
  ///
  /// Once this function is called, the instance of [FuchsiaRemoteConnection]
  /// returned will keep all associated DartVM connections opened over the
  /// lifetime of the object.
  ///
  /// At its current state Dart VM connections will not be added or removed over
  /// the lifetime of this object.
  ///
  /// Throws an [ArgumentError] if the supplied `address` is not valid IPv6 or
  /// IPv4.
  ///
  /// If `address` is IPv6 link local (usually starts with `fe80::`), then
  /// `interface` will probably need to be set in order to connect successfully
  /// (that being the outgoing interface of your machine, not the interface on
  /// the target machine).
  ///
  /// Attempts to set `address` via the environment variable
  /// `FUCHSIA_DEVICE_URL` in the event that the argument is not passed.
  /// If `address` is not supplied, `interface` is also ignored, as the format
  /// is expected to contain the interface as well (in the event that it is
  /// link-local), like the following:
  ///
  /// ```none
  /// fe80::1%eth0
  /// ```
  ///
  /// In the event that `FUCHSIA_SSH_CONFIG` is set in the environment, that
  /// will be used when `sshConfigPath` isn't supplied.
  static Future<FuchsiaRemoteConnection> connect([
    String? address,
    String interface = '',
    String? sshConfigPath,
  ]) async {
    address ??= Platform.environment['FUCHSIA_DEVICE_URL'];
    sshConfigPath ??= Platform.environment['FUCHSIA_SSH_CONFIG'];
    if (address == null) {
      throw FuchsiaRemoteConnectionError(
        r'No address supplied, and $FUCHSIA_DEVICE_URL not found.',
      );
    }
    const interfaceDelimiter = '%';
    if (address.contains(interfaceDelimiter)) {
      final List<String> addressAndInterface = address.split(interfaceDelimiter);
      address = addressAndInterface[0];
      interface = addressAndInterface[1];
    }

    return FuchsiaRemoteConnection.connectWithSshCommandRunner(
      SshCommandRunner(address: address, interface: interface, sshConfigPath: sshConfigPath),
    );
  }

  /// Closes all open connections.
  ///
  /// Any objects that this class returns (including any child objects from
  /// those objects) will subsequently have its connection closed as well, so
  /// behavior for them will be undefined.
  Future<void> stop() async {
    for (final PortForwarder pf in _forwardedVmServicePorts) {
      // Closes VM service first to ensure that the connection is closed cleanly
      // on the target before shutting down the forwarding itself.
      final Uri uri = _getDartVmUri(pf);
      final DartVm? vmService = _dartVmCache[uri];
      _dartVmCache[uri] = null;
      await vmService?.stop();
      await pf.stop();
    }
    for (final PortForwarder pf in _dartVmPortMap.values) {
      final Uri uri = _getDartVmUri(pf);
      final DartVm? vmService = _dartVmCache[uri];
      _dartVmCache[uri] = null;
      await vmService?.stop();
      await pf.stop();
    }
    _dartVmCache.clear();
    _forwardedVmServicePorts.clear();
    _dartVmPortMap.clear();
    _pollDartVms = false;
  }

  /// Helper method for [getMainIsolatesByPattern].
  ///
  /// Called when either there are no Isolates that exist that match
  /// `pattern`, or there are not yet any active Dart VM's on the system
  /// (possible when the Isolate we're attempting to connect to is in the only
  /// instance of the Dart VM and its service port has not yet opened).
  Future<List<IsolateRef>> _waitForMainIsolatesByPattern([
    Pattern? pattern,
    Duration timeout = _kIsolateFindTimeout,
    Duration vmConnectionTimeout = _kDartVmConnectionTimeout,
  ]) async {
    final completer = Completer<List<IsolateRef>>();
    _onDartVmEvent.listen(
      (DartVmEvent event) async {
        if (event.eventType == DartVmEventType.started) {
          _log.fine(
            'New VM found on port: ${event.servicePort}. Searching '
            'for Isolate: $pattern',
          );
          final DartVm? vmService = await _getDartVm(event.uri);
          // If the VM service is null, set the result to the empty list.
          final List<IsolateRef> result =
              await vmService?.getMainIsolatesByPattern(pattern!) ?? <IsolateRef>[];
          if (result.isNotEmpty) {
            if (!completer.isCompleted) {
              completer.complete(result);
            } else {
              _log.warning(
                'Found more than one Dart VM containing Isolates '
                'that match the pattern "$pattern".',
              );
            }
          }
        }
      },
      onDone: () {
        if (!completer.isCompleted) {
          _log.warning(
            'Terminating isolate search for "$pattern"'
            ' before timeout reached.',
          );
        }
      },
    );
    return completer.future.timeout(timeout);
  }

  /// Returns all Isolates running `main()` as matched by the [Pattern].
  ///
  /// If there are no live Dart VM's or the Isolate cannot be found, waits until
  /// either `timeout` is reached, or a Dart VM starts up with a name that
  /// matches `pattern`.
  Future<List<IsolateRef>> getMainIsolatesByPattern(
    Pattern pattern, {
    Duration timeout = _kIsolateFindTimeout,
    Duration vmConnectionTimeout = _kDartVmConnectionTimeout,
  }) async {
    // If for some reason there are no Dart VM's that are alive, wait for one to
    // start with the Isolate in question.
    if (_dartVmPortMap.isEmpty) {
      _log.fine('No live Dart VMs found. Awaiting new VM startup');
      return _waitForMainIsolatesByPattern(pattern, timeout, vmConnectionTimeout);
    }
    // Accumulate a list of eventual IsolateRef lists so that they can be loaded
    // simultaneously via Future.wait.
    final isolates = <Future<List<IsolateRef>>>[];
    for (final PortForwarder fp in _dartVmPortMap.values) {
      final DartVm? vmService = await _getDartVm(_getDartVmUri(fp), timeout: vmConnectionTimeout);
      if (vmService == null) {
        continue;
      }
      isolates.add(vmService.getMainIsolatesByPattern(pattern));
    }
    final List<IsolateRef> result = await Future.wait<List<IsolateRef>>(isolates)
        .timeout(timeout)
        .then<List<IsolateRef>>((List<List<IsolateRef>> listOfLists) {
          final mutableListOfLists = List<List<IsolateRef>>.from(listOfLists)
            ..retainWhere((List<IsolateRef> list) => list.isNotEmpty);
          // Folds the list of lists into one flat list.
          return mutableListOfLists.fold<List<IsolateRef>>(<IsolateRef>[], (
            List<IsolateRef> accumulator,
            List<IsolateRef> element,
          ) {
            accumulator.addAll(element);
            return accumulator;
          });
        });

    // If no VM instance anywhere has this, it's possible it hasn't spun up
    // anywhere.
    //
    // For the time being one Flutter Isolate runs at a time in each VM, so for
    // now this will wait until the timer runs out or a new Dart VM starts that
    // contains the Isolate in question.
    //
    // TODO(awdavies): Set this up to handle multiple Isolates per Dart VM.
    if (result.isEmpty) {
      _log.fine('No instance of the Isolate found. Awaiting new VM startup');
      return _waitForMainIsolatesByPattern(pattern, timeout, vmConnectionTimeout);
    }
    return result;
  }

  /// Returns a list of [FlutterView] objects.
  ///
  /// This is run across all connected Dart VM connections that this class is
  /// managing.
  Future<List<FlutterView>> getFlutterViews() async {
    if (_dartVmPortMap.isEmpty) {
      return <FlutterView>[];
    }
    final List<List<FlutterView>> flutterViewLists = await _invokeForAllVms<List<FlutterView>>((
      DartVm vmService,
    ) async {
      return vmService.getAllFlutterViews();
    });
    final List<FlutterView> results = flutterViewLists.fold<List<FlutterView>>(<FlutterView>[], (
      List<FlutterView> acc,
      List<FlutterView> element,
    ) {
      acc.addAll(element);
      return acc;
    });
    return List<FlutterView>.unmodifiable(results);
  }

  // Calls all Dart VM's, returning a list of results.
  //
  // A side effect of this function is that internally tracked port forwarding
  // will be updated in the event that ports are found to be broken/stale: they
  // will be shut down and removed from tracking.
  Future<List<E>> _invokeForAllVms<E>(
    Future<E> Function(DartVm vmService) vmFunction, [
    bool queueEvents = true,
  ]) async {
    final result = <E>[];

    // Helper function loop.
    Future<void> shutDownPortForwarder(PortForwarder pf) async {
      await pf.stop();
      _stalePorts.add(pf.remotePort);
      if (queueEvents) {
        _dartVmEventController.add(
          DartVmEvent._(
            eventType: DartVmEventType.stopped,
            servicePort: pf.remotePort,
            uri: _getDartVmUri(pf),
          ),
        );
      }
    }

    for (final PortForwarder pf in _dartVmPortMap.values) {
      final DartVm? service = await _getDartVm(_getDartVmUri(pf));
      if (service == null) {
        await shutDownPortForwarder(pf);
      } else {
        result.add(await vmFunction(service));
      }
    }
    _stalePorts.forEach(_dartVmPortMap.remove);
    return result;
  }

  Uri _getDartVmUri(PortForwarder pf) {
    String? addr;
    if (pf.openPortAddress == null) {
      addr = _useIpV6 ? '[$_ipv6Loopback]' : _ipv4Loopback;
    } else {
      addr = isIpV6Address(pf.openPortAddress!) ? '[${pf.openPortAddress}]' : pf.openPortAddress;
    }
    final uri = Uri.http('$addr:${pf.port}', '/');
    return uri;
  }

  /// Attempts to create a connection to a Dart VM.
  ///
  /// Returns null if either there is an [HttpException] or a
  /// [TimeoutException], else a [DartVm] instance.
  Future<DartVm?> _getDartVm(Uri uri, {Duration timeout = _kDartVmConnectionTimeout}) async {
    if (!_dartVmCache.containsKey(uri)) {
      // When raising an HttpException this means that there is no instance of
      // the Dart VM to communicate with. The TimeoutException is raised when
      // the Dart VM instance is shut down in the middle of communicating.
      try {
        final DartVm dartVm = await DartVm.connect(uri, timeout: timeout);
        _dartVmCache[uri] = dartVm;
      } on HttpException {
        _log.warning('HTTP Exception encountered connecting to new VM');
        return null;
      } on TimeoutException {
        _log.warning('TimeoutException encountered connecting to new VM');
        return null;
      }
    }
    return _dartVmCache[uri];
  }

  /// Checks for changes in the list of Dart VM instances.
  ///
  /// If there are new instances of the Dart VM, then connections will be
  /// attempted (after clearing out stale connections).
  Future<void> _pollVms() async {
    await _checkPorts();
    final List<int> servicePorts = await getDeviceServicePorts();
    for (final servicePort in servicePorts) {
      if (!_stalePorts.contains(servicePort) && !_dartVmPortMap.containsKey(servicePort)) {
        _dartVmPortMap[servicePort] = await fuchsiaPortForwardingFunction(
          _sshCommandRunner.address,
          servicePort,
          _sshCommandRunner.interface,
          _sshCommandRunner.sshConfigPath,
        );

        _dartVmEventController.add(
          DartVmEvent._(
            eventType: DartVmEventType.started,
            servicePort: servicePort,
            uri: _getDartVmUri(_dartVmPortMap[servicePort]!),
          ),
        );
      }
    }
  }

  /// Runs a dummy heartbeat command on all Dart VM instances.
  ///
  /// Removes any failing ports from the cache.
  Future<void> _checkPorts([bool queueEvents = true]) async {
    // Filters out stale ports after connecting. Ignores results.
    await _invokeForAllVms<void>((DartVm vmService) async {
      await vmService.ping();
    }, queueEvents);
  }

  /// Forwards a series of open ports to the remote device.
  ///
  /// When this function is run, all existing forwarded ports and connections
  /// are reset by way of [stop].
  Future<void> _forwardOpenPortsToDeviceServicePorts() async {
    await stop();
    final List<int> servicePorts = await getDeviceServicePorts();
    final List<PortForwarder?> forwardedVmServicePorts = await Future.wait<PortForwarder?>(
      servicePorts.map<Future<PortForwarder?>>((int deviceServicePort) {
        return fuchsiaPortForwardingFunction(
          _sshCommandRunner.address,
          deviceServicePort,
          _sshCommandRunner.interface,
          _sshCommandRunner.sshConfigPath,
        );
      }),
    );

    for (final pf in forwardedVmServicePorts) {
      // TODO(awdavies): Handle duplicates.
      _dartVmPortMap[pf!.remotePort] = pf;
    }

    // Don't queue events, since this is the initial forwarding.
    await _checkPorts(false);

    _pollDartVms = true;
  }

  /// Helper for getDeviceServicePorts() to extract the vm_service_port from
  /// json response.
  List<int> getVmServicePortFromInspectSnapshot(dynamic inspectSnapshot) {
    final snapshot = List<Map<String, dynamic>>.from(inspectSnapshot as List<dynamic>);
    final ports = <int>[];

    for (final item in snapshot) {
      if (!item.containsKey('payload') || item['payload'] == null) {
        continue;
      }
      final payload = Map<String, dynamic>.from(item['payload'] as Map<String, dynamic>);

      if (!payload.containsKey('root') || payload['root'] == null) {
        continue;
      }
      final root = Map<String, dynamic>.from(payload['root'] as Map<String, dynamic>);

      if (!root.containsKey('vm_service_port') || root['vm_service_port'] == null) {
        continue;
      }

      final int? port = int.tryParse(root['vm_service_port'] as String);
      if (port != null) {
        ports.add(port);
      }
    }
    return ports;
  }

  /// Gets the open Dart VM service ports on a remote Fuchsia device.
  ///
  /// The method attempts to get service ports through an SSH connection. Upon
  /// successfully getting the VM service ports, returns them as a list of
  /// integers. If an empty list is returned, then no Dart VM instances could be
  /// found. An exception is thrown in the event of an actual error when
  /// attempting to acquire the ports.
  Future<List<int>> getDeviceServicePorts() async {
    final List<String> inspectResult = await _sshCommandRunner.run(
      "iquery --format json show '**:root:vm_service_port'",
    );
    final dynamic inspectOutputJson = jsonDecode(inspectResult.join('\n'));
    final List<int> ports = getVmServicePortFromInspectSnapshot(inspectOutputJson);

    if (ports.length > 1) {
      throw StateError('More than one Dart VM service port found');
    }
    return ports;
  }
}

/// Defines an interface for port forwarding.
///
/// When a port forwarder is initialized, it is intended to save a port through
/// which a connection is persisted along the lifetime of this object.
///
/// To shut down a port forwarder you must call the [stop] function.
abstract class PortForwarder {
  /// Determines the port which is being forwarded.
  int get port;

  /// The address on which the open port is accessible. Defaults to null to
  /// indicate local loopback.
  String? get openPortAddress => null;

  /// The destination port on the other end of the port forwarding tunnel.
  int get remotePort;

  /// Shuts down and cleans up port forwarding.
  Future<void> stop();
}

/// Instances of this class represent a running SSH tunnel.
///
/// The SSH tunnel is from the host to a VM service running on a Fuchsia device.
class _SshPortForwarder implements PortForwarder {
  _SshPortForwarder._(
    this._remoteAddress,
    this._remotePort,
    this._localSocket,
    this._interface,
    this._sshConfigPath,
    this._ipV6,
  );

  final String _remoteAddress;
  final int _remotePort;
  final ServerSocket _localSocket;
  final String? _sshConfigPath;
  final String? _interface;
  final bool _ipV6;

  @override
  int get port => _localSocket.port;

  @override
  String get openPortAddress => _ipV6 ? _ipv6Loopback : _ipv4Loopback;

  @override
  int get remotePort => _remotePort;

  /// Starts SSH forwarding through a subprocess, and returns an instance of
  /// [_SshPortForwarder].
  static Future<_SshPortForwarder> start(
    String address,
    int remotePort, [
    String? interface,
    String? sshConfigPath,
  ]) async {
    final bool isIpV6 = isIpV6Address(address);
    final ServerSocket? localSocket = await _createLocalSocket();
    if (localSocket == null || localSocket.port == 0) {
      _log.warning(
        '_SshPortForwarder failed to find a local port for '
        '$address:$remotePort',
      );
      throw StateError('Unable to create a socket or no available ports.');
    }
    // TODO(awdavies): The square-bracket enclosure for using the IPv6 loopback
    // didn't appear to work, but when assigning to the IPv4 loopback device,
    // netstat shows that the local port is actually being used on the IPv6
    // loopback (::1). Therefore, while the IPv4 loopback can be used for
    // forwarding to the destination IPv6 interface, when connecting to the
    // websocket, the IPV6 loopback should be used.
    final formattedForwardingUrl = '${localSocket.port}:$_ipv4Loopback:$remotePort';
    final targetAddress = isIpV6 && interface!.isNotEmpty ? '$address%$interface' : address;
    const dummyRemoteCommand = 'true';
    final command = <String>[
      'ssh',
      if (isIpV6) '-6',
      if (sshConfigPath != null) ...<String>['-F', sshConfigPath],
      '-nNT',
      '-f',
      '-L',
      formattedForwardingUrl,
      targetAddress,
      dummyRemoteCommand,
    ];
    _log.fine("_SshPortForwarder running '${command.join(' ')}'");
    // Must await for the port forwarding function to completer here, as
    // forwarding must be completed before surfacing VM events (as the user may
    // want to connect immediately after an event is surfaced).
    final ProcessResult processResult = await _processManager.run(command);
    _log.fine(
      "'${command.join(' ')}' exited with exit code "
      '${processResult.exitCode}',
    );
    if (processResult.exitCode != 0) {
      throw StateError('Unable to start port forwarding');
    }
    final result = _SshPortForwarder._(
      address,
      remotePort,
      localSocket,
      interface,
      sshConfigPath,
      isIpV6,
    );
    _log.fine(
      'Set up forwarding from ${localSocket.port} '
      'to $address port $remotePort',
    );
    return result;
  }

  /// Kills the SSH forwarding command, then to ensure no ports are forwarded,
  /// runs the SSH 'cancel' command to shut down port forwarding completely.
  @override
  Future<void> stop() async {
    // Cancel the forwarding request. See [start] for commentary about why this
    // uses the IPv4 loopback.
    final formattedForwardingUrl = '${_localSocket.port}:$_ipv4Loopback:$_remotePort';
    final String targetAddress = _ipV6 && _interface!.isNotEmpty
        ? '$_remoteAddress%$_interface'
        : _remoteAddress;
    final String? sshConfigPath = _sshConfigPath;
    final command = <String>[
      'ssh',
      if (sshConfigPath != null) ...<String>['-F', sshConfigPath],
      '-O',
      'cancel',
      '-L',
      formattedForwardingUrl,
      targetAddress,
    ];
    _log.fine('Shutting down SSH forwarding with command: ${command.join(' ')}');
    final ProcessResult result = await _processManager.run(command);
    if (result.exitCode != 0) {
      _log.warning(
        'Command failed:\nstdout: ${result.stdout}'
        '\nstderr: ${result.stderr}',
      );
    }
    _localSocket.close();
  }

  /// Attempts to find an available port.
  ///
  /// If successful returns a valid [ServerSocket] (which must be disconnected
  /// later).
  static Future<ServerSocket?> _createLocalSocket() async {
    try {
      return await ServerSocket.bind(_ipv4Loopback, 0);
    } catch (e) {
      // We should not be catching all errors arbitrarily here, this might hide real errors.
      // TODO(ianh): Determine which exceptions to catch here.
      _log.warning('_createLocalSocket failed: $e');
      return null;
    }
  }
}
