Run Windows watcher in an isolate

pkgs/watcher/CHANGELOG.md

@@ -6,6 +6,10 @@
 - `DirectoryWatcher` on Windows performance: reduce 100ms buffering of events
   before reporting to 5ms, the larger buffer isn't needed for correctness after
   the various fixes.
+- `DirectoryWatcher` on Windows watches in a separate Isolate to make buffer
+  exhaustion, "Directory watcher closed unexpectedly", much less likely. The old
+  implementation which does not use a separate Isolate is available as
+  `DirectoryWatcher(path, runInIsolateOnWindows: false)`.
 - Bug fix: while listing directories skip symlinks that lead to a directory
   that has already been listed. This prevents a severe performance regression on
   MacOS and Linux when there are more than a few symlink loops.

pkgs/watcher/lib/src/directory_watcher.dart

@@ -8,7 +8,7 @@ import '../watcher.dart';
 import 'custom_watcher_factory.dart';
 import 'directory_watcher/linux.dart';
 import 'directory_watcher/mac_os.dart';
-import 'directory_watcher/windows.dart';
+import 'directory_watcher/windows_resubscribable_watcher.dart';
 
 /// Watches the contents of a directory and emits [WatchEvent]s when something
 /// in the directory has changed.
@@ -20,6 +20,8 @@ import 'directory_watcher/windows.dart';
 /// the message "Directory watcher closed unexpectedly" on the event stream. The
 /// code using the watcher needs to do additional work to account for the
 /// dropped events, for example by recomputing interesting files from scratch.
+/// By default, the watcher is started in a separate isolate to make this less
+/// likely. Pass `runInIsolateOnWindows = false` to not launch an isolate.
 ///
 /// On Linux, the underlying SDK `Directory.watch` fails if the system limit on
 /// watchers has been reached. If this happens the SDK exception is thrown, it
@@ -40,7 +42,11 @@ abstract class DirectoryWatcher implements Watcher {
   /// shorter will give more immediate feedback at the expense of doing more IO
   /// and higher CPU usage. Defaults to one second. Ignored for non-polling
   /// watchers.
-  factory DirectoryWatcher(String directory, {Duration? pollingDelay}) {
+  ///
+  /// On Windows, pass [runInIsolateOnWindows] `false` to not run the watcher
+  /// in a separate isolate to reduce buffer exhaustion failures.
+  factory DirectoryWatcher(String directory,
+      {Duration? pollingDelay, bool runInIsolateOnWindows = true}) {
     if (FileSystemEntity.isWatchSupported) {
       var customWatcher = createCustomDirectoryWatcher(
         directory,
@@ -49,7 +55,10 @@ abstract class DirectoryWatcher implements Watcher {
       if (customWatcher != null) return customWatcher;
       if (Platform.isLinux) return LinuxDirectoryWatcher(directory);
       if (Platform.isMacOS) return MacOSDirectoryWatcher(directory);
-      if (Platform.isWindows) return WindowsDirectoryWatcher(directory);
+      if (Platform.isWindows) {
+        return WindowsDirectoryWatcher(directory,
+            runInIsolate: runInIsolateOnWindows);
+      }
     }
     return PollingDirectoryWatcher(directory, pollingDelay: pollingDelay);
   }

pkgs/watcher/lib/src/directory_watcher/directory_list.dart

@@ -142,12 +142,17 @@ class _ResolvedDirectory {
     } on FileSystemException catch (e, s) {
       // The first operation on a directory is to resolve symbolic links, which
       // fails with a general FileSystemException if the file is not found.
-      // Convert that into a PathNotFoundException as that makes more sense
-      // to the caller, who didn't ask for anything to do with symbolic links.
-      if (e.message.contains('Cannot resolve symbolic links') &&
-          e.osError?.errorCode == 2) {
-        throw Error.throwWithStackTrace(
-            PathNotFoundException(directory.path, e.osError!), s);
+      // Convert that into a PathNotFoundException or PathAccessException
+      // as that makes more sense to the caller, who didn't ask for anything to
+      // do with symbolic links.
+      if (e.message.contains('Cannot resolve symbolic links')) {
+        if (e.osError?.errorCode == 2) {
+          throw Error.throwWithStackTrace(
+              PathNotFoundException(directory.path, e.osError!), s);
+        } else if (e.osError?.errorCode == 5) {
+          throw Error.throwWithStackTrace(
+              PathAccessException(directory.path, e.osError!), s);
+        }
       }
       rethrow;
     }

pkgs/watcher/lib/src/directory_watcher/windows.dart

@@ -22,7 +22,8 @@ class WindowsDirectoryWatcher extends ResubscribableWatcher
   String get directory => path;
 
   WindowsDirectoryWatcher(String directory)
-      : super(directory, () => _WindowsDirectoryWatcher(directory));
+      : super(
+            directory, () => WindowsManuallyClosedDirectoryWatcher(directory));
 }
 
 /// Windows directory watcher.
@@ -48,7 +49,7 @@ class WindowsDirectoryWatcher extends ResubscribableWatcher
 /// On my machine, the test failure rate due to the type drops from 150/1000
 /// at 900us to 0/10000 at 1000us. So, 1000us = 1ms is sufficient. Use 5ms to
 /// give a margin for error for different machine performance and load.
-class _WindowsDirectoryWatcher
+class WindowsManuallyClosedDirectoryWatcher
     implements DirectoryWatcher, ManuallyClosedWatcher {
   @override
   String get directory => path;
@@ -94,7 +95,7 @@ class _WindowsDirectoryWatcher
   final Set<StreamSubscription<FileSystemEntity>> _listSubscriptions =
       HashSet<StreamSubscription<FileSystemEntity>>();
 
-  _WindowsDirectoryWatcher(this.path) : _files = PathSet(path) {
+  WindowsManuallyClosedDirectoryWatcher(this.path) : _files = PathSet(path) {
     // Before we're ready to emit events, wait for [_listDir] to complete.
     _listDir().then((_) {
       _startWatch();
@@ -181,7 +182,9 @@ class _WindowsDirectoryWatcher
             event.type == EventType.createDirectory,
         modified: event.type == EventType.modifyFile ||
             event.type == EventType.modifyDirectory,
-        deleted: event.type == EventType.delete,
+        deleted: event.type == EventType.delete ||
+            event.type == EventType.moveFile ||
+            event.type == EventType.moveDirectory,
         movedOnto: false);
     final destination = event.destination;
     if (destination != null) {
@@ -208,13 +211,15 @@ class _WindowsDirectoryWatcher
   void _poll(_PendingPoll poll) {
     final path = poll.path;
     final events = _eventsBasedOnFileSystem(path,
+        reportCreate: poll.created || poll.movedOnto,
+        reportDelete: poll.deleted,
         // A modification can be reported due to a modification event, a
         // create+delete together, or if the path is a move destination.
         // The important case where the file is present, an event arrives
         // for the file and a modification is _not_ reported is when the file
         // was already discovered by listing a new directory, then the "add"
         // event for it is processed afterwards.
-        reportNoChangeAsModification:
+        reportModification:
             poll.modified || (poll.created && poll.deleted) || poll.movedOnto);
 
     for (final event in events) {
@@ -264,10 +269,13 @@ class _WindowsDirectoryWatcher
   /// This returns a list whose order should be reflected in the events emitted
   /// to the user, unlike the batched events from [Directory.watch].
   ///
-  /// [reportNoChangeAsModification] determines whether to report a modification
-  /// if there was a file at [path] and there is still a file at [path].
+  ///
+  /// [reportCreate], [reportModification] and [reportDelete] restrict the types
+  /// of events that can be emitted.
   List<Event> _eventsBasedOnFileSystem(String path,
-      {required bool reportNoChangeAsModification}) {
+      {required bool reportCreate,
+      required bool reportModification,
+      required bool reportDelete}) {
     var fileExisted = _files.contains(path);
     var dirExisted = _files.containsDir(path);
 
@@ -285,26 +293,26 @@ class _WindowsDirectoryWatcher
     var events = <Event>[];
     if (fileExisted) {
       if (fileExists) {
-        if (reportNoChangeAsModification) events.add(Event.modifyFile(path));
+        if (reportModification) events.add(Event.modifyFile(path));
       } else {
-        events.add(Event.delete(path));
+        if (reportDelete) events.add(Event.delete(path));
       }
     } else if (dirExisted) {
       if (dirExists) {
         // If we got contradictory events for a directory that used to exist and
         // still exists, we need to rescan the whole thing in case it was
         // replaced with a different directory.
-        events.add(Event.delete(path));
-        events.add(Event.createDirectory(path));
+        if (reportDelete) events.add(Event.delete(path));
+        if (reportCreate) events.add(Event.createDirectory(path));
       } else {
-        events.add(Event.delete(path));
+        if (reportDelete) events.add(Event.delete(path));
       }
     }
 
     if (!fileExisted && fileExists) {
-      events.add(Event.createFile(path));
+      if (reportCreate) events.add(Event.createFile(path));
     } else if (!dirExisted && dirExists) {
-      events.add(Event.createDirectory(path));
+      if (reportCreate) events.add(Event.createDirectory(path));
     }
 
     return events;

pkgs/watcher/lib/src/directory_watcher/windows_isolate_directory_watcher.dart

@@ -0,0 +1,170 @@
+// Copyright (c) 2025, 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.
+
+import 'dart:async';
+import 'dart:isolate';
+
+import '../resubscribable.dart';
+import '../watch_event.dart';
+import 'windows.dart';
+
+/// Runs [WindowsManuallyClosedDirectoryWatcher] in an isolate to work around
+/// a platform limitation.
+///
+/// On Windows, Directory.watch fails if too many events arrive without being
+/// processed by the Dart VM. See `directory_watcher/windows_test.dart` for code
+/// that reliably triggers the failure by doing file writes in a synchronous
+/// block that prevents the Dart VM from processing the events caused.
+///
+/// Running the watcher in an isolate makes buffer exhaustion much less likely
+/// as there is no unrelated work happening in the isolate that would block
+/// processing of events.
+class WindowsIsolateDirectoryWatcher implements ManuallyClosedWatcher {
+  @override
+  final String path;
+  final ReceivePort _receivePort = ReceivePort();
+  final Completer<SendPort> _sendPortCompleter = Completer();
+
+  final StreamController<WatchEvent> _eventsController =
+      StreamController.broadcast();
+  final Completer<void> _readyCompleter = Completer();
+
+  WindowsIsolateDirectoryWatcher(this.path) {
+    _startIsolate(path, _receivePort.sendPort);
+    _receivePort.listen((event) => _receiveFromIsolate(event as Event));
+  }
+
+  void _receiveFromIsolate(Event event) {
+    switch (event.type) {
+      case EventType.sendPort:
+        _sendPortCompleter.complete(event.sendPort);
+      case EventType.ready:
+        _readyCompleter.complete();
+      case EventType.watchEvent:
+        _eventsController.add(event.watchEvent!);
+      case EventType.close:
+        _eventsController.close();
+        _receivePort.close();
+      case EventType.error:
+        _eventsController.addError(event.error!, event.stackTrace);
+    }
+  }
+
+  @override
+  void close() {
+    // The "close" event is the only event sent to the isolate, just send
+    // `null`.
+    _sendPortCompleter.future.then((sendPort) => sendPort.send(null));
+  }
+
+  @override
+  Stream<WatchEvent> get events => _eventsController.stream;
+
+  @override
+  bool get isReady => _readyCompleter.isCompleted;
+
+  @override
+  Future<void> get ready => _readyCompleter.future;
+}
+
+/// Starts watching [path] in an isolate.
+///
+/// [sendPort] is the port from isolate to host, see `_WatcherIsolate`
+/// constructor implementation for the events that will be sent.
+void _startIsolate(String path, SendPort sendPort) async {
+  unawaited(
+      Isolate.run(() async => await _WatcherIsolate(path, sendPort).closed));
+}
+
+class _WatcherIsolate {
+  final String path;
+  final WindowsManuallyClosedDirectoryWatcher watcher;
+  final SendPort sendPort;
+
+  // The isolate stays open until this future completes.
+  Future<void> get closed => _closeCompleter.future;
+  final Completer<void> _closeCompleter = Completer();
+
+  _WatcherIsolate(this.path, this.sendPort)
+      : watcher = WindowsManuallyClosedDirectoryWatcher(path) {
+    final receivePort = ReceivePort();
+
+    // Six types of event are sent to the host.
+
+    // The `SendPort` for host to isolate communication on startup.
+    sendPort.send(Event.sendPort(receivePort.sendPort));
+
+    // `Event.ready` when the watcher is ready.
+    watcher.ready.then((_) {
+      sendPort.send(Event.ready());
+    });
+
+    watcher.events.listen((event) {
+      // The watcher events.
+      sendPort.send(Event.watchEvent(event));
+    }, onDone: () {
+      // `Event.close` if the watcher event stream closes.
+      sendPort.send(Event.close());
+    }, onError: (Object e, StackTrace s) {
+      // `Event.error` on error.
+      sendPort.send(Event.error(e, s));
+    });
+
+    receivePort.listen((event) {
+      // The only event sent from the host to the isolate is "close", no need
+      // to check the value.
+      watcher.close();
+      _closeCompleter.complete();
+      receivePort.close();
+    });
+  }
+}
+
+/// Event sent from the isolate to the host.
+class Event {
+  final EventType type;
+  final SendPort? sendPort;
+  final WatchEvent? watchEvent;
+  final Object? error;
+  final StackTrace? stackTrace;
+
+  Event.sendPort(this.sendPort)
+      : type = EventType.sendPort,
+        watchEvent = null,
+        error = null,
+        stackTrace = null;
+
+  Event.ready()
+      : type = EventType.ready,
+        sendPort = null,
+        watchEvent = null,
+        error = null,
+        stackTrace = null;
+
+  Event.watchEvent(this.watchEvent)
+      : type = EventType.watchEvent,
+        sendPort = null,
+        error = null,
+        stackTrace = null;
+
+  Event.close()
+      : type = EventType.close,
+        sendPort = null,
+        watchEvent = null,
+        error = null,
+        stackTrace = null;
+
+  Event.error(this.error, this.stackTrace)
+      : type = EventType.error,
+        sendPort = null,
+        watchEvent = null;
+}
+
+enum EventType {
+  sendPort,
+  ready,
+  watchEvent,
+  close,
+  error;
+}

pkgs/watcher/lib/src/directory_watcher/windows_resubscribable_watcher.dart

@@ -0,0 +1,25 @@
+// Copyright (c) 2025, 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.
+
+import '../directory_watcher.dart';
+import '../resubscribable.dart';
+import 'windows.dart';
+import 'windows_isolate_directory_watcher.dart';
+
+class WindowsDirectoryWatcher extends ResubscribableWatcher
+    implements DirectoryWatcher {
+  @override
+  String get directory => path;
+
+  /// Watches [directory].
+  ///
+  /// If [runInIsolate], runs the watcher in an isolate to reduce the chance of
+  /// hitting the Windows-specific buffer exhaustion failure.
+  WindowsDirectoryWatcher(String directory, {bool runInIsolate = true})
+      : super(
+            directory,
+            () => runInIsolate
+                ? WindowsIsolateDirectoryWatcher(directory)
+                : WindowsManuallyClosedDirectoryWatcher(directory));
+}

pkgs/watcher/lib/src/resubscribable.dart

@@ -45,6 +45,7 @@ abstract class ResubscribableWatcher implements Watcher {
 
     _eventsController = StreamController<WatchEvent>.broadcast(
         onListen: () async {
+          final completer = _readyCompleter;
           watcher = _factory();
           subscription = watcher.events.listen(_eventsController.add,
               onError: _eventsController.addError,
@@ -54,7 +55,7 @@ abstract class ResubscribableWatcher implements Watcher {
           // the time [onListen] is called, as opposed to the value when
           // [watcher.ready] fires. A new completer may be created by that time.
           await watcher.ready;
-          _readyCompleter.complete();
+          completer.complete();
         },
         onCancel: () {
           // Cancel the subscription before closing the watcher so that the