Check file sizes as well as "last modified" times.

pkgs/watcher/CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 1.1.5-wip
 
+- Polling watchers now check file sizes as well as "last modified" times, so
+  they are less likely to miss changes on platforms with low resolution
+  timestamps.
 - Bug fix: with `FileWatcher` on MacOS, a modify event was sometimes reported if
   the file was created immediately before the watcher was created. Now, if the
   file exists when the watcher is created then this modify event is not sent.

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

@@ -7,12 +7,15 @@ import 'dart:io';
 
 import '../async_queue.dart';
 import '../directory_watcher.dart';
+import '../polling.dart';
 import '../resubscribable.dart';
-import '../stat.dart';
 import '../utils.dart';
 import '../watch_event.dart';
 
 /// Periodically polls a directory for changes.
+///
+/// Changes are noticed if the "last modified" time of a file changes or if its
+/// size changes.
 class PollingDirectoryWatcher extends ResubscribableWatcher
     implements DirectoryWatcher {
   @override
@@ -53,10 +56,7 @@ class _PollingDirectoryWatcher
   /// directory contents.
   final Duration _pollingDelay;
 
-  /// The previous modification times of the files in the directory.
-  ///
-  /// Used to tell which files have been modified.
-  final _lastModifieds = <String, DateTime?>{};
+  final _previousPollResults = <String, PollResult>{};
 
   /// The subscription used while [directory] is being listed.
   ///
@@ -78,7 +78,8 @@ class _PollingDirectoryWatcher
   /// The set of files that have been seen in the current directory listing.
   ///
   /// Used to tell which files have been removed: files that are in
-  /// [_lastModifieds] but not in here when a poll completes have been removed.
+  /// [_previousPollResults] but not in here when a poll completes have been
+  /// removed.
   final _polledFiles = <String>{};
 
   _PollingDirectoryWatcher(this.path, this._pollingDelay) {
@@ -95,7 +96,7 @@ class _PollingDirectoryWatcher
     // Don't process any remaining files.
     _filesToProcess.clear();
     _polledFiles.clear();
-    _lastModifieds.clear();
+    _previousPollResults.clear();
   }
 
   /// Scans the contents of the directory once to see which files have been
@@ -145,14 +146,14 @@ class _PollingDirectoryWatcher
       return;
     }
 
-    final modified = await modificationTime(file);
+    final pollResult = await PollResult.poll(file);
 
     if (_events.isClosed) return;
 
-    var lastModified = _lastModifieds[file];
+    var previousPollResult = _previousPollResults[file];
 
     // If its modification time hasn't changed, assume the file is unchanged.
-    if (lastModified != null && lastModified == modified) {
+    if (previousPollResult != null && previousPollResult == pollResult) {
       // The file is still here.
       _polledFiles.add(file);
       return;
@@ -161,17 +162,17 @@ class _PollingDirectoryWatcher
     if (_events.isClosed) return;
 
     _polledFiles.add(file);
-    if (modified == null) {
+    if (!pollResult.fileExists) {
       // The file was in the directory listing but has been removed since then.
-      // Don't add to _lastModifieds, it will be reported as a REMOVE.
+      // Don't add to _previousPollResults, it will be reported as a REMOVE.
       return;
     }
-    _lastModifieds[file] = modified;
+    _previousPollResults[file] = pollResult;
 
     // Only notify if we're ready to emit events.
     if (!isReady) return;
 
-    var type = lastModified == null ? ChangeType.ADD : ChangeType.MODIFY;
+    var type = previousPollResult == null ? ChangeType.ADD : ChangeType.MODIFY;
     _events.add(WatchEvent(type, file));
   }
 
@@ -180,10 +181,11 @@ class _PollingDirectoryWatcher
   Future<void> _completePoll() async {
     // Any files that were not seen in the last poll but that we have a
     // status for must have been removed.
-    var removedFiles = _lastModifieds.keys.toSet().difference(_polledFiles);
+    var removedFiles =
+        _previousPollResults.keys.toSet().difference(_polledFiles);
     for (var removed in removedFiles) {
       if (isReady) _events.add(WatchEvent(ChangeType.REMOVE, removed));
-      _lastModifieds.remove(removed);
+      _previousPollResults.remove(removed);
     }
 
     if (!isReady) _readyCompleter.complete();

pkgs/watcher/lib/src/file_watcher/polling.dart

@@ -6,8 +6,8 @@ import 'dart:async';
 import 'dart:io';
 
 import '../file_watcher.dart';
+import '../polling.dart';
 import '../resubscribable.dart';
-import '../stat.dart';
 import '../watch_event.dart';
 
 /// Periodically polls a file for changes.
@@ -37,10 +37,7 @@ class _PollingFileWatcher implements FileWatcher, ManuallyClosedWatcher {
   /// The timer that controls polling.
   late final Timer _timer;
 
-  /// The previous modification time of the file.
-  ///
-  /// `null` indicates the file does not (or did not on the last poll) exist.
-  DateTime? _lastModified;
+  PollResult _previousPollResult = PollResult.notAFile();
 
   _PollingFileWatcher(this.path, Duration pollingDelay) {
     _timer = Timer.periodic(pollingDelay, (_) => _poll());
@@ -55,39 +52,40 @@ class _PollingFileWatcher implements FileWatcher, ManuallyClosedWatcher {
     var pathExists = await File(path).exists();
     if (_eventsController.isClosed) return;
 
-    if (_lastModified != null && !pathExists) {
+    if (_previousPollResult.fileExists && !pathExists) {
       _flagReady();
       _eventsController.add(WatchEvent(ChangeType.REMOVE, path));
       unawaited(close());
       return;
     }
 
-    DateTime? modified;
+    PollResult pollResult;
     try {
-      modified = await modificationTime(path);
+      pollResult = await PollResult.poll(path);
     } on FileSystemException catch (error, stackTrace) {
       if (!_eventsController.isClosed) {
         _flagReady();
         _eventsController.addError(error, stackTrace);
         await close();
       }
+      return;
     }
     if (_eventsController.isClosed) {
       _flagReady();
       return;
     }
 
     if (!isReady) {
-      // If this is the first poll, don't emit an event, just set the last mtime
-      // and complete the completer.
-      _lastModified = modified;
+      // If this is the first poll, don't emit an event, just set the poll
+      // result and complete the completer.
+      _previousPollResult = pollResult;
       _flagReady();
       return;
     }
 
-    if (_lastModified == modified) return;
+    if (_previousPollResult == pollResult) return;
 
-    _lastModified = modified;
+    _previousPollResult = pollResult;
     _eventsController.add(WatchEvent(ChangeType.MODIFY, path));
   }
 

pkgs/watcher/lib/src/polling.dart

@@ -0,0 +1,32 @@
+// 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:io';
+
+/// Result of polling a path.
+///
+/// If it's a file, the result is combined from the file's "last modification"
+/// time and size, so that a change to either can be noticed as a change.
+///
+/// If the path is not a file, [fileExists] return `false`.
+extension type PollResult._(int _value) {
+  /// A [PollResult] with [fileExists] `false`.
+  factory PollResult.notAFile() => PollResult._(0);
+
+  static Future<PollResult> poll(String path) async {
+    final stat = await FileStat.stat(path);
+    if (stat.type != FileSystemEntityType.file) return PollResult.notAFile();
+
+    // Construct the poll result from the "last modified" time and size.
+    // It should be very likely to change if either changes. Both are 64 bit
+    // ints with the interesting bits in the low bits. Swap the 32 bit sections
+    // of `microseconds` so the interesting bits don't clash, then XOR them.
+    var microseconds = stat.modified.microsecondsSinceEpoch;
+    microseconds = microseconds << 32 | microseconds >>> 32;
+    return PollResult._(microseconds ^ stat.size);
+  }
+
+  /// Whether the path exists and is a file.
+  bool get fileExists => _value != 0;
+}

pkgs/watcher/test/directory_watcher/file_tests.dart

@@ -67,6 +67,7 @@ void _fileTests({required bool isNative}) {
     writeFile('b.txt', contents: 'before');
     await startWatcher();
 
+    if (!isNative) sleepUntilNewModificationTime();
     writeFile('a.txt', contents: 'same');
     writeFile('b.txt', contents: 'after');
     await inAnyOrder([isModifyEvent('a.txt'), isModifyEvent('b.txt')]);
@@ -139,7 +140,7 @@ void _fileTests({required bool isNative}) {
 
     test('notifies when a file is moved onto an existing one', () async {
       writeFile('from.txt');
-      writeFile('to.txt');
+      writeFile('to.txt', contents: 'different');
       await startWatcher();
 
       renameFile('from.txt', 'to.txt');

pkgs/watcher/test/directory_watcher/link_tests.dart

@@ -32,6 +32,7 @@ void _linkTests({required bool isNative}) {
     createDir('targets');
     createDir('links');
     writeFile('targets/a.target');
+    sleepUntilNewModificationTime();
     writeFile('targets/b.target');
     writeLink(link: 'links/a.link', target: 'targets/a.target');
     await startWatcher(path: 'links');
@@ -46,7 +47,7 @@ void _linkTests({required bool isNative}) {
       'notifies when a link is replaced with a link to a different target '
       'with different contents', () async {
     writeFile('targets/a.target', contents: 'a');
-    writeFile('targets/b.target', contents: 'b');
+    writeFile('targets/b.target', contents: 'ab');
     writeLink(link: 'links/a.link', target: 'targets/a.target');
     await startWatcher(path: 'links');
 

pkgs/watcher/test/directory_watcher/polling_test.dart

@@ -15,63 +15,45 @@ import 'link_tests.dart';
 void main() {
   // Use a short delay to make the tests run quickly.
   watcherFactory = (dir) => PollingDirectoryWatcher(dir,
-      pollingDelay: const Duration(milliseconds: 100));
-
-  // Filesystem modification times can be low resolution, mock them.
-  group('with mock mtime', () {
-    setUp(enableMockModificationTimes);
-
-    fileTests(isNative: false);
-    linkTests(isNative: false);
-
-    test('does not notify if the modification time did not change', () async {
-      writeFile('a.txt', contents: 'before');
-      writeFile('b.txt', contents: 'before');
-      await startWatcher();
-      writeFile('a.txt', contents: 'after', updateModified: false);
-      writeFile('b.txt', contents: 'after');
-      await expectModifyEvent('b.txt');
-    });
-
-    // A poll does an async directory list then checks mtime on each file. Check
-    // handling of a file that is deleted between the two.
-    test('deletes during poll', () async {
-      await startWatcher();
-
-      for (var i = 0; i != 300; ++i) {
-        writeFile('$i');
-      }
-      // A series of deletes with delays in between for 300ms, which will
-      // intersect with the 100ms polling multiple times.
-      for (var i = 0; i != 300; ++i) {
-        deleteFile('$i');
-        await Future<void>.delayed(const Duration(milliseconds: 1));
+      pollingDelay: const Duration(milliseconds: 10));
+
+  /// See [enableSleepUntilNewModificationTime] for a note about the "polling"
+  /// tests.
+  setUp(enableSleepUntilNewModificationTime);
+
+  fileTests(isNative: false);
+  linkTests(isNative: false);
+
+  // A poll does an async directory list that runs "stat" on each file. Check
+  // handling of a file that is deleted between the two.
+  test('deletes during poll', () async {
+    await startWatcher();
+
+    for (var i = 0; i != 300; ++i) {
+      writeFile('$i');
+    }
+    // A series of deletes with delays in between for 300ms, which will
+    // intersect with the 10ms polling multiple times.
+    for (var i = 0; i != 300; ++i) {
+      deleteFile('$i');
+      await Future<void>.delayed(const Duration(milliseconds: 1));
+    }
+
+    final events =
+        await takeEvents(duration: const Duration(milliseconds: 500));
+
+    // Events should be adds and removes that pair up, with no modify events.
+    final adds = <String>{};
+    final removes = <String>{};
+    for (var event in events) {
+      if (event.type == ChangeType.ADD) {
+        adds.add(event.path);
+      } else if (event.type == ChangeType.REMOVE) {
+        removes.add(event.path);
+      } else {
+        fail('Unexpected event: $event');
       }
-
-      final events =
-          await takeEvents(duration: const Duration(milliseconds: 500));
-
-      // Events should be adds and removes that pair up, with no modify events.
-      final adds = <String>{};
-      final removes = <String>{};
-      for (var event in events) {
-        if (event.type == ChangeType.ADD) {
-          adds.add(event.path);
-        } else if (event.type == ChangeType.REMOVE) {
-          removes.add(event.path);
-        } else {
-          fail('Unexpected event: $event');
-        }
-      }
-      expect(adds, removes);
-    });
-  });
-
-  // Also test with delayed writes and real mtimes.
-  group('with real mtime', () {
-    setUp(enableWaitingForDifferentModificationTimes);
-
-    fileTests(isNative: false);
-    linkTests(isNative: false);
+    }
+    expect(adds, removes);
   });
 }