Fix unwanted file modify event on MacOS on startup.

Author: davidmorgan

pkgs/watcher/CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 1.1.4-wip
 
+- 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 create event is ignored,
+ which matches the Linux native and polling (Windows) watchers.
 - Improve handling of subdirectories: ignore `PathNotFoundException` due to
  subdirectory deletion racing with watcher internals, instead of raising
  it on the event stream.

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

@@ -34,6 +34,8 @@ class _NativeFileWatcher implements FileWatcher, ManuallyClosedWatcher {
  final _readyCompleter = Completer<void>();
 
  StreamSubscription<List<FileSystemEvent>>? _subscription;
+ Future<bool>? _existedAtStartupFuture;
+ bool? _existedAtStartup;
 
  _NativeFileWatcher(this.path) {
  _listen();
@@ -44,20 +46,37 @@ class _NativeFileWatcher implements FileWatcher, ManuallyClosedWatcher {
  }
 
  void _listen() {
+ var file = File(path);
+
  // Batch the events together so that we can dedup them.
- _subscription = File(path)
+ _subscription = file
  .watch()
  .batchEvents()
  .listen(_onBatch, onError: _eventsController.addError, onDone: _onDone);
+
+ // On MacOS events can arrive from file changes before the `watch`. Check
+ // existence on startup to differentiate creation before and after the
+ // `watch`.
+ if (Platform.isMacOS) _existedAtStartupFuture = file.exists();
  }
 
- void _onBatch(List<FileSystemEvent> batch) {
+ void _onBatch(List<FileSystemEvent> batch) async {
  if (batch.any((event) => event.type == FileSystemEvent.delete)) {
  // If the file is deleted, the underlying stream will close. We handle
  // emitting our own REMOVE event in [_onDone].
  return;
  }
 
+ if (Platform.isMacOS) {
+ var existedAtStartup =
+ _existedAtStartup ??= await _existedAtStartupFuture!;
+ if (existedAtStartup &&
+ batch.every((event) => event.type == FileSystemEvent.create)) {
+ // It's a create event from before `watch` was called, ignore it.
+ return;
+ }
+ }
+
  _eventsController.add(WatchEvent(ChangeType.MODIFY, path));
  }
 

pkgs/watcher/test/file_watcher/startup_race_tests.dart

@@ -2,8 +2,6 @@
 // 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';
-
 import 'package:test/test.dart';
 
 import '../utils.dart';
@@ -27,13 +25,6 @@ void startupRaceTests({required bool isNative}) {
  );
  }
  await Future.wait(futures);
-
- // TODO(davidmorgan): the MacOS watcher currently does get unwanted events,
- // fix it.
- if (isNative && Platform.isMacOS) {
- expect(events, greaterThan(50));
- } else {
- expect(events, 0);
- }
+ expect(events, 0);
  });
 }