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);
   });
 }