Fix file watcher startup race on MacOS (#2176)

Author: davidmorgan

pkgs/watcher/CHANGELOG.md

@@ -1,3 +1,10 @@
+## 1.1.5-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 modify event is not sent.
+  This matches the Linux native and polling (Windows) watchers.
+
 ## 1.1.4
 
 - Improve handling of subdirectories: ignore `PathNotFoundException` due to

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

@@ -35,6 +35,9 @@ class _NativeFileWatcher implements FileWatcher, ManuallyClosedWatcher {
 
   StreamSubscription<List<FileSystemEvent>>? _subscription;
 
+  /// On MacOS only, whether the file existed on startup.
+  bool? _existedAtStartup;
+
   _NativeFileWatcher(this.path) {
     _listen();
 
@@ -43,12 +46,23 @@ class _NativeFileWatcher implements FileWatcher, ManuallyClosedWatcher {
     _readyCompleter.complete();
   }
 
-  void _listen() {
-    // Batch the events together so that we can dedup them.
-    _subscription = File(path)
-        .watch()
-        .batchEvents()
-        .listen(_onBatch, onError: _eventsController.addError, onDone: _onDone);
+  void _listen() async {
+    var file = File(path);
+
+    // Batch the events together so that we can dedupe them.
+    var stream = file.watch().batchEvents();
+
+    if (Platform.isMacOS) {
+      var existedAtStartupFuture = file.exists();
+      // Delay processing watch events until the existence check finishes.
+      stream = stream.asyncMap((event) async {
+        _existedAtStartup ??= await existedAtStartupFuture;
+        return event;
+      });
+    }
+
+    _subscription = stream.listen(_onBatch,
+        onError: _eventsController.addError, onDone: _onDone);
   }
 
   void _onBatch(List<FileSystemEvent> batch) {
@@ -58,6 +72,16 @@ class _NativeFileWatcher implements FileWatcher, ManuallyClosedWatcher {
       return;
     }
 
+    if (Platform.isMacOS) {
+      // On MacOS, a spurious `create` event can be received for a file that is
+      // created just before the `watch`. If the file existed at startup then it
+      // should be ignored.
+      if (_existedAtStartup! &&
+          batch.every((event) => event.type == FileSystemEvent.create)) {
+        return;
+      }
+    }
+
     _eventsController.add(WatchEvent(ChangeType.MODIFY, path));
   }
 

pkgs/watcher/pubspec.yaml

@@ -1,5 +1,5 @@
 name: watcher
-version: 1.1.4
+version: 1.1.5-wip
 description: >-
   A file system watcher. It monitors changes to contents of directories and
   sends notifications when files have been added, removed, or modified.

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(10));
-    } else {
-      expect(events, 0);
-    }
+    expect(events, 0);
   });
 }