Use FSEvents for recursive file watch on Mac

base/files/file_path_watcher_fsevents.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_FILES_FILE_PATH_WATCHER_FSEVENTS_H_
+#define BASE_FILES_FILE_PATH_WATCHER_FSEVENTS_H_
+
+#include <CoreServices/CoreServices.h>
+
+#include <vector>
+
+#include "base/files/file_path.h"
+#include "base/files/file_path_watcher.h"
+
+namespace base {
+
+// Mac-specific file watcher implementation based on FSEvents.
+// There are trade-offs between the FSEvents implementation and a kqueue
+// implementation. The biggest issues are that FSEvents on 10.6 sometimes drops

[Mark Mentovai, 2014/06/08 15:29:02]

FSEvents can drop events on any OS. Who are the callers and what are their
requirements? Who can tolerate flakiness? Can anyone not tolerate flakiness? Who
needs file-modify events?

[vandebo (ex-Chrome), 2014/06/10 00:52:06]

There are not existing callers for recursive watching because it is not
supported before this CL (all current code uses kqueue). Recursive capability
will initially be used by the media galleries API to notify consumers when files
are changed or added in a directory tree.

My understanding is that post 10.6, FSEvents is reliable. If it is not reliable,
but only drops events when they are too many in too short a time, that would
still be acceptable (we don't care exactly what changed, as long as
notifications of some change are reliable). 

Since kqueue does not trigger for file-modify events, no current consumers need
it.

+// events and kqueue does not trigger for modifications to a file in a watched
+// directory. See file_path_watcher_mac.cc for the code that decides when to
+// use which one.
+class FilePathWatcherFSEvents : public FilePathWatcher::PlatformDelegate {
+ public:
+  FilePathWatcherFSEvents();
+
+  // Called from the FSEvents callback whenever there is a change to the paths.
+  void OnFilePathsChanged(const std::vector<FilePath>& paths);
+
+  // (Re-)Initialize the event stream to start reporting events from
+  // |start_event|.
+  void UpdateEventStream(FSEventStreamEventId start_event);
+
+  // Returns true if resolving the target path got a different result than
+  // last time it was done.
+  bool ResolveTargetPath();
+
+  // FilePathWatcher::PlatformDelegate overrides.
+  virtual bool Watch(const FilePath& path,
+                     bool recursive,
+                     const FilePathWatcher::Callback& callback) OVERRIDE;
+  virtual void Cancel() OVERRIDE;
+
+ private:
+  virtual ~FilePathWatcherFSEvents();
+
+  // Destroy the event stream.
+  void DestroyEventStream();
+
+  // Start watching the FSEventStream.
+  void StartEventStream(FSEventStreamEventId start_event);
+
+  // Cleans up and stops the event stream.
+  virtual void CancelOnMessageLoopThread() OVERRIDE;
+
+  // Callback to notify upon changes.
+  FilePathWatcher::Callback callback_;
+
+  // Target path to watch (passed to callback).
+  FilePath target_;
+
+  // Target path with all symbolic links resolved.
+  FilePath resolved_target_;
+
+  // Backend stream we receive event callbacks from (strong reference).
+  FSEventStreamRef fsevent_stream_;
+
+  DISALLOW_COPY_AND_ASSIGN(FilePathWatcherFSEvents);
+};
+
+}  // namespace base
+
+#endif  // BASE_FILES_FILE_PATH_WATCHER_FSEVENTS_H_