kqueue implementation of FilePathWatcher on Mac.

content/common/file_path_watcher/file_path_watcher_mac.cc

 // Copyright (c) 2011 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.
 
 #include "content/common/file_path_watcher/file_path_watcher.h"
 
-#include <CoreServices/CoreServices.h>
-#include <set>
-
-#include "base/file_path.h"
+#include <fcntl.h>
+#include <sys/event.h>
+#include <sys/param.h>
+
+#include <vector>
+
 #include "base/file_util.h"
-#include "base/logging.h"
-#include "base/mac/scoped_cftyperef.h"
-#include "base/memory/singleton.h"
 #include "base/message_loop.h"
-#include "base/time.h"
-
-// Note to future well meaning engineers. Unless kqueue semantics have changed
-// considerably, do NOT try to reimplement this class using kqueue. The main
-// problem is that this class requires the ability to watch a directory
-// and notice changes to any files within it. A kqueue on a directory can watch
-// for creation and deletion of files, but not for modifications to files within
-// the directory. To do this with the current kqueue semantics would require
-// kqueueing every file in the directory, and file descriptors are a limited
-// resource. If you have a good idea on how to get around this, the source for a
-// reasonable implementation of this class using kqueues is attached here:
-// http://code.google.com/p/chromium/issues/detail?id=54822#c13
+#include "base/message_loop_proxy.h"
+#include "base/stringprintf.h"
 
 namespace {
 
-// The latency parameter passed to FSEventsStreamCreate().
-const CFAbsoluteTime kEventLatencySeconds = 0.3;
-
-// Mac-specific file watcher implementation based on the FSEvents API.
+// Mac-specific file watcher implementation based on kqueue.
+// Originally it was based on FSEvents so that the semantics were equivalent
+// on Linux, OSX and Windows where it was able to detect:
+// - file creation/deletion/modification in a watched directory
+// - file creation/deletion/modification for a watched file
+// - modifications to the paths to a watched object that would affect the
+//   object such as renaming/attibute changes etc.
+// The FSEvents version did all of the above except handling attribute changes
+// to path components. Unfortunately FSEvents appears to have an issue where the
+// current implementation (Mac OS X 10.6.7) sometimes drops events and doesn't
+// send notifications. See
+// http://code.google.com/p/chromium/issues/detail?id=54822#c31 for source that
+// will reproduce the problem. FSEvents also required having a CFRunLoop
+// backing the thread that it was running on, that caused added complexity
+// in the interfaces.
+// The kqueue implementation will handle all of the items in the list above
+// except for detecting modifications to files in a watched directory. It will
+// detect the creation and deletion of files, just not the modification of
+// files. It does however detect the attribute changes that the FSEvents impl
+// would miss.
 class FilePathWatcherImpl : public FilePathWatcher::PlatformDelegate,
+                            public MessageLoopForIO::Watcher,
                             public MessageLoop::DestructionObserver {
  public:
-  FilePathWatcherImpl();
-
-  // Called from the FSEvents callback whenever there is a change to the paths
-  void OnFilePathChanged();
-
-  // (Re-)Initialize the event stream to start reporting events from
-  // |start_event|.
-  void UpdateEventStream(FSEventStreamEventId start_event);
+  FilePathWatcherImpl() : kqueue_(-1) {}
+  virtual ~FilePathWatcherImpl() {}
+
+  // MessageLoopForIO::Watcher overrides.
+  virtual void OnFileCanReadWithoutBlocking(int fd) OVERRIDE;
+  virtual void OnFileCanWriteWithoutBlocking(int fd) OVERRIDE;
+
+  // MessageLoop::DestructionObserver overrides.
+  virtual void WillDestroyCurrentMessageLoop() OVERRIDE;
 
   // FilePathWatcher::PlatformDelegate overrides.
   virtual bool Watch(const FilePath& path,
-                     FilePathWatcher::Delegate* delegate,
-                     base::MessageLoopProxy* loop) OVERRIDE;
+                     FilePathWatcher::Delegate* delegate) OVERRIDE;
   virtual void Cancel() OVERRIDE;
 
-  // Deletion of the FilePathWatcher will call Cancel() to dispose of this
-  // object in the right thread. This also observes destruction of the required
-  // cleanup thread, in case it quits before Cancel() is called.
-  virtual void WillDestroyCurrentMessageLoop() OVERRIDE;
-
-  scoped_refptr<base::MessageLoopProxy> run_loop_message_loop() {
-    return run_loop_message_loop_;
-  }
-
  private:
-  virtual ~FilePathWatcherImpl() {}
-
-  // Destroy the event stream.
-  void DestroyEventStream();
-
-  // Start observing the destruction of the |run_loop_message_loop_| thread,
-  // and watching the FSEventStream.
-  void StartObserverAndEventStream(FSEventStreamEventId start_event);
-
-  // Cleans up and stops observing the |run_loop_message_loop_| thread.
-  void CancelOnMessageLoopThread() OVERRIDE;
-
-  // Delegate to notify upon changes.
+  class EventData {
+   public:
+    EventData(const FilePath& path, const FilePath::StringType& subdir)
+        : path_(path), subdir_(subdir) { }
+    FilePath path_;  // Full path to this item.
+    FilePath::StringType subdir_;  // Path to any sub item.
+  };
+  typedef std::vector<struct kevent> EventVector;
+
+  // Can only be called on |io_message_loop_|'s thread.
+  virtual void CancelOnMessageLoopThread() OVERRIDE;
+
+  // Returns true if the kevent values are error free.
+  bool AreKeventValuesValid(struct kevent* kevents, int count);
+
+  // Respond to a change of attributes of the path component represented by
+  // |event|. Sets |target_file_affected| to true if |target_| is affected.
+  // Sets |update_watches| to true if |events_| need to be updated.
+  void HandleAttributesChange(const EventVector::iterator& event,
+                              bool* target_file_affected,
+                              bool* update_watches);
+
+  // Respond to a move of deletion of the path component represented by
+  // |event|. Sets |target_file_affected| to true if |target_| is affected.
+  // Sets |update_watches| to true if |events_| need to be updated.
+  void HandleDeleteOrMoveChange(const EventVector::iterator& event,
+                                bool* target_file_affected,
+                                bool* update_watches);
+
+  // Respond to a creation of an item in the path component represented by
+  // |event|. Sets |target_file_affected| to true if |target_| is affected.
+  // Sets |update_watches| to true if |events_| need to be updated.
+  void HandleCreateItemChange(const EventVector::iterator& event,
+                              bool* target_file_affected,
+                              bool* update_watches);
+
+  // Update |events_| with the current status of the system.
+  // Sets |target_file_affected| to true if |target_| is affected.
+  // Returns false if an error occurs.
+  bool UpdateWatches(bool* target_file_affected);
+
+  // Fills |events| with one kevent per component in |path|.
+  // Returns the number of valid events created where a valid event is
+  // defined as one that has a ident (file descriptor) field != -1.
+  static int EventsForPath(FilePath path, EventVector *events);
+
+  // Release a kevent generated by EventsForPath.
+  static void ReleaseEvent(struct kevent& event);
+
+  // Returns a file descriptor that will not block the system from deleting
+  // the file it references.
+  static int FileDescriptorForPath(const FilePath& path);
+
+  // Closes |*fd| and sets |*fd| to -1.
+  static void CloseFileDescriptor(int* fd);
+
+  // Returns true if kevent has open file descriptor.
+  static bool IsKeventFileDescriptorOpen(const struct kevent& event) {
+    return event.ident != static_cast<uintptr_t>(-1);
+  }
+
+  static EventData* EventDataForKevent(const struct kevent& event) {
+    return reinterpret_cast<EventData*>(event.udata);
+  }
+
+  EventVector events_;
+  scoped_refptr<base::MessageLoopProxy> io_message_loop_;
+  MessageLoopForIO::FileDescriptorWatcher kqueue_watcher_;
   scoped_refptr<FilePathWatcher::Delegate> delegate_;
-
-  // Target path to watch (passed to delegate).
   FilePath target_;
-
-  // Keep track of the last modified time of the file.  We use nulltime
-  // to represent the file not existing.
-  base::Time last_modified_;
-
-  // The time at which we processed the first notification with the
-  // |last_modified_| time stamp.
-  base::Time first_notification_;
-
-  // Backend stream we receive event callbacks from (strong reference).
-  FSEventStreamRef fsevent_stream_;
-
-  // Run loop for FSEventStream to run on.
-  scoped_refptr<base::MessageLoopProxy> run_loop_message_loop_;
+  int kqueue_;
 
   DISALLOW_COPY_AND_ASSIGN(FilePathWatcherImpl);
 };
 
-// The callback passed to FSEventStreamCreate().
-void FSEventsCallback(ConstFSEventStreamRef stream,
-                      void* event_watcher, size_t num_events,
-                      void* event_paths, const FSEventStreamEventFlags flags[],
-                      const FSEventStreamEventId event_ids[]) {
-  FilePathWatcherImpl* watcher =
-      reinterpret_cast<FilePathWatcherImpl*>(event_watcher);
-  DCHECK(watcher->run_loop_message_loop()->BelongsToCurrentThread());
-
-  bool root_changed = false;
-  FSEventStreamEventId root_change_at = FSEventStreamGetLatestEventId(stream);
-  for (size_t i = 0; i < num_events; i++) {
-    if (flags[i] & kFSEventStreamEventFlagRootChanged)
-      root_changed = true;
-    if (event_ids[i])
-      root_change_at = std::min(root_change_at, event_ids[i]);
-  }
-
-  // Reinitialize the event stream if we find changes to the root. This is
-  // necessary since FSEvents doesn't report any events for the subtree after
-  // the directory to be watched gets created.
-  if (root_changed) {
-    // Resetting the event stream from within the callback fails (FSEvents spews
-    // bad file descriptor errors), so post a task to do the reset.
-    watcher->run_loop_message_loop()->PostTask(FROM_HERE,
-        NewRunnableMethod(watcher, &FilePathWatcherImpl::UpdateEventStream,
-                          root_change_at));
-  }
-
-  watcher->OnFilePathChanged();
-}
-
-// FilePathWatcherImpl implementation:
-
-FilePathWatcherImpl::FilePathWatcherImpl()
-    : fsevent_stream_(NULL) {
-}
-
-void FilePathWatcherImpl::OnFilePathChanged() {
-  // Switch to the CFRunLoop based thread if necessary, so we can tear down
-  // the event stream.
-  if (!message_loop()->BelongsToCurrentThread()) {
-    message_loop()->PostTask(
-        FROM_HERE,
-        NewRunnableMethod(this, &FilePathWatcherImpl::OnFilePathChanged));
+void FilePathWatcherImpl::ReleaseEvent(struct kevent& event) {
+  CloseFileDescriptor(reinterpret_cast<int*>(&event.ident));
+  EventData* entry = EventDataForKevent(event);
+  delete entry;
+  event.udata = NULL;
+}
+
+int FilePathWatcherImpl::EventsForPath(FilePath path, EventVector* events) {
+  DCHECK(MessageLoopForIO::current());
+  // Make sure that we are working with a clean slate.
+  DCHECK(events->empty());
+
+  std::vector<FilePath::StringType> components;
+  path.GetComponents(&components);
+
+  if (components.size() < 1) {
+    return -1;
+  }
+
+  int last_existing_entry = 0;
+  FilePath built_path;
+  bool path_still_exists = true;
+  for(std::vector<FilePath::StringType>::iterator i = components.begin();
+      i != components.end(); ++i) {
+    if (i == components.begin()) {
+      built_path = FilePath(*i);
+    } else {
+      built_path = built_path.Append(*i);
+    }
+    int fd = -1;
+    if (path_still_exists) {
+      fd = FileDescriptorForPath(built_path);
+      if (fd == -1) {
+        path_still_exists = false;
+      } else {
+        ++last_existing_entry;
+      }
+    }
+    FilePath::StringType subdir = (i != (components.end() - 1)) ? *(i + 1) : "";
+    EventData* data = new EventData(built_path, subdir);
+    struct kevent event;
+    EV_SET(&event, fd, EVFILT_VNODE, (EV_ADD | EV_CLEAR | EV_RECEIPT),
+           (NOTE_DELETE | NOTE_WRITE | NOTE_ATTRIB |
+            NOTE_RENAME | NOTE_REVOKE | NOTE_EXTEND), 0, data);
+    events->push_back(event);
+  }
+  return last_existing_entry;
+}
+
+int FilePathWatcherImpl::FileDescriptorForPath(const FilePath& path) {
+  return HANDLE_EINTR(open(path.value().c_str(), O_EVTONLY));
+}
+
+void FilePathWatcherImpl::CloseFileDescriptor(int *fd) {
+  if (*fd == -1) {
     return;
   }
 
-  DCHECK(message_loop()->BelongsToCurrentThread());
-  DCHECK(!target_.empty());
-
-  base::PlatformFileInfo file_info;
-  bool file_exists = file_util::GetFileInfo(target_, &file_info);
-  if (file_exists && (last_modified_.is_null() ||
-      last_modified_ != file_info.last_modified)) {
-    last_modified_ = file_info.last_modified;
-    first_notification_ = base::Time::Now();
+  if (HANDLE_EINTR(close(*fd)) != 0) {
+    PLOG(ERROR) << "close";
+  }
+  *fd = -1;
+}
+
+bool FilePathWatcherImpl::AreKeventValuesValid(struct kevent* kevents,
+                                               int count) {
+  if (count < 0) {
+    PLOG(ERROR) << "kevent";
+    return false;
+  }
+  bool valid = true;
+  for (int i = 0; i < count; ++i) {
+    if (kevents[i].flags & EV_ERROR && kevents[i].data) {
+      // Find the kevent in |events_| that matches the kevent with the error.
+      EventVector::iterator event = events_.begin();
+      for (; event != events_.end(); ++event) {
+        if (event->ident == kevents[i].ident) {
+          break;
+        }
+      }
+      std::string path_name;
+      if (event != events_.end()) {
+        EventData* event_data = EventDataForKevent(*event);
+        if (event_data != NULL) {
+          path_name = event_data->path_.value();
+        }
+      }
+      if (path_name.empty()) {
+        path_name = base::StringPrintf(
+            "fd %d", *reinterpret_cast<int*>(&kevents[i].ident));
+      }
+      LOG(ERROR) << "Error: " << kevents[i].data << " for " << path_name;
+      valid = false;
+    }
+  }
+  return valid;
+}
+
+void FilePathWatcherImpl::HandleAttributesChange(
+    const EventVector::iterator& event,
+    bool* target_file_affected,
+    bool* update_watches) {
+  EventVector::iterator next_event = event + 1;
+  EventData* next_event_data = EventDataForKevent(*next_event);
+  // Check to see if the next item in path is still accessible.
+  int have_access = FileDescriptorForPath(next_event_data->path_);
+  if (have_access == -1) {
+    *target_file_affected = true;
+    *update_watches = true;
+    EventVector::iterator local_event(event);
+    for (; local_event != events_.end(); ++local_event) {
+      // Close all nodes from the event down. This has the side effect of
+      // potentially rendering other events in |updates| invalid.
+      // There is no need to remove the events from |kqueue_| because this
+      // happens as a side effect of closing the file descriptor.
+      CloseFileDescriptor(reinterpret_cast<int*>(&local_event->ident));
+    }
+  } else {
+    CloseFileDescriptor(&have_access);
+  }
+}
+
+void FilePathWatcherImpl::HandleDeleteOrMoveChange(
+    const EventVector::iterator& event,
+    bool* target_file_affected,
+    bool* update_watches) {
+  *target_file_affected = true;
+  *update_watches = true;
+  EventVector::iterator local_event(event);
+  for (; local_event != events_.end(); ++local_event) {
+    // Close all nodes from the event down. This has the side effect of
+    // potentially rendering other events in |updates| invalid.
+    // There is no need to remove the events from |kqueue_| because this
+    // happens as a side effect of closing the file descriptor.
+    CloseFileDescriptor(reinterpret_cast<int*>(&local_event->ident));
+  }
+}
+
+void FilePathWatcherImpl::HandleCreateItemChange(
+    const EventVector::iterator& event,
+    bool* target_file_affected,
+    bool* update_watches) {
+  // Get the next item in the path.
+  EventVector::iterator next_event = event + 1;
+  EventData* next_event_data = EventDataForKevent(*next_event);
+
+  // Check to see if it already has a valid file descriptor.
+  if (!IsKeventFileDescriptorOpen(*next_event)) {
+    // If not, attempt to open a file descriptor for it.
+    next_event->ident = FileDescriptorForPath(next_event_data->path_);
+    if (IsKeventFileDescriptorOpen(*next_event)) {
+      *update_watches = true;
+      if (next_event_data->subdir_.empty()) {
+        *target_file_affected = true;
+      }
+    }
+  }
+}
+
+bool FilePathWatcherImpl::UpdateWatches(bool* target_file_affected) {
+  // Iterate over events adding kevents for items that exist to the kqueue.
+  // Then check to see if new components in the path have been created.
+  // Repeat until no new components in the path are detected.
+  // This is to get around races in directory creation in a watched path.
+  bool update_watches = true;
+  while (update_watches) {
+    size_t valid;
+    for (valid = 0; valid < events_.size(); ++valid) {
+      if (!IsKeventFileDescriptorOpen(events_[valid])) {
+        break;
+      }
+    }
+    if (valid == 0) {
+      // The root of the file path is inaccessible?
+      return false;
+    }
+
+    EventVector updates(valid);
+    int count = HANDLE_EINTR(kevent(kqueue_, &events_[0], valid, &updates[0],
+                                    valid, NULL));
+    if (!AreKeventValuesValid(&updates[0], count)) {
+      return false;
+    }
+    update_watches = false;
+    for (; valid < events_.size(); ++valid) {
+      EventData* event_data = EventDataForKevent(events_[valid]);
+      events_[valid].ident = FileDescriptorForPath(event_data->path_);
+      if (IsKeventFileDescriptorOpen(events_[valid])) {
+        update_watches = true;
+        if (event_data->subdir_.empty()) {
+          *target_file_affected = true;
+        }
+      } else {
+        break;
+      }
+    }
+  }
+  return true;
+}
+
+void FilePathWatcherImpl::OnFileCanReadWithoutBlocking(int fd) {
+  DCHECK(MessageLoopForIO::current());
+  CHECK_EQ(fd, kqueue_);
+  CHECK(events_.size());
+
+  // Request the file system update notifications that have occurred and return
+  // them in |updates|. |count| will contain the number of updates that have
+  // occurred.
+  EventVector updates(events_.size());
+  struct timespec timeout = {0, 0};
+  int count = HANDLE_EINTR(kevent(kqueue_, NULL, 0, &updates[0], updates.size(),
+                                  &timeout));
+
+  // Error values are stored within updates, so check to make sure that no
+  // errors occurred.
+  if (!AreKeventValuesValid(&updates[0], count)) {
+    delegate_->OnFilePathError(target_);
+    Cancel();
+    return;
+  }
+
+  bool update_watches = false;
+  bool send_notification = false;
+
+  // Iterate through each of the updates and react to them.
+  for (int i = 0; i < count; ++i) {
+    // Find our kevent record that matches the update notification.
+    EventVector::iterator event = events_.begin();
+    for (; event != events_.end(); ++event) {
+      if (!IsKeventFileDescriptorOpen(*event) ||
+          event->ident == updates[i].ident) {
+        break;
+      }
+    }
+    if (!IsKeventFileDescriptorOpen(*event) || event == events_.end()) {
+      // The event may no longer exist in |events_| because another event
+      // modified |events_| in such a way to make it invalid. For example if
+      // the path is /foo/bar/bam and foo is deleted, NOTE_DELETE events for
+      // foo, bar and bam will be sent. If foo is processed first, then
+      // the file descriptors for bar and bam will already be closed and set
+      // to -1 before they get a chance to be processed.
+      continue;
+    }
+
+    EventData* event_data = EventDataForKevent(*event);
+
+    // If the subdir is empty, this is the last item on the path and is the
+    // target file.
+    bool target_file_affected = event_data->subdir_.empty();
+    if ((updates[i].fflags & NOTE_ATTRIB) && !target_file_affected) {

[Mark Mentovai, 2011/04/01 18:13:37]

Rationalize your style here so everything is consistent. You don’t strictly need
parentheses around |updates[i].fflags & NOTE_ATTRIB| but you’ve used them here;
but then you didn’t use them in the same situation on line 390. Personally, I
think they improve readability. I don’t mind if you choose to not use them,
though, as long as you do the same thing here and on 390.

+      HandleAttributesChange(event, &target_file_affected, &update_watches);
+    }
+    if (updates[i].fflags & (NOTE_DELETE | NOTE_REVOKE | NOTE_RENAME)) {
+      HandleDeleteOrMoveChange(event, &target_file_affected, &update_watches);
+    }
+    if (updates[i].fflags & NOTE_WRITE && !target_file_affected) {
+      HandleCreateItemChange(event, &target_file_affected, &update_watches);
+    }
+    send_notification |= target_file_affected;
+  }
+
+  if (update_watches) {
+    if (!UpdateWatches(&send_notification)) {
+      delegate_->OnFilePathError(target_);
+      Cancel();
+    }
+  }
+
+  if (send_notification) {
     delegate_->OnFilePathChanged(target_);
-  } else if (file_exists && !first_notification_.is_null()) {
-    // The target's last modification time is equal to what's on record. This
-    // means that either an unrelated event occurred, or the target changed
-    // again (file modification times only have a resolution of 1s). Comparing
-    // file modification times against the wall clock is not reliable to find
-    // out whether the change is recent, since this code might just run too
-    // late. Moreover, there's no guarantee that file modification time and wall
-    // clock times come from the same source.
-    //
-    // Instead, the time at which the first notification carrying the current
-    // |last_notified_| time stamp is recorded. Later notifications that find
-    // the same file modification time only need to be forwarded until wall
-    // clock has advanced one second from the initial notification. After that
-    // interval, client code is guaranteed to having seen the current revision
-    // of the file.
-    if (base::Time::Now() - first_notification_ >
-        base::TimeDelta::FromSeconds(1)) {
-      // Stop further notifications for this |last_modification_| time stamp.
-      first_notification_ = base::Time();
-    }
-    delegate_->OnFilePathChanged(target_);
-  } else if (!file_exists && !last_modified_.is_null()) {
-    last_modified_ = base::Time();
-    delegate_->OnFilePathChanged(target_);
-  }
+  }
+}
+
+void FilePathWatcherImpl::OnFileCanWriteWithoutBlocking(int fd) {
+  NOTREACHED();
+}
+
+void FilePathWatcherImpl::WillDestroyCurrentMessageLoop() {
+  CancelOnMessageLoopThread();
 }
 
 bool FilePathWatcherImpl::Watch(const FilePath& path,
-                                FilePathWatcher::Delegate* delegate,
-                                base::MessageLoopProxy* loop) {
-  DCHECK(target_.value().empty());
+                                FilePathWatcher::Delegate* delegate) {
   DCHECK(MessageLoopForIO::current());
-
-  set_message_loop(base::MessageLoopProxy::CreateForCurrentThread());
-  run_loop_message_loop_ = loop;
+  DCHECK(target_.value().empty());  // Can only watch one path.
+  DCHECK(delegate);
+  DCHECK_EQ(kqueue_, -1);
+
+  delegate_ = delegate;
   target_ = path;
-  delegate_ = delegate;
-
-  FSEventStreamEventId start_event = FSEventsGetCurrentEventId();
-
-  base::PlatformFileInfo file_info;
-  if (file_util::GetFileInfo(target_, &file_info)) {
-    last_modified_ = file_info.last_modified;
-    first_notification_ = base::Time::Now();
-  }
-
-  run_loop_message_loop()->PostTask(FROM_HERE,
-      NewRunnableMethod(this, &FilePathWatcherImpl::StartObserverAndEventStream,
-                        start_event));
-
-  return true;
-}
-
-void FilePathWatcherImpl::StartObserverAndEventStream(
-    FSEventStreamEventId start_event) {
-  DCHECK(run_loop_message_loop()->BelongsToCurrentThread());
+
   MessageLoop::current()->AddDestructionObserver(this);
-  UpdateEventStream(start_event);
+  io_message_loop_ = base::MessageLoopProxy::CreateForCurrentThread();
+
+  kqueue_ = kqueue();
+  if (kqueue_ == -1) {
+    PLOG(ERROR) << "kqueue";
+    return false;
+  }
+
+  int last_entry = EventsForPath(target_, &events_);
+  CHECK_NE(last_entry, 0);
+
+  EventVector responses(last_entry);
+
+  int count = HANDLE_EINTR(kevent(kqueue_, &events_[0], last_entry,
+                                  &responses[0], last_entry, NULL));
+  if (!AreKeventValuesValid(&responses[0], count)) {
+    // Calling Cancel() here to close any file descriptors that were opened.
+    // This would happen in the destructor anyways, but FilePathWatchers tend to
+    // be long lived, and if an error has occurred, there is no reason to waste
+    // the file descriptors.
+    Cancel();
+    return false;
+  }
+
+  return MessageLoopForIO::current()->WatchFileDescriptor(
+      kqueue_, true, MessageLoopForIO::WATCH_READ, &kqueue_watcher_, this);
 }
 
 void FilePathWatcherImpl::Cancel() {
-  if (!run_loop_message_loop().get()) {
-    // Watch was never called, so exit.
+  base::MessageLoopProxy* proxy = io_message_loop_.get();
+  if (!proxy) {
     set_cancelled();
     return;
   }
-
-  // Switch to the CFRunLoop based thread if necessary, so we can tear down
-  // the event stream.
-  if (!run_loop_message_loop()->BelongsToCurrentThread()) {
-    run_loop_message_loop()->PostTask(FROM_HERE,
-                                      new FilePathWatcher::CancelTask(this));
-  } else {
-    CancelOnMessageLoopThread();
-  }
+  if (!proxy->BelongsToCurrentThread()) {
+    proxy->PostTask(FROM_HERE,
+                    NewRunnableMethod(this, &FilePathWatcherImpl::Cancel));
+    return;
+  }
+  CancelOnMessageLoopThread();
 }
 
 void FilePathWatcherImpl::CancelOnMessageLoopThread() {
-  set_cancelled();
-  if (fsevent_stream_) {
-    DestroyEventStream();
+  DCHECK(MessageLoopForIO::current());
+  if (!is_cancelled()) {
+    set_cancelled();
+    kqueue_watcher_.StopWatchingFileDescriptor();
+    CloseFileDescriptor(&kqueue_);
+    std::for_each(events_.begin(), events_.end(), ReleaseEvent);
+    events_.clear();
+    io_message_loop_.release();
     MessageLoop::current()->RemoveDestructionObserver(this);
     delegate_ = NULL;
   }
 }
 
-void FilePathWatcherImpl::WillDestroyCurrentMessageLoop() {
-  CancelOnMessageLoopThread();
-}
-
-void FilePathWatcherImpl::UpdateEventStream(FSEventStreamEventId start_event) {
-  DCHECK(run_loop_message_loop()->BelongsToCurrentThread());
-  DCHECK(MessageLoopForUI::current());
-
-  // It can happen that the watcher gets canceled while tasks that call this
-  // function are still in flight, so abort if this situation is detected.
-  if (is_cancelled())
-    return;
-
-  if (fsevent_stream_)
-    DestroyEventStream();
-
-  base::mac::ScopedCFTypeRef<CFStringRef> cf_path(CFStringCreateWithCString(
-      NULL, target_.value().c_str(), kCFStringEncodingMacHFS));
-  base::mac::ScopedCFTypeRef<CFStringRef> cf_dir_path(CFStringCreateWithCString(
-      NULL, target_.DirName().value().c_str(), kCFStringEncodingMacHFS));
-  CFStringRef paths_array[] = { cf_path.get(), cf_dir_path.get() };
-  base::mac::ScopedCFTypeRef<CFArrayRef> watched_paths(CFArrayCreate(
-      NULL, reinterpret_cast<const void**>(paths_array), arraysize(paths_array),
-      &kCFTypeArrayCallBacks));
-
-  FSEventStreamContext context;
-  context.version = 0;
-  context.info = this;
-  context.retain = NULL;
-  context.release = NULL;
-  context.copyDescription = NULL;
-
-  fsevent_stream_ = FSEventStreamCreate(NULL, &FSEventsCallback, &context,
-                                        watched_paths,
-                                        start_event,
-                                        kEventLatencySeconds,
-                                        kFSEventStreamCreateFlagWatchRoot);
-  FSEventStreamScheduleWithRunLoop(fsevent_stream_, CFRunLoopGetCurrent(),
-                                   kCFRunLoopDefaultMode);
-  if (!FSEventStreamStart(fsevent_stream_)) {
-    message_loop()->PostTask(FROM_HERE,
-        NewRunnableMethod(delegate_.get(),
-                          &FilePathWatcher::Delegate::OnError));
-  }
-}
-
-void FilePathWatcherImpl::DestroyEventStream() {
-  FSEventStreamStop(fsevent_stream_);
-  FSEventStreamUnscheduleFromRunLoop(fsevent_stream_, CFRunLoopGetCurrent(),
-                                     kCFRunLoopDefaultMode);
-  FSEventStreamRelease(fsevent_stream_);
-  fsevent_stream_ = NULL;
-}
-
 }  // namespace
 
 FilePathWatcher::FilePathWatcher() {
   impl_ = new FilePathWatcherImpl();
 }