base/files/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 "base/files/file_path_watcher.h"

#include <fcntl.h>
#include <sys/event.h>
#include <sys/param.h>

#include <vector>

#include "base/file_util.h"
#include "base/message_loop.h"
#include "base/message_loop_proxy.h"
#include "base/stringprintf.h"

namespace base {
namespace files {

namespace {

// 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() : 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) OVERRIDE;
  virtual void Cancel() OVERRIDE;

 private:
  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_;
  FilePath target_;
  int kqueue_;

  DISALLOW_COPY_AND_ASSIGN(FilePathWatcherImpl);
};

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;
  }

  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) {
      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_);
  }
}

void FilePathWatcherImpl::OnFileCanWriteWithoutBlocking(int fd) {
  NOTREACHED();
}

void FilePathWatcherImpl::WillDestroyCurrentMessageLoop() {
  CancelOnMessageLoopThread();
}

bool FilePathWatcherImpl::Watch(const FilePath& path,
                                FilePathWatcher::Delegate* delegate) {
  DCHECK(MessageLoopForIO::current());
  DCHECK(target_.value().empty());  // Can only watch one path.
  DCHECK(delegate);
  DCHECK_EQ(kqueue_, -1);

  delegate_ = delegate;
  target_ = path;

  MessageLoop::current()->AddDestructionObserver(this);
  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() {
  base::MessageLoopProxy* proxy = io_message_loop_.get();
  if (!proxy) {
    set_cancelled();
    return;
  }
  if (!proxy->BelongsToCurrentThread()) {
    proxy->PostTask(FROM_HERE,
                    NewRunnableMethod(this, &FilePathWatcherImpl::Cancel));
    return;
  }
  CancelOnMessageLoopThread();
}

void FilePathWatcherImpl::CancelOnMessageLoopThread() {
  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_ = NULL;
    MessageLoop::current()->RemoveDestructionObserver(this);
    delegate_ = NULL;
  }
}

}  // namespace

FilePathWatcher::FilePathWatcher() {
  impl_ = new FilePathWatcherImpl();
}

}  // namespace files
}  // namespace base