base/files/file_path_watcher_win.cc

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "base/files/file_path_watcher.h"
   6
   7 #include "base/bind.h"
   8 #include "base/file_util.h"
   9 #include "base/files/file.h"
  10 #include "base/files/file_path.h"
  11 #include "base/logging.h"
  12 #include "base/memory/ref_counted.h"
  13 #include "base/message_loop/message_loop_proxy.h"
  14 #include "base/time/time.h"
  15 #include "base/win/object_watcher.h"
  16
  17 namespace base {
  18
  19 namespace {
  20
  21 class FilePathWatcherImpl : public FilePathWatcher::PlatformDelegate,
  22                             public base::win::ObjectWatcher::Delegate,
  23                             public MessageLoop::DestructionObserver {
  24  public:
  25   FilePathWatcherImpl()
  26       : handle_(INVALID_HANDLE_VALUE),
  27         recursive_watch_(false) {}
  28
  29   // FilePathWatcher::PlatformDelegate overrides.
  30   virtual bool Watch(const FilePath& path,
  31                      bool recursive,
  32                      const FilePathWatcher::Callback& callback) OVERRIDE;
  33   virtual void Cancel() OVERRIDE;
  34
  35   // Deletion of the FilePathWatcher will call Cancel() to dispose of this
  36   // object in the right thread. This also observes destruction of the required
  37   // cleanup thread, in case it quits before Cancel() is called.
  38   virtual void WillDestroyCurrentMessageLoop() OVERRIDE;
  39
  40   // Callback from MessageLoopForIO.
  41   virtual void OnObjectSignaled(HANDLE object);
  42
  43  private:
  44   virtual ~FilePathWatcherImpl() {}
  45
  46   // Setup a watch handle for directory |dir|. Set |recursive| to true to watch
  47   // the directory sub trees. Returns true if no fatal error occurs. |handle|
  48   // will receive the handle value if |dir| is watchable, otherwise
  49   // INVALID_HANDLE_VALUE.
  50   static bool SetupWatchHandle(const FilePath& dir,
  51                                bool recursive,
  52                                HANDLE* handle) WARN_UNUSED_RESULT;
  53
  54   // (Re-)Initialize the watch handle.
  55   bool UpdateWatch() WARN_UNUSED_RESULT;
  56
  57   // Destroy the watch handle.
  58   void DestroyWatch();
  59
  60   // Cleans up and stops observing the |message_loop_| thread.
  61   void CancelOnMessageLoopThread() OVERRIDE;
  62
  63   // Callback to notify upon changes.
  64   FilePathWatcher::Callback callback_;
  65
  66   // Path we're supposed to watch (passed to callback).
  67   FilePath target_;
  68
  69   // Handle for FindFirstChangeNotification.
  70   HANDLE handle_;
  71
  72   // ObjectWatcher to watch handle_ for events.
  73   base::win::ObjectWatcher watcher_;
  74
  75   // Set to true to watch the sub trees of the specified directory file path.
  76   bool recursive_watch_;
  77
  78   // Keep track of the last modified time of the file.  We use nulltime
  79   // to represent the file not existing.
  80   Time last_modified_;
  81
  82   // The time at which we processed the first notification with the
  83   // |last_modified_| time stamp.
  84   Time first_notification_;
  85
  86   DISALLOW_COPY_AND_ASSIGN(FilePathWatcherImpl);
  87 };
  88
  89 bool FilePathWatcherImpl::Watch(const FilePath& path,
  90                                 bool recursive,
  91                                 const FilePathWatcher::Callback& callback) {
  92   DCHECK(target_.value().empty());  // Can only watch one path.
  93
  94   set_message_loop(MessageLoopProxy::current());
  95   callback_ = callback;
  96   target_ = path;
  97   recursive_watch_ = recursive;
  98   MessageLoop::current()->AddDestructionObserver(this);
  99
 100   File::Info file_info;
 101   if (GetFileInfo(target_, &file_info)) {
 102     last_modified_ = file_info.last_modified;
 103     first_notification_ = Time::Now();
 104   }
 105
 106   if (!UpdateWatch())
 107     return false;
 108
 109   watcher_.StartWatching(handle_, this);
 110
 111   return true;
 112 }
 113
 114 void FilePathWatcherImpl::Cancel() {
 115   if (callback_.is_null()) {
 116     // Watch was never called, or the |message_loop_| has already quit.
 117     set_cancelled();
 118     return;
 119   }
 120
 121   // Switch to the file thread if necessary so we can stop |watcher_|.
 122   if (!message_loop()->BelongsToCurrentThread()) {
 123     message_loop()->PostTask(FROM_HERE,
 124                              Bind(&FilePathWatcher::CancelWatch,
 125                                   make_scoped_refptr(this)));
 126   } else {
 127     CancelOnMessageLoopThread();
 128   }
 129 }
 130
 131 void FilePathWatcherImpl::CancelOnMessageLoopThread() {
 132   DCHECK(message_loop()->BelongsToCurrentThread());
 133   set_cancelled();
 134
 135   if (handle_ != INVALID_HANDLE_VALUE)
 136     DestroyWatch();
 137
 138   if (!callback_.is_null()) {
 139     MessageLoop::current()->RemoveDestructionObserver(this);
 140     callback_.Reset();
 141   }
 142 }
 143
 144 void FilePathWatcherImpl::WillDestroyCurrentMessageLoop() {
 145   CancelOnMessageLoopThread();
 146 }
 147
 148 void FilePathWatcherImpl::OnObjectSignaled(HANDLE object) {
 149   DCHECK(object == handle_);
 150   // Make sure we stay alive through the body of this function.
 151   scoped_refptr<FilePathWatcherImpl> keep_alive(this);
 152
 153   if (!UpdateWatch()) {
 154     callback_.Run(target_, true /* error */);
 155     return;
 156   }
 157
 158   // Check whether the event applies to |target_| and notify the callback.
 159   File::Info file_info;
 160   bool file_exists = GetFileInfo(target_, &file_info);
 161   if (recursive_watch_) {
 162     // Only the mtime of |target_| is tracked but in a recursive watch,
 163     // some other file or directory may have changed so all notifications
 164     // are passed through. It is possible to figure out which file changed
 165     // using ReadDirectoryChangesW() instead of FindFirstChangeNotification(),
 166     // but that function is quite complicated:
 167     // http://qualapps.blogspot.com/2010/05/understanding-readdirectorychangesw.html
 168     callback_.Run(target_, false);
 169   } else if (file_exists && (last_modified_.is_null() ||
 170              last_modified_ != file_info.last_modified)) {
 171     last_modified_ = file_info.last_modified;
 172     first_notification_ = Time::Now();
 173     callback_.Run(target_, false);
 174   } else if (file_exists && last_modified_ == file_info.last_modified &&
 175              !first_notification_.is_null()) {
 176     // The target's last modification time is equal to what's on record. This
 177     // means that either an unrelated event occurred, or the target changed
 178     // again (file modification times only have a resolution of 1s). Comparing
 179     // file modification times against the wall clock is not reliable to find
 180     // out whether the change is recent, since this code might just run too
 181     // late. Moreover, there's no guarantee that file modification time and wall
 182     // clock times come from the same source.
 183     //
 184     // Instead, the time at which the first notification carrying the current
 185     // |last_notified_| time stamp is recorded. Later notifications that find
 186     // the same file modification time only need to be forwarded until wall
 187     // clock has advanced one second from the initial notification. After that
 188     // interval, client code is guaranteed to having seen the current revision
 189     // of the file.
 190     if (Time::Now() - first_notification_ > TimeDelta::FromSeconds(1)) {
 191       // Stop further notifications for this |last_modification_| time stamp.
 192       first_notification_ = Time();
 193     }
 194     callback_.Run(target_, false);
 195   } else if (!file_exists && !last_modified_.is_null()) {
 196     last_modified_ = Time();
 197     callback_.Run(target_, false);
 198   }
 199
 200   // The watch may have been cancelled by the callback.
 201   if (handle_ != INVALID_HANDLE_VALUE)
 202     watcher_.StartWatching(handle_, this);
 203 }
 204
 205 // static
 206 bool FilePathWatcherImpl::SetupWatchHandle(const FilePath& dir,
 207                                            bool recursive,
 208                                            HANDLE* handle) {
 209   *handle = FindFirstChangeNotification(
 210       dir.value().c_str(),
 211       recursive,
 212       FILE_NOTIFY_CHANGE_FILE_NAME | FILE_NOTIFY_CHANGE_SIZE |
 213       FILE_NOTIFY_CHANGE_LAST_WRITE | FILE_NOTIFY_CHANGE_DIR_NAME |
 214       FILE_NOTIFY_CHANGE_ATTRIBUTES | FILE_NOTIFY_CHANGE_SECURITY);
 215   if (*handle != INVALID_HANDLE_VALUE) {
 216     // Make sure the handle we got points to an existing directory. It seems
 217     // that windows sometimes hands out watches to directories that are
 218     // about to go away, but doesn't sent notifications if that happens.
 219     if (!DirectoryExists(dir)) {
 220       FindCloseChangeNotification(*handle);
 221       *handle = INVALID_HANDLE_VALUE;
 222     }
 223     return true;
 224   }
 225
 226   // If FindFirstChangeNotification failed because the target directory
 227   // doesn't exist, access is denied (happens if the file is already gone but
 228   // there are still handles open), or the target is not a directory, try the
 229   // immediate parent directory instead.
 230   DWORD error_code = GetLastError();
 231   if (error_code != ERROR_FILE_NOT_FOUND &&
 232       error_code != ERROR_PATH_NOT_FOUND &&
 233       error_code != ERROR_ACCESS_DENIED &&
 234       error_code != ERROR_SHARING_VIOLATION &&
 235       error_code != ERROR_DIRECTORY) {
 236     DPLOG(ERROR) << "FindFirstChangeNotification failed for "
 237                  << dir.value();
 238     return false;
 239   }
 240
 241   return true;
 242 }
 243
 244 bool FilePathWatcherImpl::UpdateWatch() {
 245   if (handle_ != INVALID_HANDLE_VALUE)
 246     DestroyWatch();
 247
 248   // Start at the target and walk up the directory chain until we succesfully
 249   // create a watch handle in |handle_|. |child_dirs| keeps a stack of child
 250   // directories stripped from target, in reverse order.
 251   std::vector<FilePath> child_dirs;
 252   FilePath watched_path(target_);
 253   while (true) {
 254     if (!SetupWatchHandle(watched_path, recursive_watch_, &handle_))
 255       return false;
 256
 257     // Break if a valid handle is returned. Try the parent directory otherwise.
 258     if (handle_ != INVALID_HANDLE_VALUE)
 259       break;
 260
 261     // Abort if we hit the root directory.
 262     child_dirs.push_back(watched_path.BaseName());
 263     FilePath parent(watched_path.DirName());
 264     if (parent == watched_path) {
 265       DLOG(ERROR) << "Reached the root directory";
 266       return false;
 267     }
 268     watched_path = parent;
 269   }
 270
 271   // At this point, handle_ is valid. However, the bottom-up search that the
 272   // above code performs races against directory creation. So try to walk back
 273   // down and see whether any children appeared in the mean time.
 274   while (!child_dirs.empty()) {
 275     watched_path = watched_path.Append(child_dirs.back());
 276     child_dirs.pop_back();
 277     HANDLE temp_handle = INVALID_HANDLE_VALUE;
 278     if (!SetupWatchHandle(watched_path, recursive_watch_, &temp_handle))
 279       return false;
 280     if (temp_handle == INVALID_HANDLE_VALUE)
 281       break;
 282     FindCloseChangeNotification(handle_);
 283     handle_ = temp_handle;
 284   }
 285
 286   return true;
 287 }
 288
 289 void FilePathWatcherImpl::DestroyWatch() {
 290   watcher_.StopWatching();
 291   FindCloseChangeNotification(handle_);
 292   handle_ = INVALID_HANDLE_VALUE;
 293 }
 294
 295 }  // namespace
 296
 297 FilePathWatcher::FilePathWatcher() {
 298   impl_ = new FilePathWatcherImpl();
 299 }
 300
 301 }  // namespace base