Handle FileSystemWatcherInterface::AddPath errors.

On Linux systems, failure to watch a path may be caused by the limit set in
/proc/sys/fs/inotify/max_user_watches. This can be demonstrated by creating
a directory with a large number of empty subdirectories and adding that test
directory as a library.

Check that a file is readable before adding a watch. If adding the watch fails,
report the error to the user only once. Only add the path to subdir_mapping_
if watch succeeds.

src/devices/filesystemdevice.cpp

@@ -64,6 +64,7 @@ FilesystemDevice::FilesystemDevice(const QUrl& url, DeviceLister* lister,
   connect(watcher_, SIGNAL(CompilationsNeedUpdating()), backend_.get(),
           SLOT(UpdateCompilations()));
   connect(watcher_, SIGNAL(ScanStarted(int)), SIGNAL(TaskStarted(int)));
+  connect(watcher_, &LibraryWatcher::Error, app, &Application::AddError);
 }
 
 void FilesystemDevice::Init() {

src/library/library.cpp

@@ -161,6 +161,7 @@ void Library::Init() {
           backend_.get(), SLOT(AddOrUpdateSubdirs(SubdirectoryList)));
   connect(watcher_, SIGNAL(CompilationsNeedUpdating()), backend_.get(),
           SLOT(UpdateCompilations()));
+  connect(watcher_, &LibraryWatcher::Error, app_, &Application::AddError);
   connect(app_->playlist_manager(), SIGNAL(CurrentSongChanged(Song)),
           SLOT(CurrentSongChanged(Song)));
   connect(app_->player(), SIGNAL(Stopped()), SLOT(Stopped()));

src/library/librarywatcher.cpp

@@ -588,11 +588,34 @@ uint LibraryWatcher::GetMtimeForCue(const QString& cue_path) {
 }
 
 void LibraryWatcher::AddWatch(const Directory& dir, const QString& path) {
-  if (!QFile::exists(path)) return;
+  QFileInfo info(path);
+  if (!info.exists() || !info.isReadable()) return;
 
   connect(fs_watcher_, SIGNAL(PathChanged(const QString&)), this,
           SLOT(DirectoryChanged(const QString&)), Qt::UniqueConnection);
-  fs_watcher_->AddPath(path);
+  if (!fs_watcher_->AddPath(path)) {
+    // Since this may be a system error, don't spam the user.
+    static int errCount = 0;
+    if (errCount++ == 0) {
+#ifdef Q_OS_LINUX
+      // The Linux implementation of QFileSystemWatcher utilizes inotify, so
+      // the limit in /proc/sys/fs/inotify/max_user_watches may be a problem
+      // in large libraries.
+      const char* fmt =
+          "Failed to watch %1\n"
+          "On a Linux system, this may be due to the inotify max_user_watches "
+          "limit.\n\n"
+          "This error will not be shown again during this session.";
+#else
+      const char* fmt =
+          "Failed to watch %1 for unknown reasons.\n\n"
+          "This error will not be shown again during this session.";
+#endif
+      emit Error(tr(fmt).arg(path));
+    }
+    return;
+  }
+
   subdir_mapping_[path] = dir;
 }
 

src/library/librarywatcher.h

@@ -68,6 +68,8 @@ signals:
 
   void ScanStarted(int task_id);
 
+  void Error(const QString& message);
+
  public slots:
   void ReloadSettings();
   void AddDirectory(const Directory& dir, const SubdirectoryList& subdirs);