Cancel scan when directory is removed.

When a directory is removed from the library during a scan, the scan continues
until complete. This change cancels the scan immediately, unblocking the watcher
thread, then signals the watcher to remove the directory.

A second issue occurs when a previously scanned device is removed during a
scan. All remaining files will be marked as deleted. This change mitigates
this issue, but a timing hole still remains here.

src/devices/filesystemdevice.cpp

@@ -49,8 +49,9 @@ FilesystemDevice::FilesystemDevice(const QUrl& url, DeviceLister* lister,
   connect(backend_.get(),
           SIGNAL(DirectoryDiscovered(Directory, SubdirectoryList)), watcher_,
           SLOT(AddDirectory(Directory, SubdirectoryList)));
-  connect(backend_.get(), SIGNAL(DirectoryDeleted(Directory)), watcher_,
-          SLOT(RemoveDirectory(Directory)));
+  // RemoveDirectory should be called from the sender's thread.
+  connect(backend_.get(), &LibraryBackend::DirectoryDeleted, watcher_,
+          &LibraryWatcher::RemoveDirectory, Qt::DirectConnection);
   connect(watcher_, SIGNAL(NewOrUpdatedSongs(SongList)), backend_.get(),
           SLOT(AddOrUpdateSongs(SongList)));
   connect(watcher_, SIGNAL(SongsMTimeUpdated(SongList)), backend_.get(),

src/library/library.cpp

@@ -141,8 +141,9 @@ void Library::Init() {
   connect(backend_.get(),
           SIGNAL(DirectoryDiscovered(Directory, SubdirectoryList)), watcher_,
           SLOT(AddDirectory(Directory, SubdirectoryList)));
-  connect(backend_.get(), SIGNAL(DirectoryDeleted(Directory)), watcher_,
-          SLOT(RemoveDirectory(Directory)));
+  // RemoveDirectory should be called from the sender's thread.
+  connect(backend_.get(), &LibraryBackend::DirectoryDeleted, watcher_,
+          &LibraryWatcher::RemoveDirectory, Qt::DirectConnection);
   connect(backend_.get(), SIGNAL(SongsRatingChanged(SongList)),
           SLOT(SongsRatingChanged(SongList)));
   connect(backend_.get(), SIGNAL(SongsStatisticsChanged(SongList)),

src/library/librarywatcher.cpp

@@ -663,6 +663,12 @@ void LibraryWatcher::RemoveWatch(const Directory& dir,
 }
 
 void LibraryWatcher::RemoveDirectory(const Directory& dir) {
+  watched_dirs_.Stop(dir);
+  // Invoke the DoRemoveDirectory slot on the watcher's thread.
+  QMetaObject::invokeMethod(this, "DoRemoveDirectory", Q_ARG(Directory, dir));
+}
+
+void LibraryWatcher::DoRemoveDirectory(const Directory& dir) {
   rescan_queue_.remove(dir.id);
   watched_dirs_.Remove(dir.id);
 

src/library/librarywatcher.h

@@ -55,6 +55,10 @@ class LibraryWatcher : public QObject {
   void FullScanAsync();
   void SetRescanPausedAsync(bool pause);
   void ReloadSettingsAsync();
+  // This thread-safe method will cause a scan of this directory to cancel to
+  // unblock the watcher thread. It will then invoke the DoRemoveDirectory on
+  // the watcher's thread to complete the removal.
+  void RemoveDirectory(const Directory& dir);
 
   void Stop() { watched_dirs_.StopAll(); }
 
@@ -74,7 +78,6 @@ class LibraryWatcher : public QObject {
  public slots:
   void ReloadSettings();
   void AddDirectory(const Directory& dir, const SubdirectoryList& subdirs);
-  void RemoveDirectory(const Directory& dir);
   void SetRescanPaused(bool pause);
 
  private:
@@ -158,6 +161,7 @@ class LibraryWatcher : public QObject {
   void RescanPathsNow();
   void ScanSubdirectory(const QString& path, const Subdirectory& subdir,
                         ScanTransaction* t, bool force_noincremental = false);
+  void DoRemoveDirectory(const Directory& dir);
 
  private:
   static bool FindSongByPath(const SongList& list, const QString& path,