Prevent deletion of LibraryModel object while async query is running.

Create a thread pool for each LibraryModel object and block destruction until
all threads that are operating on this object are complete.

Note that this is not a complete solution. The async query also uses the library
backend which may still be deleted before the thread exits. This will be
addressed in a future change.

src/library/librarymodel.cpp

@@ -88,6 +88,7 @@ LibraryModel::LibraryModel(LibraryBackend* backend, Application* app,
       playlists_dir_icon_(IconLoader::Load("folder-sound", IconLoader::Base)),
       playlist_icon_(IconLoader::Load("x-clementine-albums", IconLoader::Base)),
       icon_cache_(new QNetworkDiskCache(this)),
+      thread_pool_(this),
       init_task_id_(-1),
       use_pretty_covers_(false),
       show_dividers_(true) {
@@ -129,7 +130,10 @@ LibraryModel::LibraryModel(LibraryBackend* backend, Application* app,
   backend_->UpdateTotalSongCountAsync();
 }
 
-LibraryModel::~LibraryModel() { delete root_; }
+LibraryModel::~LibraryModel() {
+  thread_pool_.waitForDone();
+  delete root_;
+}
 
 void LibraryModel::set_pretty_covers(bool use_pretty_covers) {
   if (use_pretty_covers != use_pretty_covers_) {
@@ -769,7 +773,7 @@ void LibraryModel::LazyPopulate(LibraryItem* parent, bool signal) {
 
 void LibraryModel::ResetAsync() {
   QFuture<LibraryModel::QueryResult> future =
-      QtConcurrent::run(this, &LibraryModel::RunQuery, root_);
+      QtConcurrent::run(&thread_pool_, this, &LibraryModel::RunQuery, root_);
   NewClosure(future, this,
              SLOT(ResetAsyncQueryFinished(QFuture<LibraryModel::QueryResult>)),
              future);

src/library/librarymodel.h

@@ -21,6 +21,7 @@
 #include <QAbstractItemModel>
 #include <QIcon>
 #include <QNetworkDiskCache>
+#include <QThreadPool>
 
 #include "libraryitem.h"
 #include "libraryquery.h"
@@ -292,6 +293,8 @@ signals:
 
   QNetworkDiskCache* icon_cache_;
 
+  QThreadPool thread_pool_;
+
   int init_task_id_;
 
   bool use_pretty_covers_;