From dc787ba51b2aa450c98d4e873de6817959af4be2 Mon Sep 17 00:00:00 2001
From: Romain Vimont <rom1v@videolabs.io>
Date: Fri, 13 Nov 2020 14:04:42 +0100
Subject: [PATCH] qt: medialib: define a medialib thread pool

The database queries, initiated by list models, must be executed
asynchronously (on a thread different from the UI thread).

When a list model is destroyed (from the UI thread), some database
queries might still be executing. It would be incorrect for the list
model to wait for them (it would block the UI thread), so they must be
executed on an external thread pool.

Since the queries themselves require the medialib to be executed, they
must not outlive it (or the asynchronous code might crash). Therefore,
the right scope for executing asynchronous queries is the MediaLib
instance.

Signed-off-by: Pierre Lamot <pierre@videolabs.io>
---
 modules/gui/qt/medialibrary/medialib.cpp | 3 +++
 modules/gui/qt/medialibrary/medialib.hpp | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/modules/gui/qt/medialibrary/medialib.cpp b/modules/gui/qt/medialibrary/medialib.cpp
index c0b06339a9..cdccf4b1b9 100644
--- a/modules/gui/qt/medialibrary/medialib.cpp
+++ b/modules/gui/qt/medialibrary/medialib.cpp
@@ -35,6 +35,9 @@ MediaLib::MediaLib(intf_thread_t *_intf, QObject *_parent)
 {
     m_event_cb.reset( vlc_ml_event_register_callback( m_ml, MediaLib::onMediaLibraryEvent,
                                                       this ) );
+
+    /* https://xkcd.com/221/ */
+    m_threadPool.setMaxThreadCount(4);
 }
 
 void MediaLib::addToPlaylist(const QString& mrl, const QStringList* options)
diff --git a/modules/gui/qt/medialibrary/medialib.hpp b/modules/gui/qt/medialibrary/medialib.hpp
index d7646ea999..2274c359cd 100644
--- a/modules/gui/qt/medialibrary/medialib.hpp
+++ b/modules/gui/qt/medialibrary/medialib.hpp
@@ -28,6 +28,7 @@
 #include <QMetaObject>
 #include <QMetaMethod>
 #include <QQmlEngine>
+#include <QThreadPool>
 
 #include <memory>
 
@@ -65,6 +66,8 @@ public:
 
     vlc_medialibrary_t* vlcMl();
 
+    QThreadPool &threadPool() { return m_threadPool; }
+
 signals:
     void reloadStarted();
     void reloadCompleted();
@@ -90,4 +93,5 @@ private:
     vlc_medialibrary_t* m_ml;
     std::unique_ptr<vlc_ml_event_callback_t, std::function<void(vlc_ml_event_callback_t*)>> m_event_cb;
 
+    QThreadPool m_threadPool;
 };