qt: introduce WorkerThreadSet

modules/gui/qt/Makefile.am

@@ -277,6 +277,7 @@ libqt_plugin_la_SOURCES = \
 	util/asynctask.hpp \
 	util/audio_device_model.cpp  \
 	util/audio_device_model.hpp \
+	util/workerthreadset.hpp util/workerthreadset.cpp \
 	util/base_model.hpp util/base_model_p.hpp util/base_model.cpp \
 	util/color_scheme_model.cpp util/color_scheme_model.hpp \
 	util/color_svg_image_provider.cpp util/color_svg_image_provider.hpp \
@@ -443,6 +444,7 @@ nodist_libqt_plugin_la_SOURCES = \
 	playlist/playlist_model.moc.cpp \
 	util/asynctask.moc.cpp \
 	util/audio_device_model.moc.cpp \
+	util/workerthreadset.moc.cpp \
 	util/base_model.moc.cpp \
 	util/color_scheme_model.moc.cpp \
 	util/color_svg_image_provider.moc.cpp \

modules/gui/qt/meson.build

@@ -121,6 +121,7 @@ moc_headers = files(
     'playlist/playlist_model.hpp',
     'util/asynctask.hpp',
     'util/audio_device_model.hpp',
+    'util/workerthreadset.hpp',
     'util/base_model.hpp',
     'util/color_scheme_model.hpp',
     'util/color_svg_image_provider.hpp',
@@ -408,6 +409,8 @@ some_sources = files(
     'util/asynctask.hpp',
     'util/audio_device_model.cpp',
     'util/audio_device_model.hpp',
+    'util/workerthreadset.cpp',
+    'util/workerthreadset.hpp',
     'util/base_model.cpp',
     'util/base_model.hpp',
     'util/base_model_p.hpp',

modules/gui/qt/util/workerthreadset.cpp

@@ -0,0 +1,103 @@
+/*****************************************************************************
+ * Copyright (C) 2024 VLC authors and VideoLAN
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * ( at your option ) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
+ *****************************************************************************/
+
+#include "workerthreadset.hpp"
+
+#include <QTimer>
+
+WorkerThreadSet::~WorkerThreadSet()
+{
+    for (auto worker : workers)
+    {
+        if (!worker.thread)
+            continue;
+
+        finish( worker );
+    }
+}
+
+void WorkerThreadSet::assignToWorkerThread(QObject *obj)
+{
+    auto worker = reserve();
+    obj->moveToThread( worker );
+
+    QObject::connect(obj, &QObject::destroyed, worker, [this, worker]()
+    {
+        unreserve( worker );
+    });
+}
+
+void WorkerThreadSet::finish(Worker &worker)
+{
+    worker.thread->quit();
+    worker.thread->wait();
+    delete worker.thread;
+}
+
+QThread *WorkerThreadSet::reserve()
+{
+    auto itr = std::min_element(std::begin(workers)
+                              , std::end(workers)
+                              , [](const Worker &l, const Worker &r)
+    {
+        return l.load < r.load;
+    });
+
+    assert(itr != std::end(workers));
+
+    if (!itr->thread)
+    {
+        itr->thread = new QThread;
+        itr->thread->start();
+    }
+
+    itr->load++;
+    itr->inactiveTime.invalidate();
+    return itr->thread;
+}
+
+void WorkerThreadSet::unreserve(QThread *thread)
+{
+    auto itr = std::find_if(std::begin(workers)
+                            , std::end(workers)
+                            , [thread](const Worker &i) { return i.thread == thread; });
+
+    if (itr == std::end(workers)) return; // impossible?
+
+    const int load = --itr->load;
+    if (load != 0)
+        return;
+
+    itr->inactiveTime.start();
+    QTimer::singleShot(CLEANUP_TIMEOUT, this, &WorkerThreadSet::cleanupInactiveWorker);
+}
+
+void WorkerThreadSet::cleanupInactiveWorker()
+{
+    for (auto &worker : workers)
+    {
+        if ((worker.load == 0)
+                && worker.inactiveTime.hasExpired(MAX_INACTIVE_TIME)
+                && worker.thread)
+        {
+            finish( worker );
+
+            worker.thread = nullptr;
+        }
+    }
+}

modules/gui/qt/util/workerthreadset.hpp

@@ -0,0 +1,63 @@
+/*****************************************************************************
+ * Copyright (C) 2024 VLC authors and VideoLAN
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * ( at your option ) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
+ *****************************************************************************/
+
+#pragma once
+
+#include <QObject>
+#include <QThread>
+
+// maintains a set of reusable worker threads
+// class is not thread safe and must be accessed from Main thread
+class WorkerThreadSet : public QObject
+{
+    Q_OBJECT
+public:
+    using QObject::QObject;
+
+    ~WorkerThreadSet();
+
+    // changes the thread affinity of 'obj' to a worker thread
+    void assignToWorkerThread(QObject *obj);
+
+private:
+    struct Worker
+    {
+        QThread *thread = nullptr;
+        int load = 0;
+        QElapsedTimer inactiveTime;
+    };
+
+    const static int MAX_WORKER = 2;
+    const static int CLEANUP_TIMEOUT = 10000; // 10seconds
+    const static int MAX_INACTIVE_TIME = 6000; // 6seconds
+
+    void finish(Worker &worker);
+
+    // returns a worker thread after increasing it's load
+    QThread *reserve();
+
+    // reduces load of the previously allocated 'thread'
+    // and makes it available for future operations
+    // threads are automatically freed when they remain
+    // inactive for extended amount of time
+    void unreserve(QThread *thread);
+
+    void cleanupInactiveWorker();
+
+    Worker workers[MAX_WORKER] {};
+};