QWindowSystemInterface: fix mem leak and race

There was a race where QGuiApplicationPrivate::processMouseEvent accessed
QWindowSystemInterfacePrivate::windowSystemEventQueue without holding
QWindowSystemInterfacePrivate::queueMutex.

There was a memory leak where QWindowSystemInterfacePrivate::windowSystemEventQueue
would not delete events contained in it when it was destroyed.

Fix both of these by properly encapsulating the QList/QMutex pair
in a small class, WindowSystemEventList, that allows only properly protected access
to the internal QList and calls qDeleteAll() in its dtor.

Change-Id: Ifaa9968c9272096df2f7109a7a6cf1c8e5fa736c
Reviewed-by: David Faure <faure@kde.org>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>

src/gui/kernel/qwindowsysteminterface.cpp

@@ -58,8 +58,7 @@ QElapsedTimer QWindowSystemInterfacePrivate::eventTime;
 // Callback functions for plugins:
 //
 
-QList<QWindowSystemInterfacePrivate::WindowSystemEvent *> QWindowSystemInterfacePrivate::windowSystemEventQueue;
-QMutex QWindowSystemInterfacePrivate::queueMutex;
+QWindowSystemInterfacePrivate::WindowSystemEventList QWindowSystemInterfacePrivate::windowSystemEventQueue;
 
 extern QPointer<QWindow> qt_last_mouse_receiver;
 
@@ -330,29 +329,17 @@ QWindowSystemInterfacePrivate::ExposeEvent::ExposeEvent(QWindow *exposed, const
 
 int QWindowSystemInterfacePrivate::windowSystemEventsQueued()
 {
-    queueMutex.lock();
-    int ret = windowSystemEventQueue.count();
-    queueMutex.unlock();
-    return ret;
+    return windowSystemEventQueue.count();
 }
 
 QWindowSystemInterfacePrivate::WindowSystemEvent * QWindowSystemInterfacePrivate::getWindowSystemEvent()
 {
-    queueMutex.lock();
-    QWindowSystemInterfacePrivate::WindowSystemEvent *ret;
-    if (windowSystemEventQueue.isEmpty())
-        ret = 0;
-    else
-        ret = windowSystemEventQueue.takeFirst();
-    queueMutex.unlock();
-    return ret;
+    return windowSystemEventQueue.takeFirstOrReturnNull();
 }
 
 void QWindowSystemInterfacePrivate::queueWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent *ev)
 {
-    queueMutex.lock();
     windowSystemEventQueue.append(ev);
-    queueMutex.unlock();
 
     QAbstractEventDispatcher *dispatcher = QGuiApplicationPrivate::qt_qpa_core_dispatcher();
     if (dispatcher)

src/gui/kernel/qwindowsysteminterface_p.h

@@ -45,6 +45,8 @@
 
 #include <QElapsedTimer>
 #include <QPointer>
+#include <QMutex>
+#include <QList>
 
 QT_BEGIN_HEADER
 
@@ -323,8 +325,27 @@ public:
         qint64 uid;
     };
 
-    static QList<WindowSystemEvent *> windowSystemEventQueue;
-    static QMutex queueMutex;
+    class WindowSystemEventList {
+        QList<WindowSystemEvent *> impl;
+        mutable QMutex mutex;
+    public:
+        WindowSystemEventList() : impl(), mutex() {}
+        ~WindowSystemEventList()
+        { const QMutexLocker locker(&mutex); qDeleteAll(impl); impl.clear(); }
+
+        void prepend(WindowSystemEvent *e)
+        { const QMutexLocker locker(&mutex); impl.prepend(e); }
+        WindowSystemEvent *takeFirstOrReturnNull()
+        { const QMutexLocker locker(&mutex); return impl.empty() ? 0 : impl.takeFirst(); }
+        void append(WindowSystemEvent *e)
+        { const QMutexLocker locker(&mutex); impl.append(e); }
+        int count() const
+        { const QMutexLocker locker(&mutex); return impl.count(); }
+    private:
+        Q_DISABLE_COPY(WindowSystemEventList);
+    };
+
+    static WindowSystemEventList windowSystemEventQueue;
 
     static int windowSystemEventsQueued();
     static WindowSystemEvent * getWindowSystemEvent();