Fixed race condition in raster paint engine.

We need to protect the gradient cache accesses with a mutex.

Task-number: QTBUG-14614
Reviewed-by: Bradley T. Hughes

Author: Samuel Rødal

src/gui/painting/qpaintengine_raster.cpp

@@ -4811,6 +4811,7 @@ class QGradientCache
         for (int i = 0; i < stops.size() && i <= 2; i++)
             hash_val += stops[i].second.rgba();
 
+        QMutexLocker lock(&mutex);
         QGradientColorTableHash::const_iterator it = cache.constFind(hash_val);
 
         if (it == cache.constEnd())
@@ -4844,6 +4845,7 @@ class QGradientCache
     }
 
     QGradientColorTableHash cache;
+    QMutex mutex;
 };
 
 void QGradientCache::generateGradientColorTable(const QGradient& gradient, uint *colorTable, int size, int opacity) const

tests/auto/qpainter/tst_qpainter.cpp

@@ -251,6 +251,8 @@ private slots:
 
     void QTBUG5939_attachPainterPrivate();
 
+    void QTBUG14614_gradientCacheRaceCondition();
+
 private:
     void fillData();
     void setPenColor(QPainter& p);
@@ -4458,6 +4460,36 @@ void tst_QPainter::QTBUG5939_attachPainterPrivate()
     QCOMPARE(widget->deviceTransform, proxy->deviceTransform);
 }
 
+class GradientProducer : public QThread
+{
+protected:
+    void run();
+};
+
+void GradientProducer::run()
+{
+    QImage image(1, 1, QImage::Format_RGB32);
+    QPainter p(&image);
+
+    for (int i = 0; i < 1000; ++i) {
+        QLinearGradient g;
+        g.setColorAt(0, QColor(i % 256, 0, 0));
+        g.setColorAt(1, Qt::white);
+
+        p.fillRect(image.rect(), g);
+    }
+}
+
+void tst_QPainter::QTBUG14614_gradientCacheRaceCondition()
+{
+    const int threadCount = 16;
+    GradientProducer producers[threadCount];
+    for (int i = 0; i < threadCount; ++i)
+        producers[i].start();
+    for (int i = 0; i < threadCount; ++i)
+        producers[i].wait();
+}
+
 QTEST_MAIN(tst_QPainter)
 
 #include "tst_qpainter.moc"