Fix crash when combining QOpenGLWidget, QStaticText and Qt Quick

Under certain circumstances, if you had a widget with a QOpenGLPaintEngine,
and drew QStaticText into this, and then later had Qt Quick access the same
cache and try to resize it, we would get a crash because the resize function
would have a pointer to the paint engine and try to access its shader manager
(which would now be null, since this is outside the begin()/end() phase of the
paint engine.

The solution is to reset the paint engine pointer to null on the cache once it
has been populated and it is no longer needed.

[ChangeLog][QtGui][Text] Fixed a possible crash when combining QStaticText,
QOpenGLWidget and Qt Quick in the same application.

Task-number: QTBUG-70096
Change-Id: I7383ad7456d1a72499cfcd2da09a5a808d4b3eff
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/gui/opengl/qopenglpaintengine.cpp

@@ -1741,6 +1741,7 @@ void QOpenGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngine::GlyphFormat gly
             // we may have to re-bind brush textures after filling in the cache.
             brushTextureDirty = (QT_BRUSH_TEXTURE_UNIT == glypchCacheTextureUnit);
         }
+        cache->setPaintEnginePrivate(nullptr);
     }
 
     if (cache->width() == 0 || cache->height() == 0)

src/gui/opengl/qopengltextureglyphcache_p.h

@@ -152,6 +152,11 @@ public:
 
     void clear();
 
+    QOpenGL2PaintEngineExPrivate *paintEnginePrivate() const
+    {
+        return pex;
+    }
+
 private:
     void setupVertexAttribs();
 

tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp

@@ -30,6 +30,7 @@
 #include <QtGui/QOpenGLFunctions>
 #include <QtGui/QPainter>
 #include <QtGui/QScreen>
+#include <QtGui/QStaticText>
 #include <QtWidgets/QDesktopWidget>
 #include <QtWidgets/QGraphicsView>
 #include <QtWidgets/QGraphicsScene>
@@ -40,6 +41,8 @@
 #include <QtTest/QtTest>
 #include <QSignalSpy>
 #include <private/qguiapplication_p.h>
+#include <private/qstatictext_p.h>
+#include <private/qopengltextureglyphcache_p.h>
 #include <qpa/qplatformintegration.h>
 
 class tst_QOpenGLWidget : public QObject
@@ -64,6 +67,10 @@ private slots:
     void stackWidgetOpaqueChildIsVisible();
     void offscreen();
     void offscreenThenOnscreen();
+
+#ifdef QT_BUILD_INTERNAL
+    void staticTextDanglingPointer();
+#endif
 };
 
 void tst_QOpenGLWidget::initTestCase()
@@ -675,6 +682,53 @@ void tst_QOpenGLWidget::offscreenThenOnscreen()
     QVERIFY(image.pixel(30, 40) == qRgb(0, 0, 255));
 }
 
+class StaticTextPainterWidget : public QOpenGLWidget
+{
+public:
+    StaticTextPainterWidget(QWidget *parent = nullptr)
+        : QOpenGLWidget(parent)
+    {
+    }
+
+    void paintEvent(QPaintEvent *)
+    {
+        QPainter p(this);
+        text.setText(QStringLiteral("test"));
+        p.drawStaticText(0, 0, text);
+
+        ctx = QOpenGLContext::currentContext();
+    }
+
+    QStaticText text;
+    QOpenGLContext *ctx;
+};
+
+#ifdef QT_BUILD_INTERNAL
+void tst_QOpenGLWidget::staticTextDanglingPointer()
+{
+    QWidget w;
+    StaticTextPainterWidget *glw = new StaticTextPainterWidget(&w);
+    w.resize(640, 480);
+    glw->resize(320, 200);
+    w.show();
+
+    QVERIFY(QTest::qWaitForWindowExposed(&w));
+    QStaticTextPrivate *d = QStaticTextPrivate::get(&glw->text);
+
+    QCOMPARE(d->itemCount, 1);
+    QFontEngine *fe = d->items->fontEngine();
+
+    for (int i = QFontEngine::Format_None; i <= QFontEngine::Format_ARGB; ++i) {
+        QOpenGLTextureGlyphCache *cache =
+                (QOpenGLTextureGlyphCache *) fe->glyphCache(glw->ctx,
+                                                            QFontEngine::GlyphFormat(i),
+                                                            QTransform());
+        if (cache != nullptr)
+            QCOMPARE(cache->paintEnginePrivate(), nullptr);
+    }
+}
+#endif
+
 QTEST_MAIN(tst_QOpenGLWidget)
 
 #include "tst_qopenglwidget.moc"