Do not leak textures from the backing stores

Neither the default nor the eglfs-specific backingstore release the OpenGL textures
that are in use when render-to-texture widgets are involved.

The result can be fatal on embedded devices that run out of GPU memory at after showing
and closing dialogs and popups a certain number of times.

Task-number: QTBUG-49363
Task-number: QTBUG-49399
Change-Id: Ia7471b037f147bcca0a4f1db5808ca348e230547
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Andy Nichols <andy.nichols@theqtcompany.com>

src/gui/painting/qplatformbackingstore.cpp

@@ -65,6 +65,10 @@ public:
     ~QPlatformBackingStorePrivate()
     {
 #ifndef QT_NO_OPENGL
+        QOpenGLContext *ctx = QOpenGLContext::currentContext();
+        Q_ASSERT(ctx);
+        if (textureId)
+            ctx->functions()->glDeleteTextures(1, &textureId);
         if (blitter)
             blitter->destroy();
         delete blitter;

src/platformsupport/platformcompositor/qopenglcompositorbackingstore.cpp

@@ -67,6 +67,7 @@ QOpenGLCompositorBackingStore::QOpenGLCompositorBackingStore(QWindow *window)
     : QPlatformBackingStore(window),
       m_window(window),
       m_bsTexture(0),
+      m_bsTextureContext(0),
       m_textures(new QPlatformTextureList),
       m_lockedWidgetTextures(0)
 {
@@ -74,6 +75,14 @@ QOpenGLCompositorBackingStore::QOpenGLCompositorBackingStore(QWindow *window)
 
 QOpenGLCompositorBackingStore::~QOpenGLCompositorBackingStore()
 {
+    if (m_bsTexture) {
+        QOpenGLContext *ctx = QOpenGLContext::currentContext();
+        if (ctx && m_bsTextureContext && ctx->shareGroup() == m_bsTextureContext->shareGroup())
+            glDeleteTextures(1, &m_bsTexture);
+        else
+            qWarning("QOpenGLCompositorBackingStore: Texture is not valid in the current context");
+    }
+
     delete m_textures;
 }
 
@@ -85,6 +94,8 @@ QPaintDevice *QOpenGLCompositorBackingStore::paintDevice()
 void QOpenGLCompositorBackingStore::updateTexture()
 {
     if (!m_bsTexture) {
+        m_bsTextureContext = QOpenGLContext::currentContext();
+        Q_ASSERT(m_bsTextureContext);
         glGenTextures(1, &m_bsTexture);
         glBindTexture(GL_TEXTURE_2D, m_bsTexture);
         glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

src/platformsupport/platformcompositor/qopenglcompositorbackingstore_p.h

@@ -83,6 +83,7 @@ private:
     QImage m_image;
     QRegion m_dirty;
     uint m_bsTexture;
+    QOpenGLContext *m_bsTextureContext;
     QPlatformTextureList *m_textures;
     QPlatformTextureList *m_lockedWidgetTextures;
 };

src/widgets/kernel/qwidget.cpp

@@ -72,6 +72,7 @@
 #include <QtGui/qinputmethod.h>
 #include <QtGui/qopenglcontext.h>
 #include <QtGui/private/qopenglcontext_p.h>
+#include <QtGui/qoffscreensurface.h>
 
 #include <private/qgraphicseffect_p.h>
 #include <qbackingstore.h>
@@ -1817,24 +1818,47 @@ void QWidgetPrivate::deleteSysExtra()
 {
 }
 
+static void deleteBackingStore(QWidgetPrivate *d)
+{
+    QTLWExtra *topData = d->topData();
+
+    // The context must be current when destroying the backing store as it may attempt to
+    // release resources like textures and shader programs. The window may not be suitable
+    // anymore as there will often not be a platform window underneath at this stage. Fall
+    // back to a QOffscreenSurface in this case.
+    QScopedPointer<QOffscreenSurface> tempSurface;
+#ifndef QT_NO_OPENGL
+    if (d->textureChildSeen && topData->shareContext) {
+        if (topData->window->handle()) {
+            topData->shareContext->makeCurrent(topData->window);
+        } else {
+            tempSurface.reset(new QOffscreenSurface);
+            tempSurface->setFormat(topData->shareContext->format());
+            tempSurface->create();
+            topData->shareContext->makeCurrent(tempSurface.data());
+        }
+    }
+#endif
+
+    delete topData->backingStore;
+    topData->backingStore = 0;
+
+#ifndef QT_NO_OPENGL
+    if (d->textureChildSeen && topData->shareContext)
+        topData->shareContext->doneCurrent();
+#endif
+}
+
 void QWidgetPrivate::deleteTLSysExtra()
 {
     if (extra && extra->topextra) {
         //the qplatformbackingstore may hold a reference to the window, so the backingstore
         //needs to be deleted first. If the backingstore holds GL resources, we need to
-        // make the context current here, since the platform bs does not have a reference
-        // to the widget.
+        // make the context current here. This is taken care of by deleteBackingStore().
 
-#ifndef QT_NO_OPENGL
-        if (textureChildSeen && extra->topextra->shareContext)
-            extra->topextra->shareContext->makeCurrent(extra->topextra->window);
-#endif
         extra->topextra->backingStoreTracker.destroy();
-        delete extra->topextra->backingStore;
-        extra->topextra->backingStore = 0;
+        deleteBackingStore(this);
 #ifndef QT_NO_OPENGL
-        if (textureChildSeen && extra->topextra->shareContext)
-            extra->topextra->shareContext->doneCurrent();
         delete extra->topextra->shareContext;
         extra->topextra->shareContext = 0;
 #endif
@@ -11980,7 +12004,7 @@ void QWidget::setBackingStore(QBackingStore *store)
         return;
 
     QBackingStore *oldStore = topData->backingStore;
-    delete topData->backingStore;
+    deleteBackingStore(d);
     topData->backingStore = store;
 
     QWidgetBackingStore *bs = d->maybeBackingStore();