Let QPlatformBackingStore handle its own QOpenGLContext

The resources allocated by QPlatformBackingStore are owned by the class,
and should be allocated in a context the class also owns. This removes
the asymmetry of having to pass in a context to composeAndFlush, while
having to make the same context current before destroying the platform
backingstore.

The context owned by QPlatformBackingStore is shared with the associated
window though a new QWindowPrivate::shareContext() API.

The result is that on e.g. iOS, the backingstore does not need to tie
the resource allocation of QPlatformBackingStore to the global share
context, but can instead tie them to the per-window context, and hence
clean them up after each window is closed.

Task-number: QTBUG-56653
Change-Id: Ic1bcae50dafeeafaa8d16a7febd83b840ec6367a
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

3 files changed, 11 insertions, 29 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index be9287d39b..a7258d8f7e 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -1876,39 +1876,15 @@ 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. This is taken care of by deleteBackingStore().
+        //needs to be deleted first.
 
         extra->topextra->backingStoreTracker.destroy();
         deleteBackingStore(this);
diff --git a/src/widgets/kernel/qwidgetbackingstore.cpp b/src/widgets/kernel/qwidgetbackingstore.cpp
index bb421927db..52080fe05f 100644
--- a/src/widgets/kernel/qwidgetbackingstore.cpp
+++ b/src/widgets/kernel/qwidgetbackingstore.cpp
@@ -143,10 +143,8 @@ void QWidgetBackingStore::qt_flush(QWidget *widget, const QRegion &region, QBack
         // WA_TranslucentBackground. Therefore the compositor needs to know whether the app intends
         // to rely on translucency, in order to decide if it should clear to transparent or opaque.
         const bool translucentBackground = widget->testAttribute(Qt::WA_TranslucentBackground);
-        // Use the tlw's context, not widget's. The difference is important with native child
-        // widgets where tlw != widget.
-        backingStore->handle()->composeAndFlush(widget->windowHandle(), effectiveRegion, offset, widgetTextures,
-                                                tlw->d_func()->shareContext(), translucentBackground);
+        backingStore->handle()->composeAndFlush(widget->windowHandle(), effectiveRegion, offset,
+                                                widgetTextures, translucentBackground);
         widget->window()->d_func()->sendComposeStatus(widget->window(), true);
     } else
 #endif
diff --git a/src/widgets/kernel/qwidgetwindow.cpp b/src/widgets/kernel/qwidgetwindow.cpp
index d30154410c..319eccf223 100644
--- a/src/widgets/kernel/qwidgetwindow.cpp
+++ b/src/widgets/kernel/qwidgetwindow.cpp
@@ -96,6 +96,7 @@ public:
     }
 
     QRectF closestAcceptableGeometry(const QRectF &rect) const Q_DECL_OVERRIDE;
+    QOpenGLContext *shareContext() const override;
 };
 
 QRectF QWidgetWindowPrivate::closestAcceptableGeometry(const QRectF &rect) const
@@ -127,6 +128,13 @@ QRectF QWidgetWindowPrivate::closestAcceptableGeometry(const QRectF &rect) const
     return result;
 }
 
+QOpenGLContext *QWidgetWindowPrivate::shareContext() const
+{
+    Q_Q(const QWidgetWindow);
+    const QWidgetPrivate *widgetPrivate = QWidgetPrivate::get(q->widget());
+    return widgetPrivate->shareContext();
+}
+
 QWidgetWindow::QWidgetWindow(QWidget *widget)
     : QWindow(*new QWidgetWindowPrivate(), 0)
     , m_widget(widget)