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>

src/gui/kernel/qwindow.cpp

@@ -45,6 +45,7 @@
 #ifndef QT_NO_OPENGL
 #include <qpa/qplatformopenglcontext.h>
 #include "qopenglcontext.h"
+#include "qopenglcontext_p.h"
 #endif
 #include "qscreen.h"
 
@@ -2633,6 +2634,11 @@ QWindow *QWindowPrivate::topLevelWindow() const
     return window;
 }
 
+QOpenGLContext *QWindowPrivate::shareContext() const
+{
+    return qt_gl_global_share_context();
+};
+
 /*!
     Creates a local representation of a window created by another process or by
     using native libraries below Qt.

src/gui/kernel/qwindow_p.h

@@ -130,6 +130,8 @@ public:
 
     QWindow *topLevelWindow() const;
 
+    virtual QOpenGLContext *shareContext() const;
+
     virtual QWindow *eventReceiver() { Q_Q(QWindow); return q; }
 
     virtual void setVisible(bool visible);

src/gui/painting/qplatformbackingstore.cpp

@@ -50,6 +50,7 @@
 #include <QtGui/QOpenGLFunctions>
 #ifndef QT_NO_OPENGL
 #include <QtGui/qopengltextureblitter.h>
+#include <QtGui/qoffscreensurface.h>
 #endif
 #include <qpa/qplatformgraphicsbuffer.h>
 #include <qpa/qplatformgraphicsbufferhelper.h>
@@ -95,14 +96,15 @@ public:
     ~QPlatformBackingStorePrivate()
     {
 #ifndef QT_NO_OPENGL
-        QOpenGLContext *ctx = QOpenGLContext::currentContext();
+        if (context) {
-        if (ctx) {
+            QOffscreenSurface offscreenSurface;
+            offscreenSurface.setFormat(context->format());
+            offscreenSurface.create();
+            context->makeCurrent(&offscreenSurface);
             if (textureId)
-                ctx->functions()->glDeleteTextures(1, &textureId);
+                context->functions()->glDeleteTextures(1, &textureId);
             if (blitter)
                 blitter->destroy();
-        } else if (textureId || blitter) {
-            qWarning("No context current during QPlatformBackingStore destruction, OpenGL resources not released");
         }
         delete blitter;
 #endif
@@ -110,6 +112,7 @@ public:
     QWindow *window;
     QBackingStore *backingStore;
 #ifndef QT_NO_OPENGL
+    QScopedPointer<QOpenGLContext> context;
     mutable GLuint textureId;
     mutable QSize textureSize;
     mutable bool needsSwizzle;
@@ -316,20 +319,31 @@ static void blitTextureForWidget(const QPlatformTextureList *textures, int idx,
 
 void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &region,
                                             const QPoint &offset,
-                                            QPlatformTextureList *textures, QOpenGLContext *context,
+                                            QPlatformTextureList *textures,
                                             bool translucentBackground)
 {
     if (!qt_window_private(window)->receivedExpose)
         return;
 
-    if (!context->makeCurrent(window)) {
+    if (!d_ptr->context) {
+        d_ptr->context.reset(new QOpenGLContext);
+        d_ptr->context->setFormat(d_ptr->window->requestedFormat());
+        d_ptr->context->setScreen(d_ptr->window->screen());
+        d_ptr->context->setShareContext(qt_window_private(d_ptr->window)->shareContext());
+        if (!d_ptr->context->create()) {
+            qWarning("composeAndFlush: QOpenGLContext creation failed");
+            return;
+        }
+    }
+
+    if (!d_ptr->context->makeCurrent(window)) {
         qWarning("composeAndFlush: makeCurrent() failed");
         return;
     }
 
     QWindowPrivate::get(window)->lastComposeTime.start();
 
-    QOpenGLFunctions *funcs = context->functions();
+    QOpenGLFunctions *funcs = d_ptr->context->functions();
     funcs->glViewport(0, 0, window->width() * window->devicePixelRatio(), window->height() * window->devicePixelRatio());
     funcs->glClearColor(0, 0, 0, translucentBackground ? 0 : 1);
     funcs->glClear(GL_COLOR_BUFFER_BIT);
@@ -435,7 +449,7 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
     funcs->glDisable(GL_BLEND);
     d_ptr->blitter->release();
 
-    context->swapBuffers(window);
+    d_ptr->context->swapBuffers(window);
 }
 #endif
 /*!

src/gui/painting/qplatformbackingstore.h

@@ -120,7 +120,7 @@ public:
     virtual void flush(QWindow *window, const QRegion &region, const QPoint &offset) = 0;
 #ifndef QT_NO_OPENGL
     virtual void composeAndFlush(QWindow *window, const QRegion &region, const QPoint &offset,
-                                 QPlatformTextureList *textures, QOpenGLContext *context,
+                                 QPlatformTextureList *textures,
                                  bool translucentBackground);
 #endif
     virtual QImage toImage() const;

src/platformsupport/platformcompositor/qopenglcompositorbackingstore.cpp

@@ -202,14 +202,13 @@ void QOpenGLCompositorBackingStore::flush(QWindow *window, const QRegion &region
 }
 
 void QOpenGLCompositorBackingStore::composeAndFlush(QWindow *window, const QRegion &region, const QPoint &offset,
-                                               QPlatformTextureList *textures, QOpenGLContext *context,
+                                               QPlatformTextureList *textures,
                                                bool translucentBackground)
 {
     // QOpenGLWidget/QQuickWidget content provided as textures. The raster content goes on top.
 
     Q_UNUSED(region);
     Q_UNUSED(offset);
-    Q_UNUSED(context);
     Q_UNUSED(translucentBackground);
 
     QOpenGLCompositor *compositor = QOpenGLCompositor::instance();
@@ -218,7 +217,7 @@ void QOpenGLCompositorBackingStore::composeAndFlush(QWindow *window, const QRegi
 
     // The compositor's context and the context to which QOpenGLWidget/QQuickWidget
     // textures belong are not the same. They share resources, though.
-    Q_ASSERT(context->shareGroup() == dstCtx->shareGroup());
+    Q_ASSERT(qt_window_private(window)->shareContext()->shareGroup() == dstCtx->shareGroup());
 
     QWindow *dstWin = compositor->targetWindow();
     if (!dstWin)

src/platformsupport/platformcompositor/qopenglcompositorbackingstore_p.h

@@ -75,7 +75,7 @@ public:
 
     QImage toImage() const Q_DECL_OVERRIDE;
     void composeAndFlush(QWindow *window, const QRegion &region, const QPoint &offset,
-                         QPlatformTextureList *textures, QOpenGLContext *context,
+                         QPlatformTextureList *textures,
                          bool translucentBackground) Q_DECL_OVERRIDE;
 
     const QPlatformTextureList *textures() const { return m_textures; }

src/plugins/platforms/ios/qiosbackingstore.h

@@ -55,9 +55,6 @@ public:
     ~QIOSBackingStore();
 
     void flush(QWindow *window, const QRegion &region, const QPoint &offset) Q_DECL_OVERRIDE;
-
-private:
-    QOpenGLContext *m_context;
 };
 
 QT_END_NAMESPACE

src/plugins/platforms/ios/qiosbackingstore.mm

@@ -55,7 +55,6 @@ QT_BEGIN_NAMESPACE
 */
 QIOSBackingStore::QIOSBackingStore(QWindow *window)
     : QRasterBackingStore(window)
-    , m_context(new QOpenGLContext)
 {
     // We use the surface both for raster operations and for GL drawing (when
     // we blit the raster image), so the type needs to cover both use cases.
@@ -64,22 +63,10 @@ QIOSBackingStore::QIOSBackingStore(QWindow *window)
 
     Q_ASSERT_X(window->surfaceType() != QSurface::OpenGLSurface, "QIOSBackingStore",
         "QBackingStore on iOS can only be used with raster-enabled surfaces.");
-
-    m_context->setFormat(window->requestedFormat());
-    m_context->setScreen(window->screen());
-    Q_ASSERT(QOpenGLContext::globalShareContext());
-    m_context->setShareContext(QOpenGLContext::globalShareContext());
-    m_context->create();
 }
 
 QIOSBackingStore::~QIOSBackingStore()
 {
-    // We're using composeAndFlush from QPlatformBackingStore, which
-    // need to clean up any textures in its destructor, so make the
-    // context current and keep it alive until QPlatformBackingStore
-    // has cleaned up everything.
-    m_context->makeCurrent(window());
-    m_context->deleteLater();
 }
 
 void QIOSBackingStore::flush(QWindow *window, const QRegion &region, const QPoint &offset)
@@ -98,7 +85,7 @@ void QIOSBackingStore::flush(QWindow *window, const QRegion &region, const QPoin
     }
 
     static QPlatformTextureList emptyTextureList;
-    composeAndFlush(window, region, offset, &emptyTextureList, m_context, false);
+    composeAndFlush(window, region, offset, &emptyTextureList, false);
 }
 
 QT_END_NAMESPACE

src/plugins/platforms/xcb/qxcbbackingstore.cpp

@@ -600,10 +600,10 @@ void QXcbBackingStore::flush(QWindow *window, const QRegion &region, const QPoin
 
 #ifndef QT_NO_OPENGL
 void QXcbBackingStore::composeAndFlush(QWindow *window, const QRegion &region, const QPoint &offset,
-                                       QPlatformTextureList *textures, QOpenGLContext *context,
+                                       QPlatformTextureList *textures,
                                        bool translucentBackground)
 {
-    QPlatformBackingStore::composeAndFlush(window, region, offset, textures, context, translucentBackground);
+    QPlatformBackingStore::composeAndFlush(window, region, offset, textures, translucentBackground);
 
     QXcbWindow *platformWindow = static_cast<QXcbWindow *>(window->handle());
     if (platformWindow->needsSync()) {

src/plugins/platforms/xcb/qxcbbackingstore.h

@@ -61,7 +61,7 @@ public:
     void flush(QWindow *window, const QRegion &region, const QPoint &offset) override;
 #ifndef QT_NO_OPENGL
     void composeAndFlush(QWindow *window, const QRegion &region, const QPoint &offset,
-                         QPlatformTextureList *textures, QOpenGLContext *context,
+                         QPlatformTextureList *textures,
                          bool translucentBackground) override;
 #endif
     QImage toImage() const override;

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
+        //needs to be deleted first.
-        // make the context current here. This is taken care of by deleteBackingStore().
 
         extra->topextra->backingStoreTracker.destroy();
         deleteBackingStore(this);

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
+        backingStore->handle()->composeAndFlush(widget->windowHandle(), effectiveRegion, offset,
-        // widgets where tlw != widget.
+                                                widgetTextures, translucentBackground);
-        backingStore->handle()->composeAndFlush(widget->windowHandle(), effectiveRegion, offset, widgetTextures,
-                                                tlw->d_func()->shareContext(), translucentBackground);
         widget->window()->d_func()->sendComposeStatus(widget->window(), true);
     } else
 #endif

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)