Introduced QOpenGLContext::aboutToBeDestroyed() signal.

This signal can be used to clean up OpenGL resources in a safe way
before the context is destroyed.

Task-number: QTBUG-20083
Change-Id: I45a4be01b06af4ee7196fa502116f099d50afeab
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>

src/gui/kernel/qopenglcontext.cpp

@@ -174,6 +174,8 @@ bool QOpenGLContext::create()
 void QOpenGLContext::destroy()
 {
     Q_D(QOpenGLContext);
+    if (d->platformGLContext)
+        emit aboutToBeDestroyed();
     if (QOpenGLContext::currentContext() == this)
         doneCurrent();
     if (d->shareGroup)
@@ -185,6 +187,17 @@ void QOpenGLContext::destroy()
     d->functions = 0;
 }
 
+/*!
+    \fn void QOpenGLContext::aboutToBeDestroyed()
+
+    This signal is emitted before the underlying native OpenGL context is
+    destroyed, such that users may clean up OpenGL resources that might otherwise
+    be left dangling in the case of shared OpenGL contexts.
+
+    If you wish to make the context current in order to do clean-up, make sure to
+    only connect to the signal using a direct connection.
+*/
+
 /*!
   If this is the current context for the thread, doneCurrent is called
 */

src/gui/kernel/qopenglcontext.h

@@ -118,6 +118,9 @@ public:
 
     QOpenGLFunctions *functions() const;
 
+Q_SIGNALS:
+    void aboutToBeDestroyed();
+
 private:
     friend class QGLContext;
     friend class QOpenGLContextResourceBase;

tests/auto/gui/qopengl/tst_qopengl.cpp

@@ -50,6 +50,8 @@
 
 #include <QtTest/QtTest>
 
+#include <QSignalSpy>
+
 class tst_QOpenGL : public QObject
 {
 Q_OBJECT
@@ -62,6 +64,7 @@ private slots:
     void fboRendering();
     void fboHandleNulledAfterContextDestroyed();
     void openGLPaintDevice();
+    void aboutToBeDestroyed();
 };
 
 struct SharedResourceTracker
@@ -500,5 +503,24 @@ void tst_QOpenGL::openGLPaintDevice()
     QCOMPARE(image, fbo.toImage().convertToFormat(QImage::Format_RGB32));
 }
 
+void tst_QOpenGL::aboutToBeDestroyed()
+{
+    QWindow window;
+    window.setGeometry(0, 0, 128, 128);
+    window.create();
+
+    QOpenGLContext *context = new QOpenGLContext;
+    QSignalSpy spy(context, SIGNAL(aboutToBeDestroyed()));
+
+    context->create();
+    context->makeCurrent(&window);
+
+    QCOMPARE(spy.size(), 0);
+
+    delete context;
+
+    QCOMPARE(spy.size(), 1);
+}
+
 QTEST_MAIN(tst_QOpenGL)
 #include "tst_qopengl.moc"