QOpenGLDebugLogger: strengthen the code path in case of GL context destruction

Trying to move closer to GL semantics: it is allowed to destroy
a GL context at any time and that must free all of its resources.

Change-Id: I3daa81d721cf26baf86a1a6435b77e3c28feb1a2
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

src/gui/opengl/qopengldebug.cpp

@@ -36,6 +36,7 @@
 #include <QtCore/qvarlengtharray.h>
 #include <QtGui/qopengl.h>
 #include <QtGui/qopenglfunctions.h>
+#include <QtGui/qoffscreensurface.h>
 
 #include "qopengldebug.h"
 
@@ -1281,8 +1282,41 @@ void QOpenGLDebugLoggerPrivate::controlDebugMessages(QOpenGLDebugMessage::Source
 */
 void QOpenGLDebugLoggerPrivate::_q_contextAboutToBeDestroyed()
 {
+    Q_ASSERT(context);
+
+    // Re-make our context current somehow, otherwise stopLogging will fail.
+
+    // Save the current context and its surface in case we need to set them back
+    QOpenGLContext *currentContext = QOpenGLContext::currentContext();
+    QSurface *currentSurface = 0;
+
+    QScopedPointer<QOffscreenSurface> offscreenSurface;
+
+    if (context != currentContext) {
+        // Make our old context current on a temporary surface
+        if (currentContext)
+            currentSurface = currentContext->surface();
+
+        offscreenSurface.reset(new QOffscreenSurface);
+        offscreenSurface->setFormat(context->format());
+        offscreenSurface->create();
+        if (!context->makeCurrent(offscreenSurface.data()))
+            qWarning("QOpenGLDebugLoggerPrivate::_q_contextAboutToBeDestroyed(): could not make the owning GL context current for cleanup");
+    }
+
     Q_Q(QOpenGLDebugLogger);
     q->stopLogging();
+
+    if (offscreenSurface) {
+        // We did change the current context: set it back
+        if (currentContext)
+            currentContext->makeCurrent(currentSurface);
+        else
+            context->doneCurrent();
+    }
+
+    QObject::disconnect(context, SIGNAL(aboutToBeDestroyed()), q, SLOT(_q_contextAboutToBeDestroyed()));
+    context = 0;
     initialized = false;
 }