Warning debug output when doing dangerous things

src/providers/wms/qgswmsprovider.cpp

@@ -611,7 +611,10 @@ static void _drawDebugRect( QPainter &p, const QRectF &rect, const QColor &color
 
 QImage *QgsWmsProvider::draw( QgsRectangle const &viewExtent, int pixelWidth, int pixelHeight, QgsRasterBlockFeedback *feedback )
 {
-  QgsDebugMsg( "Entering." );
+  if ( QgsApplication::instance()->thread() != QThread::currentThread() )
+  {
+    QgsDebugMsg( "Trying to draw a WMS image on the main thread. Stop it!" );
+  }
 
   // compose the URL query string for the WMS server.