[Bugfix][Server] Display composer legend with auto update like the De…

…sktop

QGIS Server was not able to display auto update legend in composer since
2.18.x. To fix it we need to deactivate *auto update* and to provide the
the project layer tree.

src/server/qgswmsprojectparser.cpp

@@ -599,8 +599,9 @@ QgsComposition* QgsWMSProjectParser::initComposition( const QString& composerTem
 #endif
       if ( legend->autoUpdateModel() )
       {
+        legend->setAutoUpdateModel( false );
         model->setRootGroup( projectLayerTreeGroup() );
-        legend->synchronizeWithModel();
+        legend->updateLegend();
       }
       // if the legend has no map
       // we will load all layers
@@ -2461,6 +2462,41 @@ QgsLayerTreeGroup* QgsWMSProjectParser::projectLayerTreeGroup() const
     return rootGroup;
   }
 
+  /*const QDomDocument* projectDoc = mProjectParser->xmlDocument();
+  if ( projectDoc )
+  {
+  QDomElement qgisElem = projectDoc->documentElement();
+  if ( qgisElem.isNull() )
+  {
+  QDomElement layerTreeElem = qgisElem.firstChildElement( "layer-tree-group" );*/
+  if ( layerTreeElem.isNull() )
+  {
+    const QHash< QString, QDomElement > &projectLayerElements = mProjectParser->projectLayerElementsById();
+    QDomNodeList treeLayerNodeList = layerTreeElem.elementsByTagName( "layer-tree-layer" );
+    for ( int i = 0; i < treeLayerNodeList.size(); ++i )
+    {
+      QDomElement treeLayerElem = treeLayerNodeList.at( i ).toElement();
+      QString layerId = treeLayerElem.attribute( "id" );
+      QgsMapLayer * layer = QgsMapLayerRegistry::instance()->mapLayer( layerId );
+      if ( layer )
+      {
+        continue;
+      }
+
+      QHash< QString, QDomElement >::const_iterator layerElemIt = projectLayerElements.find( layerId );
+      if ( layerElemIt != projectLayerElements.constEnd() )
+      {
+        layer = mProjectParser->createLayerFromElement( layerElemIt.value(), true );
+      }
+      if ( layer )
+      {
+        QgsMapLayerRegistry::instance()->addMapLayer( layer );
+      }
+    }
+  }
+  /*}
+  }*/
+
   return QgsLayerTreeGroup::readXML( layerTreeElem );
 }