Respect custom layer order for groups in GetMap

Backport of bugfix from master PR 9878

Fixes #21917

(cherry-picking commit 9ddbae6 from 3.6)

src/server/services/wms/qgswmsrenderer.cpp

@@ -2496,9 +2496,33 @@ namespace QgsWms
     if ( !groupName.isEmpty() )
     {
       mLayerGroups[groupName] = QList<QgsMapLayer *>();
-      for ( QgsLayerTreeLayer *layer : group->findLayers() )
+      const auto projectLayerTreeRoot { mProject->layerTreeRoot() };
+      const auto treeGroupLayers { group->findLayers() };
+      // Fast track if there is no custom layer order,
+      // otherwise reorder layers.
+      if ( ! projectLayerTreeRoot->hasCustomLayerOrder() )
       {
-        mLayerGroups[groupName].append( layer->layer() );
+        for ( const auto &tl : treeGroupLayers )
+        {
+          mLayerGroups[groupName].push_back( tl->layer() );
+        }
+      }
+      else
+      {
+        const auto projectLayerOrder { projectLayerTreeRoot->layerOrder() };
+        // Flat list containing the layers from the tree nodes
+        QList<QgsMapLayer *> groupLayersList;
+        for ( const auto &tl : treeGroupLayers )
+        {
+          groupLayersList << tl->layer();
+        }
+        for ( const auto &l : projectLayerOrder )
+        {
+          if ( groupLayersList.contains( l ) )
+          {
+            mLayerGroups[groupName].push_back( l );
+          }
+        }
       }
     }
 

tests/src/python/test_qgsserver_wms_getmap.py

@@ -1427,6 +1427,50 @@ def test_wms_getmap_group_regression_20810(self):
         r, h = self._result(self._execute_request(qs))
         self._img_diff_error(r, h, "WMS_GetMap_GroupedLayersDown")
 
+    def test_wms_getmap_root_custom_layer_order_regression_21917(self):
+        """When drawing root layer, custom layer order order should be respected."""
+
+        qs = "?" + "&".join(["%s=%s" % i for i in list({
+            "MAP": urllib.parse.quote(os.path.join(self.testdata_path, 'bug_21917_root_layer_order.qgs')),
+            "SERVICE": "WMS",
+            "VERSION": "1.3.0",
+            "REQUEST": "GetMap",
+            "BBOX": "44.9014,8.20346,44.9015,8.20355",
+            "CRS": "EPSG:4326",
+            "WIDTH": "400",
+            "HEIGHT": "400",
+            "LAYERS": "group",
+            "STYLES": ",",
+            "FORMAT": "image/png",
+            "DPI": "200",
+            "MAP_RESOLUTION": "200",
+            "FORMAT_OPTIONS": "dpi:200"
+        }.items())])
+
+        r, h = self._result(self._execute_request(qs))
+        self._img_diff_error(r, h, "WMS_GetMap_Group_Layer_Order")
+
+        # Check with root_layer
+        qs = "?" + "&".join(["%s=%s" % i for i in list({
+            "MAP": urllib.parse.quote(os.path.join(self.testdata_path, 'bug_21917_root_layer_order.qgs')),
+            "SERVICE": "WMS",
+            "VERSION": "1.3.0",
+            "REQUEST": "GetMap",
+            "BBOX": "44.9014,8.20346,44.9015,8.20355",
+            "CRS": "EPSG:4326",
+            "WIDTH": "400",
+            "HEIGHT": "400",
+            "LAYERS": "root_layer",
+            "STYLES": ",",
+            "FORMAT": "image/png",
+            "DPI": "200",
+            "MAP_RESOLUTION": "200",
+            "FORMAT_OPTIONS": "dpi:200"
+        }.items())])
+
+        r, h = self._result(self._execute_request(qs))
+        self._img_diff_error(r, h, "WMS_GetMap_Group_Layer_Order")
+
 
 if __name__ == '__main__':
     unittest.main()