[symbology] When rendering a multipart geometry, ensure that

geometry generator symbol is only rendered once, instead of
once per part

Fixes #23730

python/core/auto_generated/symbology/qgsgeometrygeneratorsymbollayer.sip.in

@@ -40,9 +40,12 @@ that is created by this generator.
 
     virtual void startRender( QgsSymbolRenderContext &context );
 
-
     virtual void stopRender( QgsSymbolRenderContext &context );
 
+    virtual void startFeatureRender( const QgsFeature &feature, QgsRenderContext &context );
+
+    virtual void stopFeatureRender( const QgsFeature &feature, QgsRenderContext &context );
+
 
     virtual QgsSymbolLayer *clone() const /Factory/;
 

src/core/symbology/qgsgeometrygeneratorsymbollayer.cpp

@@ -96,6 +96,17 @@ void QgsGeometryGeneratorSymbolLayer::stopRender( QgsSymbolRenderContext &contex
     mSymbol->stopRender( context.renderContext() );
 }
 
+void QgsGeometryGeneratorSymbolLayer::startFeatureRender( const QgsFeature &, QgsRenderContext & )
+{
+  mRenderingFeature = true;
+  mHasRenderedFeature = false;
+}
+
+void QgsGeometryGeneratorSymbolLayer::stopFeatureRender( const QgsFeature &, QgsRenderContext & )
+{
+  mRenderingFeature = false;
+}
+
 QgsSymbolLayer *QgsGeometryGeneratorSymbolLayer::clone() const
 {
   QgsGeometryGeneratorSymbolLayer *clone = new QgsGeometryGeneratorSymbolLayer( mExpression->expression() );
@@ -192,6 +203,9 @@ bool QgsGeometryGeneratorSymbolLayer::isCompatibleWithSymbol( QgsSymbol *symbol
 }
 void QgsGeometryGeneratorSymbolLayer::render( QgsSymbolRenderContext &context )
 {
+  if ( mRenderingFeature && mHasRenderedFeature )
+    return;
+
   if ( context.feature() )
   {
     QgsExpressionContext &expressionContext = context.renderContext().expressionContext();
@@ -205,6 +219,9 @@ void QgsGeometryGeneratorSymbolLayer::render( QgsSymbolRenderContext &context )
     subSymbolExpressionContextScope->setFeature( f );
 
     mSymbol->renderFeature( f, context.renderContext(), -1, context.selected() );
+
+    if ( mRenderingFeature )
+      mHasRenderedFeature = true;
   }
 }
 

src/core/symbology/qgsgeometrygeneratorsymbollayer.h

@@ -49,8 +49,9 @@ class CORE_EXPORT QgsGeometryGeneratorSymbolLayer : public QgsSymbolLayer
     QgsSymbol::SymbolType symbolType() const { return mSymbolType; }
 
     void startRender( QgsSymbolRenderContext &context ) override;
-
     void stopRender( QgsSymbolRenderContext &context ) override;
+    void startFeatureRender( const QgsFeature &feature, QgsRenderContext &context ) override;
+    void stopFeatureRender( const QgsFeature &feature, QgsRenderContext &context ) override;
 
     QgsSymbolLayer *clone() const override SIP_FACTORY;
 
@@ -113,6 +114,9 @@ class CORE_EXPORT QgsGeometryGeneratorSymbolLayer : public QgsSymbolLayer
      * The type of the sub symbol.
      */
     QgsSymbol::SymbolType mSymbolType;
+
+    bool mRenderingFeature = false;
+    bool mHasRenderedFeature = false;
 };
 
 #endif // QGSGEOMETRYGENERATORSYMBOLLAYER_H

tests/src/python/test_qgsgeometrygeneratorsymbollayer.py

@@ -37,7 +37,8 @@
     QgsRectangle,
     QgsGeometryGeneratorSymbolLayer,
     QgsSymbol,
-    QgsMultiRenderChecker
+    QgsMultiRenderChecker,
+    QgsMapSettings
 )
 
 from qgis.testing import start_app, unittest
@@ -163,6 +164,31 @@ def test_buffer_points(self):
         self.report += renderchecker.report()
         self.assertTrue(res)
 
+    def test_multi_poly_opacity(self):
+        # test that multi-type features are only rendered once
+
+        multipoly = QgsVectorLayer(os.path.join(TEST_DATA_DIR, 'multipatch.shp'), 'Polygons', 'ogr')
+
+        sym = QgsFillSymbol.createSimple({'color': '#77fdbf6f', 'outline_color': 'black'})
+
+        buffer_layer = QgsGeometryGeneratorSymbolLayer.create({'geometryModifier': 'buffer($geometry, -0.01)', 'outline_color': 'black'})
+        buffer_layer.setSymbolType(QgsSymbol.Fill)
+        buffer_layer.setSubSymbol(sym)
+        geom_symbol = QgsFillSymbol()
+        geom_symbol.changeSymbolLayer(0, buffer_layer)
+        multipoly.renderer().setSymbol(geom_symbol)
+
+        mapsettings = QgsMapSettings(self.mapsettings)
+        mapsettings.setExtent(multipoly.extent())
+        mapsettings.setLayers([multipoly])
+
+        renderchecker = QgsMultiRenderChecker()
+        renderchecker.setMapSettings(mapsettings)
+        renderchecker.setControlName('expected_geometrygenerator_opacity')
+        res = renderchecker.runTest('geometrygenerator_opacity')
+        self.report += renderchecker.report()
+        self.assertTrue(res)
+
 
 if __name__ == '__main__':
     unittest.main()