add QgsVectorLayer::getGeometry(fid) (#8174)

* add QgsVectorLayer::getGeometry(fid)

* update docs to say geom is invalid

python/core/auto_generated/qgsvectorlayer.sip.in

@@ -1054,6 +1054,12 @@ Query the layer for features matching a given expression.
 %Docstring
 Query the layer for the feature with the given id.
 If there is no such feature, the returned feature will be invalid.
+%End
+
+    QgsGeometry getGeometry( QgsFeatureId fid ) const;
+%Docstring
+Query the layer for the geometry at the given id.
+If there is no such feature, the returned geometry will be invalid.
 %End
 
     QgsFeatureIterator getFeatures( const QgsFeatureIds &fids );

src/core/qgsvectorlayer.cpp

@@ -941,6 +941,16 @@ QgsFeatureIterator QgsVectorLayer::getFeatures( const QgsFeatureRequest &request
   return QgsFeatureIterator( new QgsVectorLayerFeatureIterator( new QgsVectorLayerFeatureSource( this ), true, request ) );
 }
 
+QgsGeometry QgsVectorLayer::getGeometry( QgsFeatureId fid ) const
+{
+  QgsFeature feature;
+  getFeatures( QgsFeatureRequest( fid ).setFlags( QgsFeatureRequest::SubsetOfAttributes ) ).nextFeature( feature );
+  if ( feature.isValid() )
+    return feature.geometry();
+  else
+    return QgsGeometry();
+}
+
 bool QgsVectorLayer::addFeature( QgsFeature &feature, Flags )
 {
   if ( !mValid || !mEditBuffer || !mDataProvider )

src/core/qgsvectorlayer.h

@@ -1038,6 +1038,12 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
       return feature;
     }
 
+    /**
+     * Query the layer for the geometry at the given id.
+     * If there is no such feature, the returned geometry will be invalid.
+     */
+    QgsGeometry getGeometry( QgsFeatureId fid ) const;
+
     /**
      * Query the layer for the features with the given ids.
      */