Fix vector iterator number of attributes returned with virtual fields

Fix the issue of wrong number of attributes returned when the feature
request has setSubsetOfAttributes set, in that case the virtual fields
were not returned causing an inconsistency in the API response.

With this patch the virtual fields are always returned (possibly Null).

src/core/vector/qgsvectorlayerfeatureiterator.cpp

@@ -551,6 +551,8 @@ bool QgsVectorLayerFeatureIterator::fetchFeature( QgsFeature &f )
 
     if ( mHasVirtualAttributes )
       addVirtualAttributes( f );
+    else
+      f.padAttributes( mSource->mFields.count() - f.attributeCount() );
 
     if ( mRequest.filterType() == QgsFeatureRequest::FilterExpression && mProviderRequest.filterType() != QgsFeatureRequest::FilterExpression )
     {
@@ -664,6 +666,8 @@ void QgsVectorLayerFeatureIterator::useAddedFeature( const QgsFeature &src, QgsF
 
   if ( mHasVirtualAttributes )
     addVirtualAttributes( f );
+  else
+    f.padAttributes( mSource->mFields.count() - f.attributeCount() );
 }
 
 
@@ -918,6 +922,7 @@ void QgsVectorLayerFeatureIterator::prepareFields()
   {
     createOrderedJoinList();
   }
+
 }
 
 void QgsVectorLayerFeatureIterator::createOrderedJoinList()
@@ -1292,6 +1297,8 @@ bool QgsVectorLayerFeatureIterator::nextFeatureFid( QgsFeature &f )
 
     if ( mHasVirtualAttributes )
       addVirtualAttributes( f );
+    else
+      f.padAttributes( mSource->mFields.count() - f.attributeCount() );
 
     return true;
   }

tests/src/python/test_qgsvectorlayer.py

@@ -2658,6 +2658,34 @@ def testReselect(self):
         layer.reselect()
         self.assertCountEqual(layer.selectedFeatureIds(), [5])
 
+    def testGetFeaturesVirtualFieldsSubset(self):
+        """Test that when a subset is requested virtual fields are returned nullified"""
+
+        vl = QgsVectorLayer(os.path.join(unitTestDataPath(), 'points.shp'), 'Points', 'ogr')
+        virt_field_idx = vl.addExpressionField('\'Importance: \' || Importance', QgsField('virt_1', QVariant.String))
+
+        self.assertEqual(vl.fields().lookupField('virt_1'), virt_field_idx)
+
+        req = QgsFeatureRequest()
+        req.setSubsetOfAttributes([0, 1])
+        attrs = next(vl.getFeatures(req)).attributes()
+        self.assertEqual(attrs, ['Jet', 90, None, None, None, None, None])
+
+        attrs = next(vl.getFeatures()).attributes()
+        self.assertEqual(attrs, ['Jet', 90, 3.0, 2, 0, 2, 'Importance: 3'])
+
+        req.setSubsetOfAttributes([0, 2])
+        attrs = next(vl.getFeatures(req)).attributes()
+        self.assertEqual(attrs, ['Jet', None, 3.0, None, None, None, None])
+
+        req.setSubsetOfAttributes([0, 1, 6])
+        attrs = next(vl.getFeatures(req)).attributes()
+        self.assertEqual(attrs, ['Jet', 90, 3.0, None, None, None, 'Importance: 3'])
+
+        req.setSubsetOfAttributes([6])
+        attrs = next(vl.getFeatures(req)).attributes()
+        self.assertEqual(attrs, [None, None, 3.0, None, None, None, 'Importance: 3'])
+
     def testAggregate(self):
         """ Test aggregate calculation """
         layer = QgsVectorLayer("Point?field=fldint:integer", "layer", "memory")