Correctly handle edit buffer when using request with limit (fix #15505)

(cherry-picked from 2665eb5)

Author: nyalldawson

src/core/qgsvectorlayerfeatureiterator.cpp

@@ -175,6 +175,28 @@ QgsVectorLayerFeatureIterator::QgsVectorLayerFeatureIterator( QgsVectorLayerFeat
       changedIds << attIt.key();
     }
     mChangedFeaturesRequest.setFilterFids( changedIds );
+
+    if ( mChangedFeaturesRequest.limit() > 0 )
+    {
+      int providerLimit = mProviderRequest.limit();
+
+      // features may be deleted in buffer, so increase limit sent to provider
+      providerLimit += mSource->mDeletedFeatureIds.size();
+
+      if ( mProviderRequest.filterType() == QgsFeatureRequest::FilterExpression )
+      {
+        // attribute changes may mean some features no longer match expression, so increase limit sent to provider
+        providerLimit += mSource->mChangedAttributeValues.size();
+      }
+
+      if ( mProviderRequest.filterType() == QgsFeatureRequest::FilterExpression || mProviderRequest.filterType() == QgsFeatureRequest::FilterRect )
+      {
+        // geometry changes may mean some features no longer match expression or rect, so increase limit sent to provider
+        providerLimit += mSource->mChangedGeometries.size();
+      }
+
+      mProviderRequest.setLimit( providerLimit );
+    }
   }
 
   if ( request.filterType() == QgsFeatureRequest::FilterFid )

tests/src/python/test_qgsvectorlayer.py

@@ -1626,6 +1626,47 @@ def testEvaluatingDefaultExpressions(self):
         layer.setDefaultValueExpression(1, 'not a valid expression')
         self.assertFalse(layer.defaultValue(1))
 
+    def testGetFeatureLimitWithEdits(self):
+        """ test getting features with a limit, when edits are present """
+        layer = createLayerWithOnePoint()
+        # now has one feature with id 0
+
+        pr = layer.dataProvider()
+
+        f1 = QgsFeature(1)
+        f1.setAttributes(["test", 3])
+        f1.setGeometry(QgsGeometry.fromPoint(QgsPoint(300, 200)))
+        f2 = QgsFeature(2)
+        f2.setAttributes(["test", 3])
+        f2.setGeometry(QgsGeometry.fromPoint(QgsPoint(100, 200)))
+        f3 = QgsFeature(3)
+        f3.setAttributes(["test", 3])
+        f3.setGeometry(QgsGeometry.fromPoint(QgsPoint(100, 200)))
+        self.assertTrue(pr.addFeatures([f1, f2, f3]))
+
+        req = QgsFeatureRequest().setLimit(2)
+        self.assertEqual(len(list(layer.getFeatures(req))), 2)
+
+        # now delete feature f1
+        layer.startEditing()
+        self.assertTrue(layer.deleteFeature(1))
+        req = QgsFeatureRequest().setLimit(2)
+        self.assertEqual(len(list(layer.getFeatures(req))), 2)
+        layer.rollBack()
+
+        # change an attribute value required by filter
+        layer.startEditing()
+        req = QgsFeatureRequest().setFilterExpression('fldint=3').setLimit(2)
+        self.assertTrue(layer.changeAttributeValue(2, 1, 4))
+        self.assertEqual(len(list(layer.getFeatures(req))), 2)
+        layer.rollBack()
+
+        layer.startEditing()
+        req = QgsFeatureRequest().setFilterRect(QgsRectangle(50, 100, 150, 300)).setLimit(2)
+        self.assertTrue(layer.changeGeometry(2, QgsGeometry.fromPoint(QgsPoint(500, 600))))
+        self.assertEqual(len(list(layer.getFeatures(req))), 2)
+        layer.rollBack()
+
 
 # TODO:
 # - fetch rect: feat with changed geometry: 1. in rect, 2. out of rect