[Backport release-3_40] [vector layer] Fix newly-added features in the edit buffer prior to adding a new attribute will not save the new attribute's values

python/PyQt6/core/auto_generated/vector/qgsvectorlayereditbuffer.sip.in

@@ -321,7 +321,7 @@ Update feature with uncommitted geometry updates
 Update feature with uncommitted attribute updates
 %End
 
-    void handleAttributeAdded( int index );
+    void handleAttributeAdded( int index, const QgsField &field );
 %Docstring
 Update added and changed features after addition of an attribute
 %End

python/core/auto_generated/vector/qgsvectorlayereditbuffer.sip.in

@@ -321,7 +321,7 @@ Update feature with uncommitted geometry updates
 Update feature with uncommitted attribute updates
 %End
 
-    void handleAttributeAdded( int index );
+    void handleAttributeAdded( int index, const QgsField &field );
 %Docstring
 Update added and changed features after addition of an attribute
 %End

src/core/vector/qgsvectorlayereditbuffer.cpp

@@ -294,7 +294,9 @@ bool QgsVectorLayerEditBuffer::changeAttributeValue( QgsFeatureId fid, int field
   if ( field < 0 || field >= L->fields().count() ||
        L->fields().fieldOrigin( field ) == Qgis::FieldOrigin::Join ||
        L->fields().fieldOrigin( field ) == Qgis::FieldOrigin::Expression )
+  {
     return false;
+  }
 
   L->undoStack()->push( new QgsVectorLayerUndoCommandChangeAttribute( this, fid, field, newValue, oldValue ) );
   return true;
@@ -503,7 +505,7 @@ QString QgsVectorLayerEditBuffer::dumpEditBuffer()
 }
 #endif
 
-void QgsVectorLayerEditBuffer::handleAttributeAdded( int index )
+void QgsVectorLayerEditBuffer::handleAttributeAdded( int index, const QgsField &field )
 {
   // go through the changed attributes map and adapt indices
   QgsChangedAttributesMap::iterator it = mChangedAttributeValues.begin();
@@ -519,6 +521,21 @@ void QgsVectorLayerEditBuffer::handleAttributeAdded( int index )
     QgsAttributes attrs = featureIt->attributes();
     attrs.insert( index, QVariant() );
     featureIt->setAttributes( attrs );
+    QgsFields fields;
+    const QgsFields oldFields = featureIt->fields();
+    for ( int i = 0; i < oldFields.size(); i++ )
+    {
+      if ( i == index )
+      {
+        fields.append( field, L->fields().fieldOrigin( L->fields().indexFromName( field.name() ) ) );
+      }
+      fields.append( oldFields.at( i ), oldFields.fieldOrigin( i ) );
+    }
+    if ( index == oldFields.size() )
+    {
+      fields.append( field, L->fields().fieldOrigin( L->fields().indexFromName( field.name() ) ) );
+    }
+    featureIt->setFields( fields, false );
   }
 
   // go through renamed attributes and adapt
@@ -559,6 +576,9 @@ void QgsVectorLayerEditBuffer::handleAttributeDeleted( int index )
     QgsAttributes attrs = featureIt->attributes();
     attrs.remove( index );
     featureIt->setAttributes( attrs );
+    QgsFields fields = featureIt->fields();
+    fields.remove( index );
+    featureIt->setFields( fields, false );
   }
 
   // go through rename attributes and adapt
@@ -919,6 +939,7 @@ bool QgsVectorLayerEditBuffer::commitChangesAddFeatures( bool &featuresAdded, QS
     // not be sent to the data provider. Refs #18784
     for ( int i = 0; i < featuresToAdd.count(); ++i )
     {
+      // Empty the feature's fields so the up-to-date fields from the data provider is used to match attributes
       QgsVectorLayerUtils::matchAttributesToFields( featuresToAdd[i], L->dataProvider()->fields() );
     }
 

src/core/vector/qgsvectorlayereditbuffer.h

@@ -297,7 +297,7 @@ class CORE_EXPORT QgsVectorLayerEditBuffer : public QObject
     void updateChangedAttributes( QgsFeature &f );
 
     //! Update added and changed features after addition of an attribute
-    void handleAttributeAdded( int index );
+    void handleAttributeAdded( int index, const QgsField &field );
 
     //! Update added and changed features after removal of an attribute
     void handleAttributeDeleted( int index );

src/core/vector/qgsvectorlayerundocommand.cpp

@@ -334,7 +334,7 @@ void QgsVectorLayerUndoCommandAddAttribute::undo()
 void QgsVectorLayerUndoCommandAddAttribute::redo()
 {
   mBuffer->mAddedAttributes.append( mField );
-  mBuffer->handleAttributeAdded( mFieldIndex );
+  mBuffer->handleAttributeAdded( mFieldIndex, mField );
   mBuffer->updateLayerFields();
 
   emit mBuffer->attributeAdded( mFieldIndex );
@@ -391,7 +391,7 @@ void QgsVectorLayerUndoCommandDeleteAttribute::undo()
   }
 
   mBuffer->updateLayerFields();
-  mBuffer->handleAttributeAdded( mFieldIndex ); // update changed attributes + new features
+  mBuffer->handleAttributeAdded( mFieldIndex, mOldField ); // update changed attributes + new features
 
   if ( !mOldName.isEmpty() )
   {

tests/src/python/test_qgsvectorlayereditbuffer.py

@@ -13,7 +13,7 @@
 import os
 from osgeo import gdal
 
-from qgis.PyQt.QtCore import QTemporaryDir, QVariant
+from qgis.PyQt.QtCore import QMetaType, QTemporaryDir, QVariant
 from qgis.PyQt.QtTest import QSignalSpy
 from qgis.core import (
     Qgis,
@@ -856,6 +856,27 @@ def _check_feature(wkt):
 
         _test(Qgis.TransactionMode.BufferedGroups)
 
+    def testAddNewFeatureAndNewAttribute(self):
+        layer = QgsVectorLayer("Point?field=fldtxt:string", "addfeat", "memory")
+        layer.startEditing()
+
+        # Add a new feature with fields information attached to it
+        f = QgsFeature(layer.fields())
+        f.setAttributes(["test"])
+        f.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(100, 200)))
+        layer.addFeature(f)
+
+        # Add a new attribute to the layer
+        layer.addAttribute(QgsField("fldint", QMetaType.Type.Int))
+
+        # Change the newly-added attribute value for the new feature
+        layer.changeAttributeValue(f.id(), 1, 123)
+
+        # Commit changes
+        layer.commitChanges()
+
+        self.assertEqual(layer.getFeature(1).attributes(), ["test", 123])
+
 
 if __name__ == "__main__":
     unittest.main()