PyQGIS QgsProcessingAlgorithm: no output layer when cancelled

Question

For one plugin I maintain, I can't figure out how to return output layer when the user hits 'Cancel'. Basically I'm looping over a set of results in processAlgorithm which populates the output layer and I'd expect to have the output layer returned with all results calculated until 'Cancel' was clicked.

A minimal example:

def processAlgorithm(self, parameters, context, feedback):
 (sink, self.dest_id) = self.parameterAsSink(parameters, 'Output', context,
 list_of_fields,
 QgsWkbTypes.Polygon,
 CRS)
 for result in results:
 if feedback.isCanceled():
 break
 # Do smth smart
 sink.addFeature(result)
 return {'Output': self.dest_id}

However, no layer is added to the map when 'Cancel' is hit mid-way through processing.

EDIT Just to clarify, I'd have expected that if feedback.isCanceled(): break would yield return {'Output': self.dest_id}, i.e. the output layer with the added features until feedback.isCanceled() is called.

Answer 1

Asked the same question on the QGIS dev mailing list and according to @nyalldawson this is intended behavior, so as not to receive misleading partial results.