[processing] More responsive cancelation for dissolve algorithm

Author: nyalldawson

src/analysis/processing/qgsalgorithmdissolve.cpp

@@ -219,11 +219,16 @@ QVariantMap QgsDissolveAlgorithm::processAlgorithm( const QVariantMap &parameter
     // See: https://issues.qgis.org/issues/20591 - Dissolve tool failing to produce outputs
     if ( ! result.lastError().isEmpty() && parts.count() >  2 )
     {
+      if ( feedback->isCanceled() )
+        return result;
+
       feedback->pushDebugInfo( QObject::tr( "GEOS exception: taking the slower route ..." ) );
       result = QgsGeometry();
       for ( const auto &p : parts )
       {
         result = QgsGeometry::unaryUnion( QVector< QgsGeometry >() << result << p );
+        if ( feedback->isCanceled() )
+          return result;
       }
     }
     if ( ! result.lastError().isEmpty() )