Merge pull request #995 from ispyisail/revive/713-collection-cancel-race

Cancel the collection-loading map before waiting on it in ~ElementsCollectionModel

sources/ElementsCollection/elementscollectionmodel.cpp

@@ -49,6 +49,14 @@ ElementsCollectionModel::ElementsCollectionModel(QObject *parent) :
 */
 ElementsCollectionModel::~ElementsCollectionModel()
 {
+	// Without cancel(), the wait below runs the whole queued
+	// QtConcurrent::map() to completion, so closing this dialog on a
+	// large collection blocks until every remaining item has been
+	// processed -- a visible hang on the button pressed precisely to
+	// stop the work. cancel() drops the not-yet-started items so the
+	// wait that follows (still needed, so an in-flight item can't
+	// dereference this object after it's gone) is short.
+	m_future.cancel();
 	m_future.waitForFinished();
 }