presto-main/src/main/java/io/prestosql/execution/SqlQueryExecution.java/SqlQueryExecution/closeSplitSources
class SqlQueryExecution:    private static void closeSplitSources(StageExecutionPlan plan)
    {
        for (SplitSource source : plan.getSplitSources().values()) {
            try {
                source.close();
            }
            catch (Throwable t) {
                log.warn(t, "Error closing split source");
            }
        }

        for (StageExecutionPlan stage : plan.getSubStages()) {
            closeSplitSources(stage);
        }
    }