presto-main/src/main/java/io/prestosql/server/remotetask/HttpRemoteTask.java/HttpRemoteTask/getSources
class HttpRemoteTask:    private synchronized List<TaskSource> getSources()
    {
        return Stream.concat(planFragment.getPartitionedSourceNodes().stream(), planFragment.getRemoteSourceNodes().stream())
                .filter(Objects::nonNull)
                .map(PlanNode::getId)
                .map(this::getSource)
                .filter(Objects::nonNull)
                .collect(toImmutableList());
    }