presto-main/src/main/java/io/prestosql/operator/exchange/LocalExchange.java/LocalExchange/getNextSource
class LocalExchange:    public synchronized LocalExchangeSource getNextSource()
    {
        checkState(nextSourceIndex < sources.size(), "All operators already created");
        LocalExchangeSource result = sources.get(nextSourceIndex);
        nextSourceIndex++;
        return result;
    }