race condition related to ScriptOperator + UnionOperator

ScriptOperator uses a second thread to output the rows to the children operators. In a corner case which contains a union, 2 threads might be outputting data into the same operator hierarchy and caused race conditions.

CREATE TABLE tablea (cola STRING);
SELECT *
FROM (
    SELECT TRANSFORM(cola)
    USING 'cat'
    AS cola
    FROM tablea
  UNION ALL
    SELECT cola as cola
    FROM tablea
) a;