presto-main/src/main/java/io/prestosql/operator/DriverYieldSignal.java/DriverYieldSignal/toString
class DriverYieldSignal:    @Override
    public synchronized String toString()
    {
        return toStringHelper(this)
                .add("yieldScheduled", yieldFuture != null)
                .add("yield", yield.get())
                .toString();
    }