presto-main/src/main/java/io/prestosql/operator/WindowOperator.java/WindowOperator/isBlocked
class WindowOperator:    @Override
    public ListenableFuture<?> isBlocked()
    {
        // We can block e.g. because of self-triggered spill
        if (outputPages.isBlocked()) {
            return outputPages.getBlockedFuture();
        }

        return NOT_BLOCKED;
    }