This is a blocker for supporting Kudu DML with mt_dop. The expression has some mutable objects in the ScalarExpr object that needs to be moved to the evaluator.
KuduPartitioner is not thread-safe, because it uses an internal buffer. We should consider making it thread-safe or clonable, because the initialisation is expensive and does RPCs and things - https://github.com/cloudera/kudu/blob/master/src/kudu/client/partitioner-internal.cc#L40
KuduPartialRow is inherently not thread-safe because it contains thread-local data.