[GitHub] sohami commented on issue #1367: DRILL-6585: PartitionSender clones vectors, but shares field metdata

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sohami commented on issue #1367: DRILL-6585: PartitionSender clones vectors, but shares field metdata
URL: https://github.com/apache/drill/pull/1367#issuecomment-404713253
 
 
   @paul-rogers - I have couple of questions related to this change and your last comment:
   1) I don't understand what you mean by `clone` method will clear out the child field. Isn't `MaterializedField::clone` is taking care of [cloning the children fields](https://github.com/apache/drill/blob/master/exec/vector/src/main/java/org/apache/drill/exec/record/MaterializedField.java#L145) as well ? Instead `cloneEmpty` is throwing away the children fields.
   
   2) The main issue with the original PR and the logic of building outgoing batch from incoming batch of Partition Sender is something like below:
   
   - In original PR there is a change for NullableValueVectors to add the `values` and `bits` vector materialized field as child field of parent vector field. See [here](https://github.com/apache/drill/pull/1244/files#diff-646d5ac291e14369d72634752edaae84R111). From your comment it looks like because the internal `values` ValueVector mode needs to be required so you are creating another Materialized Field with that mode for internal `values` vector and adding it as child of parent vector field. But AFAIK based on the usage seen, the children of a Materialized Field is only used in case of container Type vector like Map not in case of composite vector like NullableValueVectors. For those the internal `values` and `bits` vector holds on to the parent Materialized Field. Now with change even for Nullable ValueVectors the children component of parent field will be populated. This will probably affect the serde component and compatibility too ?
   
   - Now coming to the main issue. Earlier for each incoming of the partition sender, there are multiple threads which are trying to create outgoing batches but using the same Materialized from incoming batch vectors. Since outgoing batch will have same schema as incoming batch.
   
   - When a ValueVector in incoming batch is of Map type then while creating a new ValueVector of Map type in outgoing batch it passes the Materialized Field from input Map Vector. In the constructor of [AbstractMapVector](https://github.com/apache/drill/blob/master/exec/vector/src/main/java/org/apache/drill/exec/vector/complex/AbstractMapVector.java#L49), it clones the input field and then iterate over the children of **not** the cloned field but the original input field (which is from same incoming vector). This looks to be a bug where it should [iterate on cloned field children rather than input field children](https://github.com/apache/drill/blob/master/exec/vector/src/main/java/org/apache/drill/exec/vector/complex/AbstractMapVector.java#L51).
   
   - With above if there is a child of the Map vector which is of Nullable type, then that child vector will get reference to the children field from actual input field to parent MapVector (not the reference to cloned children field).
   
   - With [new change](https://github.com/apache/drill/pull/1244/files#diff-646d5ac291e14369d72634752edaae84R111) in NullableValueVector it will try to modify it's input Materialized Field to add children for `bit` and `values` vector field.
   
   - So combination of above bug in Map vector and the new change will cause the concurrent modification exception as NullableValueVector child of MapVector will modify the Materialized field of incoming ValueVector not the new clone Materialized Field created for the outgoing vector.
   
   With your current fix I think you should still use `clone` instead of `cloneEmpty` since we need to preserve the children information too in outgoing batches of PartitionSender.

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