Add Send to ArrayBuilder

[Max-Meldrum]

Problem: I'm using a StructBuilder in Arcon to build up Arrow tables. The threading model ensures exclusive access to the builder but may move it across threads. The main problem is that ArrayBuilder does not implement Send so it fails to compile..

Solution:

// Old
pub trait ArrayBuilder: Any { .... }
// New
pub trait ArrayBuilder: Any + Send { .... }

[jorgecarleitao]

Thanks for this.

Honest curiosity: what is the use-case where the ArrayBuilder is passed around?

Also, isn't it possible to use &dyn ArrayBuilder + Send on the code to enforce this?

[Max-Meldrum]

Thanks for this.

Honest curiosity: what is the use-case where the ArrayBuilder is passed around?

Also, isn't it possible to use &dyn ArrayBuilder + Send on the code to enforce this?

Well it is not so much the ArrayBuilder being sent around, but it is a StructBuilder that is passed around which itself contains a bunch of field builders of Box<ArrayBuilder>. The use case is that a StructBuilder is used to build up columnar data in a streaming window. During the length of this window, the "worker" may move across threads as the underlying thread model uses a workstealing scheduler.

[jorgecarleitao]

makes sense, thanks!