Weston Pace created ARROW-13155:
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             Summary: [C++] MapGenerator should optionally forward reentrant 
pressure
                 Key: ARROW-13155
                 URL: https://issues.apache.org/jira/browse/ARROW-13155
             Project: Apache Arrow
          Issue Type: Bug
          Components: C++
            Reporter: Weston Pace


Currently the map generator will allow the map function to run in parallel but 
it will not forward reentrant pressure onto the source generator.  Instead it 
queues requests.

In some cases this is the right decision (if source is not async reentrant) but 
in some cases we want it to forward the pressure (so that the entire chain can 
run in parallel).

By making it an option when the mapped generator is created we can allow 
pressure to be forwarded where appropriate.

 

Phrasing it another way.  If we have source, map function A, map function B, 
map function C, and then a reentrant pull we would currently only run C in 
parallel.



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