ASF GitHub Bot commented on ARROW-1860:

wesm commented on issue #1500: ARROW-1860: [C++] Introduce ipc::PreparedMessage 
data structure to avoid making multiple passes over record batches
URL: https://github.com/apache/arrow/pull/1500#issuecomment-365091190
   So it seems that not all Python file objects support the `tell` operation, 
for example the `_io.BufferedReader` that you get calling `makefile` on a 
socket. That's the last issue here -- I think probably the best way to deal 
with this is to implement a `PyInputStream` in 
https://github.com/apache/arrow/blob/master/cpp/src/arrow/python/io.h and 
handle the stream position bookkeeping internally, if that sounds good to folks

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> [C++] Add data structure to "stage" a sequence of IPC messages from in-memory 
> data
> ----------------------------------------------------------------------------------
>                 Key: ARROW-1860
>                 URL: https://issues.apache.org/jira/browse/ARROW-1860
>             Project: Apache Arrow
>          Issue Type: New Feature
>          Components: C++
>            Reporter: Wes McKinney
>            Assignee: Wes McKinney
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 0.9.0
>         Attachments: text.html
> Currently, when you need to pre-allocate space for a record batch or a stream 
> (schema + dictionaries + record batches), you must make multiple passes over 
> the data structures of interest (and use e.g. {{MockOutputStream}} to compute 
> the size of the output buffer). It would be useful to make a single pass to 
> "prepare" the IPC payload for both sizing and writing to prevent having to 
> make multiple passes

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