lidavidm commented on pull request #8962:
URL: https://github.com/apache/arrow/pull/8962#issuecomment-748197373


   Thanks for catching & taking a look at this!
   
   I think, rather than change the IPC class, we should make the FlightData 
Protobuf deserializer act more like a real Protobuf parser, and fill in an 
empty body instead of leaving the buffer as null if there was not a buffer read 
from the wire. That should probably happen here: 
https://github.com/apache/arrow/blob/519e9da4fc1698f686525f4226295f3680a3f3db/cpp/src/arrow/flight/serialization_internal.cc#L376-L377
   
   I'll admit that even having written the code, it's hard to trace the path 
from here to where you currently have the fix...
   
   Here is where we invoke the deserializer:
   
https://github.com/apache/arrow/blob/25c736d48dc289f457e74d15d05db65f6d539447/cpp/src/arrow/flight/serialization_internal.cc#L429-L432
   That in turn is called from a peekable reader:
   
https://github.com/apache/arrow/blob/25c736d48dc289f457e74d15d05db65f6d539447/cpp/src/arrow/flight/serialization_internal.h#L133
   The reader fills in a FlightData which gets passed to the RecordBatchReader 
here:
   
https://github.com/apache/arrow/blob/519e9da4fc1698f686525f4226295f3680a3f3db/cpp/src/arrow/flight/server.cc#L190
   Calling ReadNext implicitly bounces through this glue class:
   
https://github.com/apache/arrow/blob/519e9da4fc1698f686525f4226295f3680a3f3db/cpp/src/arrow/flight/server.cc#L96-L119
   Which finally actually constructs the IPC Message in this PR:
   
https://github.com/apache/arrow/blob/25c736d48dc289f457e74d15d05db65f6d539447/cpp/src/arrow/flight/serialization_internal.cc#L382-L384


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