Describe the enhancement requested

With apache/arrow#36155, implementations of Flight RPC may not return quickly via a newly added pollFlightInfo function. Sometimes, the system implementing this function may not know the output schema for some time--for example, after a lengthy queue time as elapsed, or after planning.

In proto3, fields may not be present, and it's a coding convention to require them ¹. To support upcoming client integration work for pollFlightInfo, the schema field can be made optional so that it's not a requirement to populate the FlightInfo's schema on the first pollFlightInfo request.

We can modify our client code to allow this field to be optional. This is already the case for the Go code.

Sub-issues:

• [Java][FlightRPC] Allow FlightInfo#Schema to be nullable for long-running queries arrow#37553
• [Docs][FlightRPC] Update documentation to note that FlightInfo::schema may be null arrow#37554
• [C++][FlightRPC] Allow FlightInfo.schema to be nullable arrow#37677
• ...

CC: @lidavidm

Component(s)

C++, Java, Python