[C++] Expose Message-level IPC metadata in both read and write interfaces

[asfimport]

the Message flatbuffer type has custom_metadata but there is no API support for reading and writing values to this field.

Reporter: Wes McKinney / @wesm

Related issues:

• [C++] Implement experimental buffer compression in IPC messages (relates to)
• [C++] Record batch specific metadata is not saved in IPC file (is superceded by)

_{Note: This issue was originally created as ARROW-6940. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I implemented this in ARROW-7979

[asfimport]

Wes McKinney / @wesm:
I was wrong, there is more work to do to enable metadata to be supplied when writing a message into a stream writer or file writer. Will require a follow up patch. Something for after 0.17.0

[asfimport]

Antoine Pitrou / @pitrou:
cc @lidavidm

[asfimport]

David Li / @lidavidm:
@pitrou  sorry, what's the context for this ping? It looks like there's a few things left to do:

• Expose custom_metadata from the record batch reader/writer

• Implement it in Flight (possibly doing the (de)compression in the background to avoid blocking the consumer/producer)

  I could take a look at it when I get to trying compression with Flight.

[asfimport]

Antoine Pitrou / @pitrou:
Ah, sorry @lidavidm. I think the custom metadata addition was mainly meant for Flight? Or am I mistaken?

[asfimport]

David Li / @lidavidm:
Flight has a separate custom ("application") metadata field that was added before this was implemented. I don't think it's needed for Flight but I do think we need to take a second look at compression + Flight.