indigophox opened a new pull request, #40071:
URL: https://github.com/apache/arrow/pull/40071
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### Rationale for this change
Documentation for C++ Flight server middleware implies that SendingHeaders
is called when headers are to be sent, however it is instead called when
middleware is initialized, prior to the app's RPC handler. This is also
inconsistent with e.g. Java where the call happens post-handler.
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### What changes are included in this PR?
[In-progress] Fix for the above call ordering, so far affecting only
non-streaming handlers, including DoAction() but not yet including Handshake().
This PR should also [pending] include a more involved fix for DoGet, DoPut,
and DoExchange, for which helpers are already present.
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### Are these changes tested?
[Pending further commits and finalization of the proposed patch.]
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### Are there any user-facing changes?
Barely: There is a "breaking" change in that if a C++ RPC handler mutates
the call's middleware instance in a way that affects its SendingHeaders()
behaviour, this will now work as expected. However, apps relying on this being
broken (i.e. doing something currently moot and expecting it not to work) are
themselves fundamentally broken so I am not concerned about this breaking
aspect of this change.
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**This PR includes breaking changes to public APIs.**
See above regarding user-facing changes. This SHOULD have zero affect on
even remotely sane applications, as above.
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