[PR] GH-54916: [C++][IPC] Align metadata in Message::Open [arrow]

["Spikhalskiy (via GitHub)" <gi...@apache.org>]

Spikhalskiy opened a new pull request, #54917:
URL: https://github.com/apache/arrow/pull/54917

   ### Rationale for this change
   
   `ipc::Message::Open()` verifies and reads the Flatbuffers metadata in place. Flight passes it a `data_header` sliced zero-copy out of the gRPC message, which can start at any address, so Flatbuffers performs misaligned loads (reported by `-fsanitize=alignment`). `MaybeAlignMetadata()` already exists for this, but only `MessageDecoder` calls it.
   
   ### What changes are included in this PR?
   
   `MessageImpl::Open()` now calls `MaybeAlignMetadata()` before verifying the metadata. Aligned metadata, including everything that comes through `MessageDecoder`, is not copied.
   
   ### Are these changes tested?
   
   Yes. `TestMessage.OpenUnalignedMetadata` opens a message whose metadata starts at an odd address and checks that it round-trips and that `Message::metadata()` is 8-byte aligned. Without the fix the alignment check fails, and under `-fsanitize=alignment` the test reports misaligned accesses in `VerifyMessage` and `GetKeyValueMetadata`. With the fix it passes cleanly.
   
   ### Are there any user-facing changes?
   
   No. `Message::metadata()` may now return an aligned copy instead of the buffer passed to `Message::Open()`.
   
   ### Was AI used for this PR?
   
   **PR code and description written by:**
   
   - [ ] Human
   - [x] AI
   
   **Reviewed before submission by:**
   
   - [x] Human
   - [ ] AI
   - [ ] Not reviewed
   
   Closes #54916
   


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Re: [PR] GH-54916: [C++][IPC] Align metadata in Message::Open [arrow]

["github-actions[bot] (via GitHub)" <gi...@apache.org>]

github-actions[bot] commented on PR #54917:
URL: https://github.com/apache/arrow/pull/54917#issuecomment-6321935520

   :warning: GitHub issue #54916 **has been automatically assigned in GitHub** to PR creator.


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