[PR] fix(arrow-array): reset the lone offset of an empty FFI byte array [arrow-rs]

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

ahlag opened a new pull request, #11661:
URL: https://github.com/apache/arrow-rs/pull/11661

   # Which issue does this PR close?
   
   Closes #11585.
   
   # Rationale for this change
   
   `StringArray::slice(n, 0)` keeps the offset it was sliced at as the array's single offset. When such an array is exported and imported again, `buffer_len` sizes the values buffer of an empty array at offset 0 as 0, because that lone offset can be any value from the producer (see `test_empty_string_with_non_zero_offset`). The offset itself was kept, so the imported `ArrayData` failed `validate_full` with `First offset 2 of Utf8 is larger than values length 0`.
   
   # What changes are included in this PR?
   
   In `ImportedArrowArray::consume`, an empty `Utf8`/`Binary`/`LargeUtf8`/`LargeBinary` array at offset 0 gets a zeroed offsets buffer, which matches the empty values buffer. Sizing the values buffer from the lone offset instead would read past the buffer for producers like the one in `test_empty_string_with_non_zero_offset` (offset 123 over an empty buffer).
   
   # Are these changes tested?
   
   Yes. `test_empty_slice_of_bytes` round-trips `slice(1, 0)` of each of the four types through `to_ffi`/`from_ffi`, then checks `validate_full` and equality with the exported data. It fails without the fix with the error above. `cargo test -p arrow-array --features ffi` and `--features ffi,force_validate` pass, and `cargo clippy -p arrow-array --all-targets --all-features -- -D warnings` is clean.
   
   # Are there any user-facing changes?
   
   No API changes. Empty sliced byte arrays imported through the C Data Interface now pass validation.
   
   AI use: Claude Code (AI) helped trace the import path and wrote the first draft of the fix, the test and this description; I reviewed and ran everything.
   


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Re: [PR] fix(arrow-array): reset the lone offset of an empty FFI byte array [arrow-rs]

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

ahlag commented on PR #11661:
URL: https://github.com/apache/arrow-rs/pull/11661#issuecomment-6355405453

   @alamb Thanks for the approval! All checks have passed and the branch is up to date. Could this be merged when you get a chance?


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