Hei-MaoM opened a new pull request, #51017:
URL: https://github.com/apache/arrow/pull/51017
### Rationale for this change
`StructArray::GetFlattenedField` currently combines a struct's validity
bitmap with a child by assigning it to buffer 0. Union arrays do not have
top-level validity buffers, so flattening a union field aborts even when
the parent bitmap is all-valid. When the parent contains nulls, that
validity must instead be represented by the union's child arrays.
### What changes are included in this PR?
- Preserve the union's null top-level validity buffer and its type codes.
- Apply parent validity to every aligned child of a sparse union.
- Rebuild dense-union child segments in logical order so parent-null slots
are null without invalidating valid slots that share the same original
child offset.
- Preserve non-decreasing dense-union offsets and handle sliced arrays.
- Add sparse and dense regression tests.
### Are these changes tested?
- `cmake --build /build/cpp --target arrow-array-test -j 4`
- `/build/cpp/debug/arrow-array-test
--gtest_filter=StructArray.FlattenSparseUnion:StructArray.FlattenDenseUnionWithSharedOffsets
--gtest_brief=1`
- `/build/cpp/debug/arrow-array-test --gtest_brief=1`
- 1062 passed
- 1 skipped because the local build does not enable the required threading
configuration
- `clang-format 18.1.8`
- `cpplint 1.6.1`
- `git diff --check`
### Are there any user-facing changes?
Yes. `StructArray::GetFlattenedField` and callers such as `struct_field`
and `Flatten` no longer crash when selecting a union field from a nullable
struct. Parent nulls are represented in the appropriate union children.
There are no public API changes.
**This PR contains a "Critical Fix".** It fixes a crash on valid input.
Closes #14736.
### AI assistance disclosure
I used OpenAI Codex to help investigate the issue, draft parts of the
implementation and tests, and run verification. I reviewed the final diff
and am responsible for understanding, debugging, and maintaining the change.
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