[C++] Sliced sparse union element access can return incorrect values

[fenfeng9]

Describe the bug, including details regarding any error messages, version, and platform.

Slicing a sparse union array produces incorrect logical values.
to_pylist() and scalar access on the slice return wrong results —a non-null element is reported as null, or a value from the wrong position is returned.

Reproduce

import pyarrow as pa


def main():
    arr = pa.UnionArray.from_sparse(
        pa.array([0, 0, 0], type=pa.int8()),
        [pa.array([None, 20, 30])],
    )
    sliced = arr.slice(2)

    print(f"pyarrow: {pa.__version__}")
    print()

    # Direct access on the original array is still correct.
    expected_arr_2 = 30
    actual_arr_2 = arr[2].as_py()
    print(f"arr[2].as_py() expected: {expected_arr_2}")
    print(f"arr[2].as_py() actual:   {actual_arr_2}")
    print()

    # The sliced array should preserve the same logical value in to_pylist().
    expected_sliced_pylist = [30]
    actual_sliced_pylist = sliced.to_pylist()
    print(f"sliced.to_pylist() expected: {expected_sliced_pylist}")
    print(f"sliced.to_pylist() actual:   {actual_sliced_pylist}")
    print()

    # Scalar access on the sliced array should also return the same value.
    expected_sliced_0 = 30
    actual_sliced_0 = sliced[0].as_py()
    print(f"sliced[0].as_py() expected: {expected_sliced_0}")
    print(f"sliced[0].as_py() actual:   {actual_sliced_0}")


if __name__ == "__main__":
    main()

Result

pyarrow: 24.0.0

arr[2].as_py() expected: 30
arr[2].as_py() actual:   30

sliced.to_pylist() expected: [30]
sliced.to_pylist() actual:   [None]

sliced[0].as_py() expected: 30
sliced[0].as_py() actual:   None

Component(s)

C++

[fenfeng9]

I think this bug is in the sparse-union logical null path here:

arrow/cpp/src/arrow/array/data.cc

Lines 72 to 77 in e16067a

	bool IsNullSparseUnion(const ArrayData& data, int64_t i) {
	auto* union_type = checked_cast<const SparseUnionType*>(data.type.get());
	const auto* types = reinterpret_cast<const int8_t*>(data.buffers[1]->data());
	const int child_id = union_type->child_ids()[types[data.offset + i]];
	return data.child_data[child_id]->IsNull(i);
	}

and here:

arrow/cpp/src/arrow/array/data.cc

Lines 697 to 702 in e16067a

	bool ArraySpan::IsNullSparseUnion(int64_t i) const {
	auto* union_type = checked_cast<const SparseUnionType*>(this->type);
	const auto* types = reinterpret_cast<const int8_t*>(this->buffers[1].data);
	const int child_id = union_type->child_ids()[types[this->offset + i]];
	return this->child_data[child_id].IsNull(i);
	}

IsNullSparseUnion checks child nullness with i, but it should use data.offset + i. Likewise, ArraySpan::IsNullSparseUnion should use this->offset + i instead of i. Otherwise, sliced sparse unions can return incorrect logical values.

I have verified the fix locally and will open a PR.