arrow-row: Fix decode_fixed_size_list to apply the corrected_type step for dictionary children

[zhuqi-lucas]

Which issue does this PR close?

Closes #10413.

Rationale for this change

RowConverter::convert_rows used to fail with a schema-mismatch InvalidArgumentError on any target type that contained a FixedSizeList<Dictionary<K, V>>:

InvalidArgumentError("FixedSizeListArray expected data type Dictionary(Int32, Utf8) got Utf8 for \"item\"")

decode_fixed_size_list was building the returned array with the declared element_field (which still carried Dictionary<...>) while the children came from converter.convert_raw, which returns the flattened (non-dictionary) type. try_new_with_length validates the child type against element_field and returned Err.

The other list-like decoders — List, LargeList, ListView, LargeListView, Map — already reconcile declared-vs-actual via a corrected_type step (see list::decode<L> at list.rs:~260 on main), and Codec::Struct in lib.rs does the same via corrected_fields. decode_fixed_size_list was the only list-like decoder that skipped this pattern.

What changes are included in this PR?

• arrow-row/src/list.rs::decode_fixed_size_list: adopt the corrected_type step, mirroring the shape of every other list-like decoder in the file.
• arrow-row/src/lib.rs::tests::test_fixed_size_list_of_dictionaries_round_trips: regression test that reproduces the original failure on one row ["a", "b"] of FixedSizeList<Dictionary<Int32, Utf8>, 2>.

Diff on the decoder is ~10 lines:

let mut children = unsafe { converter.convert_raw(&mut child_rows, validate_utf8) }?;
assert_eq!(children.len(), 1);

// Since `RowConverter` flattens certain data types (i.e. `Dictionary`),
// we need to use the child's actual data type rather than the declared
// `element_field`'s. Mirrors the `corrected_type` logic in `decode<L>`
// above and `Codec::Struct`'s `corrected_fields` in `lib.rs`.
let corrected_element_field = Arc::new(
    element_field
        .as_ref()
        .clone()
        .with_data_type(children[0].data_type().clone()),
);

FixedSizeListArray::try_new_with_length(
    corrected_element_field,
    *size,
    children.pop().unwrap(),
    nulls,
    num_rows,
)

Are these changes tested?

Yes:

• test_fixed_size_list_of_dictionaries_round_trips — regression test in arrow-row/src/lib.rs. Pre-fix it panics at the first convert_rows(...).unwrap() (matching the report in arrow-row: decode_fixed_size_list panics on FixedSizeList<Dictionary> (missing corrected_type) #10413); post-fix it returns a FixedSizeList<Utf8> (same flattened shape produced today for List<Dictionary<...>>) whose values are "a", "b".
• All existing arrow-row tests continue to pass (cargo test -p arrow-row).
• cargo clippy -p arrow-row --all-targets -- -D warnings clean.

Are there any user-facing changes?

Behaviour change (bug fix): RowConverter::convert_rows now succeeds where it previously returned an InvalidArgumentError for FixedSizeList<Dictionary<...>> (and any type containing one, e.g. FSL<Struct<Dict>>, List<FSL<Dict>>). The returned array flattens the dictionary child to its native representation — same behaviour today's List<Dict> / LargeList<Dict> / Map<Dict> decoders exhibit. Callers that need the declared dictionary type back can re-encode after convert_rows (that's the documented contract).

No public API surface changes.

Downstream context

DataFusion hit this in apache/datafusion#23523 (nested-type support in GroupValuesColumn) where a GROUP BY fsl_col on FixedSizeList<Dictionary<Int32, Utf8>> would panic on emit. The DataFusion PR is landing a defensive blacklist in the meantime; this fix will let that blacklist be lifted in a follow-up.

[Copilot]

Pull request overview

Fixes arrow-row decoding for FixedSizeList<Dictionary<..>> by ensuring decode_fixed_size_list uses the actual decoded child data type (post-dictionary-flattening) when constructing the returned FixedSizeListArray, matching the existing “corrected type” behavior of other list-like decoders.

Changes:

• Update decode_fixed_size_list to rebuild the element Field with the decoded child’s (possibly flattened) DataType before calling FixedSizeListArray::try_new_with_length
• Add a regression test covering round-tripping FixedSizeList<Dictionary<Int32, Utf8>, 2> through RowConverter::{convert_columns, convert_rows}

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
arrow-row/src/list.rs	Applies “corrected type” logic to FixedSizeList decoding to avoid schema mismatch when dictionary children are flattened
arrow-row/src/lib.rs	Adds regression test ensuring convert_rows succeeds and returns a self-consistent flattened child type for FixedSizeList<Dictionary<..>>

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[Jefffrey]

the comments seem overly verbose

[zhuqi-lucas]

the comments seem overly verbose

Thanks @Jefffrey for good suggestion, applied now.

[Jefffrey]

thanks @zhuqi-lucas