Allow large strings input within the parquet make_column utility

[davidwendt]

Description

Fixes a silent-corruption bug in the Parquet reader's binary→list<uint8> conversion.

When a binary column is read with set_convert_binary_to_strings(false), the reader first materializes it as a strings column, then hands that column's offsets child directly to cudf::make_lists_column:

Any strings column may carry 64-bit offsets. A LIST column's offsets child is always 32-bit — per the Arrow columnar format (a 64-bit variant would be a distinct LargeList type which libcudf does not currently support). So this produces a malformed LIST column with an INT64 offsets child.

Because there is a guard already bounding the chars by max(int32), every offset value is representable as an int32_t, so the fix just converts the offsets.

A new gtest is added taking advantage of the LIBCUDF_LARGE_STRINGS_THRESHOLD env var to produce a strings column with INT64 offsets.

This fix was discovered as part of the work on #23607

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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• cpp/src/io/utilities/column_buffer.cpp

🚧 Files skipped from review as they are similar to previous changes (1)

• cpp/src/io/utilities/column_buffer.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Fixed conversion of binary data from large-string representations to byte-list columns.
  • Ensured offset handling remains compatible with 32-bit list offsets while preserving table contents.

• Tests

  • Added regression coverage for binary conversion involving large strings and 64-bit offsets.

Walkthrough

The string-to-binary conversion now normalizes string offsets to INT32 before creating a LIST<UINT8> column. A Parquet regression test covers binary data that passes through the large-string path.

Changes

Binary list conversion

Layer / File(s)	Summary
Offset normalization and regression coverage cpp/src/io/utilities/column_buffer.cpp, cpp/tests/io/parquet_reader_test.cpp	The conversion extracts string offsets, casts non-INT32 offsets to INT32, and uses them to construct the list column. The regression test verifies the offset type and table contents.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to e2440

The change fixes large-string Parquet conversion by narrowing offsets for LIST columns, but the returned offsets buffer still uses the current memory resource instead of the caller-selected resource, which could place allocations in an unexpected memory pool and should receive explicit owner follow-up.

Suggested reviewers: bdice, mythrocks

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title identifies the Parquet utility change and the support for large-string input addressed by the fix.
Description check	✅ Passed	The description clearly explains the silent-corruption bug, the int32 offset conversion, and the regression test.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 3

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Inline comments:
In `@cpp/src/io/utilities/column_buffer.cpp`:
- Around line 231-247: Update the cudf::detail::cast call in the offsets_col
conversion to use buffer._mr as its memory-resource argument instead of the
current device resource. Preserve the existing INT32 check and make_lists_column
flow so the returned LIST<UINT8> column uses the caller-selected resource.

In `@cpp/tests/io/parquet_reader_test.cpp`:
- Around line 1690-1722: Add focused cases to BinaryAsListLargeStringsThreshold
covering an empty binary column, a null binary row, values exactly at the
large-strings threshold boundary, and input spanning multiple blocks; verify
both converted offsets and null-mask transfer through the reader for each
required case.
- Around line 1690-1722: Add a unit benchmark alongside
BinaryAsListLargeStringsThreshold covering binary reads with default offsets and
with LIBCUDF_LARGE_STRINGS_THRESHOLD forcing the INT64-offset conversion path.
Measure and report both conversion runtime and allocation impact, reusing the
existing Parquet read setup and cleanup patterns.

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📥 Commits

Reviewing files that changed from the base of the PR and between a1dc068 and 02f684a.

📒 Files selected for processing (2)

• cpp/src/io/utilities/column_buffer.cpp
• cpp/tests/io/parquet_reader_test.cpp

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[vyasr]

Good catch. I wonder if we could eventually support large lists in a similar way to how we support large strings today.