[Matt711]

Description

Closes #24950.

cudf::make_column_from_scalar threw 0-length struct column is unsupported for a struct scalar with size == 0. The guard came from #8185, which removed the generic size == 0 -> make_empty_column shortcut so that lists could return a well-formed empty column. The struct path was not updated, so it got a guard instead. The gather that builds the children already handles an empty range, so this removes the guard.

cudf-polars hit this when broadcasting a length-1 struct column to 0 rows (broadcast in dsl/utils/reshape.py). That is how test_struct_aggstates_in_apply_expr_24168 fails on the upstream Polars suite. The test is not new, it started running with #24825

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes. (No documentation change needed.)

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• cpp/tests/column/factories_test.cpp

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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Struct scalars can now be converted into zero-row columns, including valid and null scalars, while preserving their child columns and nested structure.
  • Group-by aggregations that construct structs from literal values now work with empty frames, returning results that match expected output regardless of row order.

The struct-scalar factory no longer rejects zero-length output. C++ tests cover valid and null struct scalars. A Python test covers a group-by that aggregates a struct on an empty frame.

Changes: Zero-length struct scalar conversion

Layer / File(s)	Summary
Construct and validate zero-length struct columns cpp/src/column/column_factories.cu, cpp/tests/column/factories_test.cpp, python/cudf_polars/tests/expressions/test_struct.py	The factory proceeds with child gathering and struct-column construction at size zero. C++ tests check valid and null scalars against empty columns shaped like their one-row results. A Python test checks a struct aggregation in an empty-frame group-by.

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

Change: Bug fix · Severity of issue fixed: Low

Possibly related issues

• NVIDIA/cudf issue 24950: The issue describes a failure when converting a struct scalar to a zero-length column. This change removes the factory rejection and adds zero-length struct coverage.

Merge Risk: Minimal · up to a2abe

The implementation and focused tests cover the zero-length struct behavior, with no current merge-blocking risk established.

Pre-merge checks: 5

Check name	Status	Explanation
Title check		The title names the main change: the struct-scalar factory in cudf::make_column_from_scalar now accepts size-0 input instead of throwing. The raw summary, the PR description, and the objectives all de…
Linked Issues check		Issue #24950 requires zero-length struct scalar conversion during cudf-polars broadcast and the upstream test test_struct_aggstates_in_apply_expr_24168 to pass. In `cpp/src/column/column_factories.c…
Out of Scope Changes check		The changed source removes the guard that blocks zero-length struct columns. The libcudf tests verify the changed factory behavior. The cudf-polars test exercises a group-by aggregation that construct…
Docstring Coverage		No eligible functions found in the changed files. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…
Description check		The pull request description directly explains the zero-length struct column issue, the guard removal, the cudf-polars impact, and the added test coverage. It is relevant to the changeset.

Full details: Title check

Explanation

The title names the main change: the struct-scalar factory in cudf::make_column_from_scalar now accepts size-0 input instead of throwing. The raw summary, the PR description, and the objectives all describe removing the zero-length guard for struct scalars, so the title matches the changeset.

Full details: Linked Issues check

Explanation

Issue #24950 requires zero-length struct scalar conversion during cudf-polars broadcast and the upstream test test_struct_aggstates_in_apply_expr_24168 to pass. In cpp/src/column/column_factories.cu, the struct dispatch now gathers over the requested range, including an empty range, and constructs a STRUCT column with the requested size. The added libcudf tests cover valid and null struct scalars at size zero. The PR reports the upstream test passing with 128 passes and 2 skips.

Full details: Out of Scope Changes check

Explanation

The changed source removes the guard that blocks zero-length struct columns. The libcudf tests verify the changed factory behavior. The cudf-polars test exercises a group-by aggregation that constructs a struct from literals on an empty frame, which directly covers issue #24950. No unrelated change is demonstrated.

Full details: Docstring Coverage

Explanation

No eligible functions found in the changed files. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 0 functions across 2 files.

Finishing Touches: 💡 1

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Actionable comments posted: 1

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Prompt to fix review comments

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Inline comments:
Review comments at @python/cudf_polars/tests/utils/test_broadcast.py:
- Line 6: Ensure the test suite can collect when only the test extra is
installed by adding PyArrow to the test dependencies, or remove the PyArrow
import from test_broadcast.py and construct the struct scalar without it.

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• Source mode: Auto
• Repository: NVIDIA/cudf
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• Run ID: 569a9474-cf3a-41f3-ad7d-52b1e822cc2d

Commits

Reviewing files that changed from the base of the PR and between a52cc57 and e3c26c5.

Files selected for processing: 3

• cpp/src/column/column_factories.cu
• cpp/tests/column/factories_test.cpp
• python/cudf_polars/tests/utils/test_broadcast.py

Files with no reviewable changes: 1

• cpp/src/column/column_factories.cu

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[Matt711]

/merge