Avoid leaving multi-rank execution in a bad state in cudf-polars tests

[wence-]

Description

If a cudf-polars query raises an exception during execution, it is possible that it can leave a dangling collective that then tears down one process.

This is racy because it depends on tasks being cancelled in Python and then dropping C++ objects in a particular order.

The one test in the cudf-polars tests suite that could do this is fixed on main, so backport the relevant changes (#23235), and un-xfail the test.

Checklist

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

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between 95390db and 5509992.

📒 Files selected for processing (6)

• cpp/include/cudf/join/join.hpp
• cpp/src/join/cross_join.cu
• cpp/tests/join/cross_join_tests.cpp
• python/cudf/cudf/core/join/join.py
• python/cudf/cudf/tests/reshape/test_merge.py
• python/cudf_polars/tests/streaming/test_select.py

💤 Files with no reviewable changes (1)

• python/cudf_polars/tests/streaming/test_select.py

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Cross joins now support inputs with zero columns while preserving the correct output row count.
  • Added protection against cross-join row-count overflow with a clear error.
  • Cross-merge results now correctly retain their row count when the output has no columns.
• Tests
  • Expanded coverage for zero-column cross joins and overflow handling.
  • Updated streaming selection tests to reflect corrected behavior.

Walkthrough

Changes

The cross-join implementation now supports zero-column operands, preserves their row counts, and throws std::overflow_error when the output row count exceeds cudf::size_type. cuDF cross merges preserve equivalent row counts, and a Polars streaming test removes a version-specific xfail.

Cross-join behavior

Layer / File(s)	Summary
libcudf cross-join contract and implementation cpp/include/cudf/join/join.hpp, cpp/src/join/cross_join.cu, cpp/tests/join/cross_join_tests.cpp	Documentation, implementation, and tests now cover zero-column operands, explicit output row counts, and overflow exceptions.
cuDF cross-merge row preservation python/cudf/cudf/core/join/join.py, python/cudf/cudf/tests/reshape/test_merge.py	Cross merges pass explicit row counts through libcudf and preserve zero-column result indexes, with pandas comparison coverage.

Polars streaming test expectation

Layer / File(s)	Summary
Unconditional streaming selection test python/cudf_polars/tests/streaming/test_select.py	test_select_with_len no longer depends on a Polars version check or conditional xfail marker.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• rapidsai/cudf#22765: Related explicit row-count handling for zero-column tables.
• rapidsai/cudf#23235: Related cross-join and cross-merge zero-column handling and overflow changes.

Suggested labels: improvement

Suggested reviewers: vyasr, galipremsagar

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title matches the main goal of preventing bad multi-rank execution states in cudf-polars tests.
Description check	✅ Passed	The description is clearly about the same cudf-polars backport and test fix as the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[rjzamora]

Python changes look reasonable to me. Thanks!

[TomAugspurger]

For reference, this is part of a series of changes supporting length 0 tables in libcudf/cudf-polars (#22765, #23234). I believe this is the only bit that wasn't merged before release/26.08 was created.

[vyasr]

LGTM, thanks.

[TomAugspurger]

/merge