Fix build for MG C++ tests

[chyunsu3]

Follow-up to #8645, #8529, and #8564

Update the multi-GPU C++ tests to use the new stream API (cudaStream_t -> cuda::stream_ref)

Also update the CI to ensure that MG C++ tests build successfully.

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Reviewing files that changed from the base of the PR and between 57ee70c and 3ca8a69.

📒 Files selected for processing (6)

• .github/workflows/pr.yaml
• .github/workflows/test.yaml
• ci/build_mg_cpp_tests.sh
• cpp/tests/mg/knn_test_helper.cuh
• cpp/tests/mg/pca.cu
• dependencies.yaml

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📝 Summary

Summary by CodeRabbit

• Tests
  • Updated CUDA stream handling in multi-GPU C++ test utilities.
  • Added a dedicated CI build for multi-GPU C++ tests, with a configured CUDA build environment.
  • Pull-request checks now wait for the multi-GPU C++ test build to complete.

Walkthrough

The KNN and PCA multi-GPU tests now use CUDA stream references and pass native stream handles to APIs. The CI configuration adds a Conda environment, build script, and workflow job for cpp-mgtests.

Changes

Multi-GPU C++ test support

Layer / File(s)	Summary
KNN and PCA stream updates cpp/tests/mg/knn.cu, cpp/tests/mg/knn_test_helper.cuh, cpp/tests/mg/pca.cu	The tests store streams as cuda::stream_ref and pass native CUDA stream handles to partition, matrix, cuBLAS, formatting, and deallocation APIs.
Multi-GPU test environment and build dependencies.yaml, ci/build_mg_cpp_tests.sh	The dependency configuration adds the test_mg_cpp environment. The script creates the environment and builds cpp-mgtests with ccache for supported GPU architectures.
CI workflow integration .github/workflows/test.yaml, .github/workflows/pr.yaml	The test workflow adds the build job. The pull-request workflow runs it when the test_cpp changed-file group is true and adds it as a pr-builder dependency.

Priority: ⬇️ Low

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

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 3ca8a

The identified stream-warning and build-environment concerns do not block the multi-GPU test build. The PR is mergeable after normal checks.

🚥 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%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 4 files. (4 skipped: 4 …	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 clearly and concisely identifies the main build-related change for the MG C++ tests.
Description check	✅ Passed	The description explains the stream API migration and the CI build update. It is mostly complete, but it does not link an issue with a GitHub closing keyword as requested by the template.
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.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 4 files. (4 skipped: 4 unsupported.)

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[viclafargue]

Thanks!

I've also noticed that mg C++ tests are not being run in the CI. Is this a deliberate decision?

Yes, historically. The C++ multi-GPU tests need more than one GPU to test their intended behavior, while the Dask tests can run with multi-GPU support enabled on a single GPU. Still, we should at least build the C++ tests in CI to catch compilation errors.

[chyunsu3]

we should at least build the C++ tests in CI to catch compilation errors.

I added the step in ci/test_cpp.sh to test building cpp-mgtests. I avoided adding openmpi to the base environment, because I didn't want to add it to the devcontainer; I added it to test_cpp instead.

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Inline comments:
In `@dependencies.yaml`:
- Line 601: Add ccache to the test_cpp environment dependencies in
dependencies.yaml so the environment activated by ci/test_cpp.sh provides the
compiler launcher required by the CMake build.

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[chyunsu3]

I was able to get cpp-mgtests to build in the CI. The PR is now ready for another round of review.

[bdice]

"Ensure that cpp-mgtests can be built" is a task for a CPU runner, not a GPU runner.

Also, why isn't this part of our CI already?

[chyunsu3]

Also, why isn't this part of our CI already?

See #8683 (review)

[chyunsu3]

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[chyunsu3]

@bdice Can you take another look? I've moved the CI job to the CPU runner.

[chyunsu3]

/merge