Force stream sync in raft::device_reference

[achirkin]

Add a stream synchronization on each read/write access to raft::device_reference.

This type is used as a proxy for raft's mdarray/mdspan indexing operators; without the fix, both getter and setter may copy garbage data, because the lifetime of the host memory in many use cases smaller than it takes to actually copy the data.

See also: #3123 rapidsai/rmm#2527

[achirkin]

FYI: the most dangerous case is raft::make_device_scalar(res, val) that is used all over the place in raft and cuvs.

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Documentation
  • Clarified that device references may transfer data between host and device and synchronize streams.
  • Documented potential stream synchronization and performance impacts when creating device scalars.
• Bug Fixes
  • Added interruptible synchronization during device-to-host conversions and device assignment operations.

Walkthrough

The changes update NVIDIA copyright attributions, document host-device transfer and stream synchronization behavior, and add interruptible synchronization after device reads and assignments.

Changes

Device access behavior

Layer / File(s)	Summary
Interruptible device reference synchronization cpp/include/raft/core/device_container_policy.hpp	device_reference documentation now describes host-device transfers and stream synchronization. Device reads and assignments now perform interruptible CUDA-stream synchronization.
Device scalar synchronization documentation cpp/include/raft/core/device_mdarray.hpp, cpp/include/raft/core/device_container_policy.hpp	make_device_scalar documentation now describes implicit resource-stream synchronization and its performance effect when prior work is pending. Copyright attributions include NVIDIA affiliates and updated years.

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

Merge Risk: 🟡 Moderate · up to 58eff

Each read and write now waits for stream completion, but cancellation can still leave an asynchronous copy using a temporary host buffer after that buffer is destroyed, risking invalid memory access. Repeated accesses may also serialize GPU work and reduce performance. The PR is not merge-ready until the cancellation lifetime issue is fixed and the performance impact is explicitly accepted or addressed.

Suggested reviewers: divyegala, bdice

🚥 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 2 files.	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 change: adding stream synchronization to raft::device_reference.
Description check	✅ Passed	The description accurately explains the synchronization change, its purpose, affected indexing operations, and the risk of copying invalid data.
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.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@cpp/include/raft/core/device_container_policy.hpp`:
- Around line 59-66: Update device_reference::operator value_type() and
device_reference::operator=(T const&) to catch raft::interrupted_exception from
raft::interruptible::synchronize(stream_), synchronously drain stream_ with
RAFT_CUDA_TRY(cudaStreamSynchronize(stream_)), then rethrow; add cancellation
tests covering pending D2H and H2D copies.
- Around line 59-66: Remove unconditional stream_ synchronization from
device_reference read/write operations, including operator= and the
corresponding read path, while preserving lifetime safety for asynchronous
copies. Provide an explicit asynchronous API or opt-in synchronization
mechanism, and add a benchmark covering queued GPU work with repeated
proxy/scalar accesses, including make_device_scalar(handle, v).

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between c0eaacd and 58eff94.

📒 Files selected for processing (2)

• cpp/include/raft/core/device_container_policy.hpp
• cpp/include/raft/core/device_mdarray.hpp

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