[Changho Hwang (chhwang)]

Summary

• Make CudaStreamWithFlags and GpuStream non-copyable to prevent duplicate stream destruction and returning a pool lease more than once. No move API is added; current direct-prvalue return usage remains valid under C++20 guaranteed copy elision. This intentionally breaks callers that copy either type, including std::move expressions that previously fell back to copying.
• Free the successful verbs device list in getIBDeviceCount() and return zero if list acquisition returns null.
• Initialize empty GpuBuffer state with zero bytes and deviceId() == -1, document that behavior, and extend the existing GPU-utils test.

Validation

• Built the actual unit_tests and mp_unit_tests targets with CUDA 12.9, C++20, and IB enabled; ownership static_asserts compiled.
• Existing GpuUtilsTest.StreamPool and GpuUtilsTest.AllocShared each passed with exactly one filtered test selected. The latter includes the empty-buffer assertions.
• No full test suite or IB hardware runtime test was run.

This PR does not add locks, reference counting, synchronization, allocator changes, or benchmark/performance claims.

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[Copilot]

🟢 Approval recommended

No active review issues are reported for the stream-ownership, resource-cleanup, and empty-buffer changes.

No open issues 🎉

What changed in this PR

This PR tightens GPU stream ownership and resource cleanup in MSCCL++ utilities while defining predictable empty-buffer behavior.

Changes:

• Makes both stream wrappers non-copyable to prevent duplicate destruction or pool returns; no move API is added.
• Frees acquired InfiniBand device lists and returns zero when acquisition fails.
• Initializes empty GPU buffers with zero bytes and device ID -1, documents the behavior, and adds regression assertions.

File	Description
test/​unit/​gpu_utils_tests.cc	Adds compile-time non-copyability checks and empty-buffer assertions.
src/​core/​ib.cc	Releases the device list and handles null acquisition.
include/​mscclpp/​gpu_utils.hpp	Deletes stream copy operations and initializes and documents empty-buffer state.

_{Review effort: Balanced}

[Copilot]

🟢 Approval recommended

No active review issues are reported for the resource-ownership and cleanup changes.

No open issues 🎉

_{Review effort: Balanced}

[Copilot]

🟢 Approval recommended

There are no active review issues for the ownership and resource-cleanup changes.

No open issues 🎉

_{Review effort: Balanced}