[Integration] Reject failed Python buddy allocator backing buffer

[he-yufeng]

Summary

• stop buddy allocation when the backing raw buffer allocation or registration fails
• leave buffer_list_ and the slab free lists unchanged on failure
• propagate the failure to allocateManagedBuffer, which already returns a null address

Problem

The max-class branch appended the result of allocateRawBuffer without checking it. On allocation failure or local-memory registration failure, this inserted nullptr into buffer_list_ and derived invalid addresses into free_list_, while reporting success.

Fixes #2371.

Validation

• git diff --check
• source-path review against current upstream/main

To verify

• Mooncake integration C++ build and CI on Linux
• failure-injection coverage for raw backing-buffer allocation, where available

[gemini-code-assist]

Code Review

This pull request adds a null check after allocating a raw buffer in TransferEnginePy::doBuddyAllocate to prevent pushing a null pointer to the buffer list and avoid potential crashes. There are no review comments to address, and the change looks correct.

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[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...integration/transfer_engine/transfer_engine_py.cpp	0.00%	1 Missing ⚠️

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[jfeng18]

LGTM. Clean minimal fix — checks allocateRawBuffer return before mutating buffer_list_ and free_list_. This prevents the NameError-equivalent in C++ where nullptr seeds the buddy allocator with invalid addresses.

One observation: when allocateRawBuffer fails, the class_id split loop in allocateManagedBuffer will retry doBuddyAllocate at progressively larger class_ids until kMaxClassId, then return nullptr to the caller. This is the correct failure propagation — no further changes needed.

[alogfans]

LGTM