[BugFix] Fix MultiConnector test after HMA changes

[njhill]

#17996 broke CI because it didn't account for recent multi-connector changes.

Thanks @NickLucche for helping with the investigation.

Fixes #19281.

[github-actions]

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[gemini-code-assist]

Hello @njhill, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

Summary of Changes

Hello! Gemini or gemini-code-assist here, providing a summary of this pull request. This PR, authored by njhill, aims to fix a broken CI test (test_multi_connector.py) that resulted from recent changes in PR #17996, which introduced updates related to multi-connector functionality and Hierarchical Memory Allocation (HMA). The issue is tracked in #19281. The core problem seems to be a change in how KVCacheBlocks are structured or represented, particularly in a multi-connector context, which caused the existing test assertions and logging to fail.

Highlights

• Fixing CI Test: The primary goal of this PR is to resolve a failing CI test (test_multi_connector.py) that was broken by previous multi-connector/HMA changes.
• Adapting to KVCacheBlocks Structure Change: The changes adapt the code and tests to a likely change in the internal representation of KVCacheBlocks, specifically how empty blocks are handled and how the structure is maintained across multiple connectors.
• Adding KVCacheBlocks.new_empty(): A new method new_empty() is added to the KVCacheBlocks class to create an empty instance that preserves the structural dimensions (e.g., number of sub-lists for different layers or heads).
• Updating MultiConnector Logic: The MultiConnector's update_state_after_alloc method is updated to use the new KVCacheBlocks.new_empty() method when passing empty block information to connectors that were not chosen for a specific request.
• Updating Test Assertions: The unit test test_multi_shared_storage_connector_consistency is updated to match the new logging format for KVCacheBlocks, which now shows the lengths of the internal block lists (e.g., num_blocks=[...]) instead of a single total length.

Changelog

• tests/v1/kv_connector/unit/test_multi_connector.py
  • Modified a logging wrapper to print the lengths of the internal block lists within KVCacheBlocks.
  • Updated assertions in test_multi_shared_storage_connector_consistency to expect the new list-based format for num_blocks in logged events.
• vllm/distributed/kv_transfer/kv_connector/v1/multi_connector.py
  • Changed the creation of empty KVCacheBlocks instances passed to non-chosen connectors from KVCacheBlocks.create_empty() to using the new blocks.new_empty() method.
• vllm/v1/core/kv_cache_manager.py
  • Added a new_empty() method to the KVCacheBlocks class to create an empty instance while preserving the structure of the block lists.

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[gemini-code-assist]

Code Review

This pull request effectively addresses the CI breakage caused by recent multi-connector/HMA changes. The core of the fix involves updating how KVCacheBlocks are handled, particularly ensuring that empty block instances correctly reflect the multi-group structure.

The introduction of the KVCacheBlocks.new_empty() method is a clean and robust way to create these structured empty blocks. The corresponding updates in MultiConnector to use this method and the test adjustments for the new logging format are logical and well-executed.

Overall, the changes are clear, targeted, and appear to correctly resolve the issue. The code quality is good.

Summary of Findings

• Test Logger Formatting: In tests/v1/kv_connector/unit/test_multi_connector.py (lines 63-64), the f-string for logging num_blocks includes a list comprehension directly and is broken across lines. While functional, formatting it as block_lengths = [len(b) for b in arg.blocks]; to_log.append(f"num_blocks={block_lengths}") or letting an auto-formatter like Black handle the multi-line f-string might slightly improve readability. This is a minor stylistic point and was not commented on directly due to review settings (severity: low).
• Robustness Improvement: The change in vllm/distributed/kv_transfer/kv_connector/v1/multi_connector.py to use blocks.new_empty() (derived from the new KVCacheBlocks.new_empty() method) instead of a generic KVCacheBlocks.create_empty() is a significant improvement. It ensures that the empty blocks passed to non-chosen connectors maintain the same group structure as the primary blocks, which is crucial for consistency with HMA changes.

Merge Readiness

The pull request appears to be in good shape and addresses the described bug effectively. The changes are well-reasoned and improve the robustness of the multi-connector logic. I believe it is ready for merging after standard CI checks pass. As an AI, I am not authorized to approve pull requests, so please ensure it is reviewed and approved by a human maintainer.

[mgoin]

Thank you