[P/D] Handle Abort and Make Lifecycle Explicit

[NickLucche]

Rebasing and cleaning up @robertgshaw2-redhat work here #18096.

[github-actions]

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[gemini-code-assist]

Summary of Changes

Hello @NickLucche, 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!

This pull request significantly enhances the robustness and clarity of request lifecycle management within the distributed KV cache system. My changes introduce explicit handling for request abortion and refine the process of freeing KV cache blocks, ensuring that resources are released only after all necessary distributed operations are complete. This work builds upon existing efforts to improve the stability of the vLLM distributed architecture.

Highlights

• Request Abort Handling: I've implemented a robust mechanism to handle the abortion of requests within the distributed KV cache system. This ensures that resources are properly cleaned up even if a request is terminated prematurely, preventing potential memory leaks or stale states.
• Explicit KV Block Lifecycle Management: The lifecycle of KV cache blocks has been made more explicit. Requests now transition through a pending_kv_free_req_ids state, ensuring that KV blocks are only freed after all distributed transfer operations (like sending blocks to another worker) are confirmed complete. This prevents double-free issues and ensures data consistency.
• Refactored KV Connector Metadata: The NixlConnectorMetadata and NixlConnector have been refactored to explicitly track requests based on their state: those needing to be received, those needing to be sent, and those that have been aborted. This improves clarity and control over the distributed KV operations.
• Enhanced Unit Testing: A new comprehensive unit test (test_abort) has been added to thoroughly validate the correct lifecycle behavior of aborted remote decode requests, covering various scenarios from prefill to final cleanup.

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[gemini-code-assist]

Code Review

This pull request introduces mechanisms for handling request aborts and makes the lifecycle of remote decode requests more explicit, particularly concerning the freeing of KV cache blocks. The changes in the scheduler to manage pending_kv_free_req_ids and the KV connector's new abort_request method are significant improvements. The tests have been updated to cover these new scenarios. A minor type hint correction is suggested. Overall, the PR appears to enhance the robustness and clarity of the system.

[gemini-code-assist]

The type hint for _reqs_need_abort is set[int], but req_id is consistently a string throughout the codebase. This should be set[str] to match the type of req_id.

Suggested change

	self._reqs_need_abort: set[int] = set()
	self._reqs_need_abort: set[str] = set()

[gemini-code-assist]

The connector needs to handle aborts, so calling self.connector.abort_request(req_id) is a good addition here.

[gemini-code-assist]

Consider removing or updating this TODO comment, as the introduction of pending_kv_free_req_ids and the logic in _free_request and _update_from_kv_xfer_finished seem to address it.

[gemini-code-assist]

The current implementation calls connector.abort_request(req_id) and then proceeds to _free_request. In _free_request, _connector_finished will likely return delay_free_blocks=False because the status is FINISHED_ABORTED, leading to immediate block freeing via _free_blocks. This aligns with the comment "Since we have not yet cached the blocks in this state, we should be okay."

[gemini-code-assist]

The logic to conditionally free blocks immediately or add them to pending_kv_free_req_ids based on delay_free_blocks is clear.

[gemini-code-assist]

Refactoring _free_blocks to accept request_id instead of the Request object simplifies its responsibility.

[gemini-code-assist]

This logic correctly handles the removal of req_id from pending_kv_free_req_ids and calls _free_blocks(req_id) when the KV transfer is finished.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @NickLucche.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @NickLucche.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[NickLucche]

closing in favor of #20139