Fix KV offload crash when request is aborted mid-transfer

[ch2lab]

Summary

When a request is aborted during KV offloading, the OffloadingConnectorScheduler crashes with an AssertionError because offload_keys and block_ids have mismatched lengths in _build_store_jobs.

Context

This was discovered on SM120 (RTX 5090) with NVFP4 KV cache + MTP speculative decoding + KV offloading enabled. The abort scenario:

1. Client sends a request, server starts computing KV cache and offloading to CPU
2. Client disconnects (or request is cancelled via API)
3. vLLM scheduler marks request as finished, but request.num_computed_tokens is not updated
4. On the next schedule step, _build_store_jobs tries to create store jobs for the finished request

Root Cause

1. request.block_hashes contains entries for all allocated blocks, including pre-allocated blocks that haven't been computed yet
2. After abort, request.num_computed_tokens isn't updated, so get_block_ids_for_computed_tokens() returns empty lists
3. update_offload_keys() iterates block_hashes without checking block_ids, creating more offload_keys than there are block_ids
4. The assert len(offload_keys) == len(offload_block_ids) at _build_store_jobs expects equal lengths → crash

Fix

• Add limit_by_block_ids parameter to update_offload_keys() to cap offload_keys at the number of chunks with known block_ids
• Skip store job creation entirely for finished/aborted requests in _build_store_jobs, since their blocks are about to be freed anyway
• Replace the hard assert with a defensive clamp + diagnostic logging (debug for finished requests, warning for unexpected mismatches)
• Accept final_block_ids in request_finished() and sync to group_state.block_ids for consistency
• Pass block_ids through OffloadingConnector to the scheduler (was previously discarded)

Files Changed

• vllm/distributed/kv_transfer/kv_connector/v1/offloading/scheduler.py
• vllm/distributed/kv_transfer/kv_connector/v1/offloading_connector.py

Related

Part of SM120 NVFP4 KV cache enablement effort:

• vLLM #46329 — NVFP4 KV on SM120 via FA2 + VO-split
• vLLM #49011 — NVFP4 KV on SM120 prototype
• vLLM #49010 — FULL cudagraph + flashinfer output corruption
• FlashInfer jethac/flashinfer#1 — Re-enable split-KV for NVFP4 KV cache

Test Plan

• Start a request with KV offloading enabled (--kv-transfer-config ...)
• Abort the request mid-generation (e.g., client disconnect)
• Verify no AssertionError crash occurs
• Verify subsequent requests work normally
• Verify normal KV offload (non-aborted) still works correctly

[claude]

Claude Code Review

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ch2lab.

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

[orozery]

Thanks @ch2lab !
I believe this issue is already fixed by #49285 .

[ch2lab]

Superseded by updated #49891 (KV offload fix included as second commit).