### What this PR does / why we need it?
#### Problem
When decode node enables prefix cache and the local prefix cache fully
hits, the following assertion error occurs:
```
(EngineCore_DP3 pid=34912) File "/usr/local/python3.11.14/lib/python3.11/site-packages/vllm/v1/engine/core.py", line 520, in step_with_batch_queue
(EngineCore_DP3 pid=34912) engine_core_outputs = self.scheduler.update_from_output(
(EngineCore_DP3 pid=34912) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_DP3 pid=34912) File "/usr/local/python3.11.14/lib/python3.11/site-packages/vllm/v1/core/sched/scheduler.py", line 1520, in update_from_output
(EngineCore_DP3 pid=34912) self._update_from_kv_xfer_finished(kv_connector_output)
(EngineCore_DP3 pid=34912) File "/usr/local/python3.11.14/lib/python3.11/site-packages/vllm/v1/core/sched/scheduler.py", line 2120, in _update_from_kv_xfer_finished
(EngineCore_DP3 pid=34912) assert RequestStatus.is_finished(req.status)
(EngineCore_DP3 pid=34912) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_DP3 pid=34912) AssertionError
```
The error is triggered in scheduler.py at _update_from_kv_xfer_finished:
```
if req.status == RequestStatus.WAITING_FOR_REMOTE_KVS:
self.finished_recving_kv_req_ids.add(req_id)
else:
assert RequestStatus.is_finished(req.status)
```
#### Root Cause
When decode node has prefix cache enabled and local prefix cache fully
hits:
1. get_num_new_matched_tokens returns ext_tokens=0, load_kv_async=False
when decode prefix cache fully hits
2. Request status becomes RUNNING (not WAITING_FOR_REMOTE_KVS)
3. However, update_state_after_alloc still adds the request to
_reqs_need_recv because remote_block_ids exists in kv_transfer_params
4. Worker processes the request in _handle_request:
- _transfer_kv_cache returns immediately (no actual transfer,
local_block_ids is empty)
- finally block still calls update_done_task_count(request_id)
5. finished_recving contains this request
6. When _update_from_kv_xfer_finished processes finished_recving,
request status is RUNNING
7. Assertion fails
#### Solution
In _handle_request, only notify scheduler (update_done_task_count) when
actual KV transfer happened (local_block_ids is not empty). The signals
to notify Prefill to release KVCache
(_send_done_signal_to_free_remote_port and _send_done_recv_signal) are
still sent regardless.
### Does this PR introduce _any_ user-facing change?
### How was this patch tested?
- vLLM version: v0.17.0
- vLLM main:
4034c3d32e
Signed-off-by: LCAIZJ <leichao139636@163.com>
d9ac7e8539