[Bugfix] Finalize streaming requests at terminal sentinel

[vklimkov-nvidia]

Purpose

A resumable request uses None in streaming_queue as the terminal sentinel. _handle_stopped_request() consumes that sentinel and returns True, but leaves request.resumable set to True. The request is finished, yet teardown code and request-finished hooks still receive state that says another streaming chunk is possible.

What broke: the terminal-sentinel path returned a finished result without finalizing the request lifecycle flag.

Root cause: the None branch returned immediately after popping the sentinel and never cleared resumable.

Fix: set request.resumable = False when the terminal sentinel is consumed. A paired regression test keeps the empty-queue behavior unchanged: a resumable request with no queued update still waits for more input.

To reproduce the failure against the parent commit while keeping the new test:

git checkout HEAD^ -- vllm/v1/core/sched/scheduler.py
.venv/bin/python -m pytest -q tests/v1/streaming_input/test_scheduler_streaming.py -k handle_stopped_request
git restore --source=HEAD --staged --worktree vllm/v1/core/sched/scheduler.py

Why this is not a duplicate

I searched the open vLLM PRs for streaming_queue, terminal sentinels, and resumable-request finalization. #42502 addresses max_model_len overflow across later streaming segments, while #43683 clears stale speculative-decoding state. Neither changes the None sentinel path or finalizes the resumable flag.

AI assistance: OpenAI Codex helped inspect the scheduler lifecycle, prepare the focused tests, and draft this PR. This is a draft so I can review every changed line and reproduce the test result before marking it ready.

Model evaluation is not applicable because this does not change model execution, sampling, or generated output.

Test Plan

.venv/bin/python -m pytest -q tests/v1/streaming_input/test_scheduler_streaming.py
.venv/bin/pre-commit run --files vllm/v1/core/sched/scheduler.py tests/v1/streaming_input/test_scheduler_streaming.py

Test Result

Before the fix, the focused tests reported 1 failed, 1 passed; the terminal case failed with assert True is False for request.resumable.

After the fix:

10 passed, 17 warnings in 1.40s

All configured pre-commit hooks passed on the two changed files, including Ruff, formatting, typos, mypy, SPDX, and repository policy checks.

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[claude]

Claude Code Review

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[njhill]

@vklimkov-nvidia this seems reasonable but is it actually visible in any way? i.e. does it cause any different/incorrect behavior?

[vklimkov-nvidia]

Yes, this has visible behavior. I am working on an incremental TTS plugin in NVIDIA-NeMo/Speech#15931, and this distinction is exercised directly in its codec transfer path.

The plugin keeps partial acoustic-code buffers across incremental text segments. Its _is_true_request_finish() check is deliberately request.is_finished() and not request.resumable: at an intermediate segment stop, is_finished() is true but resumable remains true, so talker2code2wav_async_chunk() preserves the partial codec chunk and the request-wide frame/emission counters for the next text segment.

When the client sends the terminal None sentinel, that same request must become non-resumable. Without this change, the terminal event is still classified as an intermediate segment stop, which has two observable consequences:

• A short final codec chunk is not emitted. The current deployment uses an 8-frame codec cadence at 25 fps, so as many as 7 final frames (about 280 ms of audio) can remain buffered, producing an audibly truncated utterance ending.
• The request-keyed _emp_seen_frames, _emp_emitted_frames, _emp_emitted_chunks, _emp_frame_buffer_base, and _emp_frame_buffer state is not cleaned up.

The distinction is covered in that PR by test_resumable_segment_stop_does_not_flush_partial_codec_chunk() and test_async_codec_state_stays_continuous_across_resumable_segments(). The latter currently flips request.resumable = False at terminal completion to verify that the persistent state is released; this vLLM fix makes the real scheduler provide that terminal lifecycle signal.

Incremental TTS is a vital part of low-latency dialog agents: synthesis needs to begin from partial LLM text instead of waiting for the full response, while preserving continuous audio across segment boundaries and still flushing and cleaning up correctly at the true end of the utterance.