fix(cli): play a retired audio rendition's tail alongside its replacement

[kixelated]

Problem

moq play runs one task per kind, so Media::play could not start a replacement audio rendition until the retired play_audio returned. That task ended by draining its sink (the full --delay), and the replacement then opened a fresh sink that primes its own --delay of silence before the first sample sounds. The two buffers ran in series, so every audio rendition switch cost one --delay of silence: 100 ms at the default, up to 10 s at the ceiling.

Reproduced locally with an in-process harness (two Opus renditions, the retired one finished mid-stream, real speaker), measuring when the retired tail stops sounding versus when the replacement's first sample sounds:

--delay	gap before	gap after
1000 ms	1045 ms	17 ms
100 ms	122 ms	9 ms

The remaining few ms is the time to subscribe to the replacement.

Approach

• play_audio hands its sink back when the track ends instead of draining it.
• Media::play plays each retired sink out on a background tails set while the replacement fills, and waits for every tail before returning, so a finite track still plays to its end.
• One playback::Engine is shared by the playing audio rendition and any tails, so the sinks mix on one device stream. Two engines would open a second stream, which fails on an exclusive device. It is released once no audio task or tail is left, so an idle play does not hold the device.
• The speaker keeps the playout anchor while a tail sounds: a retired rendition marks restarted() so its replacement's first frame re-pins, and video takes the anchor back (stopped()) only once the last audio goes quiet. stopped() now also clears a pending restart (regression test a_stopped_speaker_drops_a_pending_restart).
• playback::joined is generic over what a task leaves behind.
• Completes and deletes quest/m1/play-audio-rendition-gap.md.

Impact

• No public API or wire changes. Internal to the moq binary's play feature.

Alternatives

• Handing the retired sink to the replacement so its ring is already at depth. That keeps one sink, but only works when the sample rate and Layout match, and an overlap in media time would play twice. Overlapping two sinks works for any pair of renditions, and it keeps Playback's one-task-per-kind state machine as it is, because a tail is not a playing half.

Open questions

• Regression test. No CI test lands for the gap itself. media.rs is behind play and needs a speaker and an EventLoopProxy, and CI only compiles it (nightly --all-features clippy), so the task structure that caused the gap cannot be exercised there. The reproduction harness above needs a display and an audio device, so it is not committed. I went with the local measurement. The alternative is a sink trait plus a wake callback in play_audio, which adds seams to production code for a test. That is proposed as a follow-up below.
• Sink reset. A mid-track sink reset (a hole too large to fill, or a publisher rewind) still drops the old sink, so its buffered audio is cut and the new sink primes a fresh delay. The tails could take that sink too. I left it out to keep this PR on rendition switches.

Follow-ups

• Deviceless play harness: a fake sink and wake handle so media.rs's task structure (rendition switches, drains, tune-in) can be tested in CI.
• Hand a reset sink to the tails instead of dropping it, so a skip does not cut the buffered audio.

(written by Opus 5.5)

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Walkthrough

Audio playback now shares one engine across renditions. When an audio task ends, its sink is drained while replacement renditions can play. The engine is released after no audio rendition is playing and no tails remain. Playback task completion now returns the task’s value with its kind. The CLI documentation describes the handoff, and the related quest documentation is removed.

Priority: ⬇️ Low

Severity of issue fixed: Low

Merge Risk: 🟡 Moderate · up to f496b

Video can get ahead of audio during a rendition switch. Keep the speaker clock aligned with the audible tail before merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 2 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: playing a retired audio rendition's tail alongside its replacement.
Description check	✅ Passed	The description directly explains the audio gap problem, the implementation approach, measured impact, limitations, and follow-ups.

Full details: Docstring Coverage

Explanation

Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 2 files. (1 skipped: 1 unsupported.)

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-24T02:43:19.376594Z	1617092	New commits

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Inline comments:
In `@rs/moq-cli/src/play/media.rs`:
- Around line 102-105: Update the audio sink retirement flow around
`Kind::Audio` and `self.presentation.lock().unwrap().stopped()` so the speaker
remains the playout anchor while buffered samples drain. Mark it stopped only
after the tail finishes or replacement audio takes over, preventing video from
advancing the clock ahead of still-audible audio.

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📒 Files selected for processing (5)

• doc/bin/cli.md
• quest/m1/README.md
• quest/m1/play-audio-rendition-gap.md
• rs/moq-cli/src/play/media.rs
• rs/moq-cli/src/play/playback.rs

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• quest/m1/play-audio-rendition-gap.md
• quest/m1/README.md

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[kixelated]

Merged as ffa5b81 (squash, pinned to 1617092). Changes made while landing:

• Merged main; the args.rs play.delay conflict took main's version (test(cli): reproduce wide-delay play tune-in stall #3946 landed the same fix), so this PR no longer touches args.rs.
• Release the shared audio Engine once no audio task or tail holds it, so an idle play does not keep the device open (Codex).
• Keep the speaker's playout anchor until the last tail drains: a retired rendition marks restarted() and stopped() runs only when all audio is quiet. stopped() now also clears a pending restart, with regression test a_stopped_speaker_drops_a_pending_restart (Codex, CodeRabbit).

Checked locally with cargo clippy -p moq-cli --all-targets --features play -- -D warnings and cargo test -p moq-cli --features play. Note the timeline.rs tests sit behind play, which CI only compiles, so they are not in the merge gate.

(Written by Claude)

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