fix(transcode): follow a source resolution change with the ladder

[kixelated]

Summary

Root cause. moq_transcode::run called resolve_rungs exactly once, at startup, and said so in a comment: the rung set was fixed for the life of the transcode, and only the passthrough entries tracked the source. So a source whose picture changed size afterwards kept a ladder sized for the picture it used to carry.

That is not a rare case. moq_video::encode::publish_capture opens its source twice by design: once at startup to probe the mode and advertise an exact rendition, and again when the first subscriber arrives. Camera modes are stable across the two opens; macOS window capture is not, since screencapture.rs derives geometry from the window on every open. The catalog recovers on its own once frames flow, because the importer republishes the SPS dimensions. The ladder did not. A reconnecting publisher and a renegotiated screen share hit the same thing later in the stream.

The two opens stay. Holding the source open from the probe to the first subscriber would close the capture case only, and it would do it by keeping an idle camera open, which is exactly what the demand gate exists to release. Letting the ladder follow the source closes every case, so that is the fix here, and both opens are untouched.

What changed

• moq-transcode: every source catalog snapshot re-resolves the ladder and diffs it against the live one. Rungs the new picture has no room for retire: their tracks finish cleanly at the next group boundary, and their catalog entries go away. Rungs it makes room for are probed and added. The rest carry on untouched, feed and encoder session included, while a codec description change rebuilds the decoder because those bytes may contain out-of-band parameter sets.
• A track name is handed out once and never again. A rung whose own picture moved (an aspect-ratio change keeps every rung height while moving every rung width) retires and its replacement is published as video/360p.2, not as video/360p again. A clean track end is terminal, and a relay keeps the finished logical track: broadcast::Consumer::track_inner drops only an aborted spliced entry, so a name the transcoder finished would serve its EOF to every later subscriber and never reach the transcoder again. Reusing it would have turned a mis-sized picture into stopped playback for anyone behind a relay. The same holds when the shared decode is rebuilt (a new source track, or new out-of-band parameter sets on the same one): every rung retires whatever it resolves to, so none of their names carries forward even when the picture never moved.
• A retiring rung stops taking new fetches but serves every one the consumer had already asked for, then drains them all below the track's final sequence. A group request queues on the track's dynamic handler the moment a consumer fetches, before anything pops it, so retirement covers what is sitting in that queue as well as what this loop had reached. Otherwise dropping the handler cancels a fetch the consumer is already waiting on, and an accepted group stalls. The cutoff is one pass with no await between pops, and then the handler is dropped, which is what closes admission: track::Dynamic admits a cache miss for as long as it is alive, so a drain that waited for a concurrency slot with it still open could be fed by a consumer holding the retired track and the rung would never finish. Dropping it releases whatever arrived after the pass; the requests already popped are held in hand and unaffected.
• The track is finished by serve, after both halves return, rather than by the live path on its way out. finish takes the live edge as the boundary, which on a rung that only ever served fetches is sequence 0, and a group at or above the boundary is refused. Finishing while a fetch was still opening its decoder rejected the very group retirement had kept the loop alive to serve. The boundary is only knowable once nothing can add a group.
• The ladder, the shared decode behind it, and the rung tasks serving off it now live in one ladder::Ladder rather than as six locals threaded through run's select!.
• moq play followed the catalog only until video and audio had both started, so a retired rung ended playback outright or left audio playing alone. It now follows the catalog for as long as the catalog lasts, and a track that ends re-arms selection for its own half; playback ends once the catalog has ended and every track it started has too.
• The retirement snapshot arrives before the track it retires ends: the ladder republishes as soon as it resolves, while the rung rides out the group it is mid-way through. So play holds onto the newest snapshot and reads it when a half needs one, rather than only when it arrives. Each half records which snapshot it read, so a track that ends with nothing newer on offer stays stopped instead of resubscribing to the rendition it just finished, and doing it again the moment that ended too. Playback is over only once no half has a snapshot left to read either: the catalog's last word can be the replacement for the rendition it retires, and the retired track outlives the catalog by the group it was mid-way through.

Notes

• Resizing a rung in place, keeping its track and changing its geometry mid-stream, would avoid the rebuffer entirely but not the name problem: the H.264/H.265 codec string carries the level, which is derived from the picture, so a resize usually changes the mimetype and with it the catalog entry a player opened its decoder from.
• The chosen source rendition is sticky by name while it is still on offer. Re-running choose_source from scratch on every snapshot would hand the ladder to a taller rendition, retiring every rung serving, the moment a publisher briefly advertised one.
• A mid-stream re-resolve that fails to probe (a picture this machine cannot encode at) logs and keeps the working ladder rather than ending the broadcast. Nothing is committed before the probe succeeds, so the next snapshot retries.
• A retired rung finishes at the next group boundary rather than mid-group, so the last thing a subscriber gets is a complete group and then a clean track end.

Public API changes

None. catalog::{Names, Published, Resolved}, ladder::Ladder, rung::Retire, and active::Producer::declare are all pub(crate); declare now takes an IntoIterator because it is called again on every re-resolve. active::Rendition::{size,bitrate,framerate} are unchanged and still fixed for the life of a rendition, since a rung re-resolved under a new picture is a new name and so a new handle.

Test plan

• nix develop --command env -u RUSTC_WRAPPER just fix
• nix develop --command env -u RUSTC_WRAPPER just check
• nix develop --command env -u RUSTC_WRAPPER just test
• nix develop --command env -u RUSTC_WRAPPER cargo nextest run -p moq-cli --no-default-features --features iroh,quinn,websocket,play (47 pass). This is the only thing that runs the play.rs tests at all. moq play sits behind the non-default play feature, so neither just test nor CI compiles this file, and nightly's just rs features only compiles --all-features rather than running it. Each play.rs test here was checked against a reverted implementation to confirm it discriminates. Closing that coverage gap is bigger than this PR and wants a quest of its own.
• moq-transcode: ladder_follows_a_source_resize republishes the source rendition at a new size and asserts the rung set changes, the passthrough entry follows, the subscriber on a retired rung sees a clean track end, and the subscriber on a rung that still fits is left alone.
• moq-transcode: a_resized_rung_takes_a_fresh_name changes the source aspect ratio so a rung keeps its height and loses its width, and asserts the replacement is published as video/120p.2 while video/120p ends cleanly and is gone from the catalog. Against an implementation that reuses the name, a subscriber behind a relay would hold the finished track forever.
• moq-transcode: a_rebuilt_decode_renames_every_rung republishes the source with new out-of-band parameter sets at an unchanged picture, so the rungs still resolve to the same size, and asserts the replacement is video/120p.2. It fails against a carry-forward that matches on shape alone.
• moq-transcode: names_are_never_reused pins the minting rule directly.
• moq-transcode: a_new_description_is_a_new_decode_stream covers decoder replacement when out-of-band codec configuration changes.
• moq-transcode: retirement_finishes_an_in_flight_fetch fetches a group while its source group is open, retires the rung, and requires the output group to finish after the source does. It timed out against an implementation that drops the fetch task, and failed with Cancel on CI against one that finished the track at sequence 0 underneath it. Consumer::fetch_group resolves as soon as the attempt is registered, well before GroupRequest::accept creates the group, so the retirement really does land while the fetch is still opening its decoder; which side of accept it lands on is the scheduler's call, so the test catches that case some of the time rather than every time, and says so. Two attempts at forcing the ordering are recorded on the review thread; neither discriminated when the bug was reintroduced, and this machine never loses the race the CI runner lost. What is not probabilistic is the fix: producer.finish() now sits after the join, so it is unreachable while a fetch is in flight by construction. It resolves the track info first, which waits for Request::accept, so a resize cannot retire the rung before the request is served at all: that is correct behavior, just not what the test is about.
• moq-cli: a_finished_track_re_arms_its_half, a_read_snapshot_is_not_read_twice, playback_ends_with_the_catalog, and a_final_snapshot_outlives_the_catalog cover the play change. The last sequences a retirement snapshot, then the catalog ending, then the retired track ending, and fails against a done that ignores an unread snapshot. The first sequences the retirement snapshot before the track EOF, which is the real order, and fails against an implementation that leaves video_started set, stops following the catalog once both halves are running, or discards a snapshot that arrived while the doomed track was still playing. The second pins the other side of that: a half must not re-read a snapshot it already read, or a track ending with nothing newer on offer resubscribes to itself in a loop.
• cargo package --locked -p moq-transcode --allow-dirty --no-verify reaches a pre-existing origin/main manifest blocker: published moq-video 0.0.22 lacks the unreleased v4l2 feature requested by the unchanged moq-transcode manifest. Version repair remains release work.

Merged from main

main moved under this branch while it was open. Three files conflicted:

• rs/moq-video/src/frame.rs: main had already dropped the same dead-code expectation on DmaBufExport::inner that this branch carried, so its version is taken and the change leaves this PR entirely.
• doc/bin/cli.md: main condensed the whole page in the docs reorg, which deleted the long play and transcode prose this branch had edited. The two behavior changes are rewritten as one sentence each against the new sections.
• quest/m0/README.md: main added a neighboring quest line; both edits keep.

Cross-package sync

• rs/moq-cli -> doc/bin/cli.md: done. The play section documented the old behavior verbatim ("keeps following catalog updates until both have started ... It doesn't switch renditions afterwards"), and the transcode section now says the ladder follows the source and names the replacement rung.
• js/hang / doc/concept: skipped, no catalog format or wire change. The transcoder republishes its own derivative catalog with a different rung set, which is an ordinary catalog update. On the browser side js/watch/src/video/source.ts already recomputes the selection from every snapshot with no stickiness toward the playing track, so a viewer on a retired rung reselects with no change needed.

Quest

Completes and deletes quest/m0/2799-moq-video-capture-negotiates-twice-so-a-window-resize.md, drops its entry from quest/m0/README.md, and removes the ## Related link to it in quest/m0/capture-window-lifecycle.md.

Closes #2799

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Walkthrough

Playback now follows catalog updates, replaces completed renditions, preserves catalog lifetime, and maintains audio timing across gaps and rewinds. Transcoding dynamically re-resolves ladders when source dimensions or codec descriptions change, retires incompatible rungs, and republishes replacements with revisioned names. Tests cover playback state, audio timing, ladder resizing, rung retirement, fetch completion, and decode-stream changes. Documentation reflects the updated behavior.

Merge Risk: 🟡 Moderate · up to ff00d

Rendition replacement can mishandle restarted timestamps, large audio gaps can add avoidable latency, and retired transcode tracks can continue accepting requests rather than completing. These media lifecycle regressions should be fixed before merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 90.77% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 65 functions across 7 files. (1 skipped: 1 …
Linked Issues check	✅ Passed	The PR addresses issue #2799 by making the transcode ladder follow source resolution changes while preserving the two-open capture behavior.
Out of Scope Changes check	✅ Passed	The documentation, quest cleanup, playback changes, and transcoder refactor support the stated objective and do not introduce unrelated scope.
Title check	✅ Passed	The title clearly and concisely summarizes the primary change: the transcode ladder now follows source resolution changes.
Description check	✅ Passed	The description is detailed and directly related to the ladder re-resolution, rung retirement, playback updates, tests, and issue #2799.

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In `@rs/moq-cli/src/play.rs`:
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rs/moq-cli/src/play.rs (1)

659-660: 🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Do not write silence into a sink that just skipped the hole.

When hole > fill_max, push returns both reset_sink = true and silence = fill_max. play_audio then drops the sink, opens a new one, and writes fill_max of silence into it before the frame. The comment at Lines 522-527 states that past the cap the sink skips the hole and the clock re-anchors. Writing the capped silence into the fresh sink adds latency_max of buffered silence on the exact path chosen to avoid that delay.

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🐛 Proposed fix

-		let silence = hole.min(fill_max);
 		let reset_sink = rewound || hole > fill_max;
+		// A skipped hole is skipped: the replacement sink starts at this frame
+		// rather than behind a cap's worth of silence.
+		let silence = if reset_sink { 0 } else { hole };

Update the expectation at Line 1321 accordingly:

 		let skipped = timeline.push(Duration::from_secs(1), 960, 48_000, 4_800);
 		assert!(skipped.reset_sink);
-		assert_eq!(skipped.silence, 4_800);
+		assert_eq!(skipped.silence, 0);

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rs/moq-transcode/src/rung.rs (1)

206-206: 🎯 Functional Correctness | 🟠 Major | ⚡ Quick win

Prioritize retirement before opening a new source group.

When listener.recv() and retire.fired() are both ready, Tokio 1.53.1 can select either branch because this tokio::select! is not biased. If it selects listener.recv(), retiring remains false, so the live path can call producer.create_group after retirement. The next iteration then drains that obsolete group.

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In `@rs/moq-transcode/src/rung.rs` at line 206, Update the tokio::select! loop in
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- Line 122: Update the duration accounting around Guard and frame_duration so
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In `@rs/moq-transcode/src/rung.rs`:
- Line 206: Update the tokio::select! loop in the listener/retirement handling
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In `@rs/moq-transcode/src/rung.rs`:
- Around line 391-399: Update the Dynamic request-admission and retirement flow
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reject all subsequent requests including earlier groups, and drain only requests
accepted before the boundary so late or repeated requests cannot be spawned
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[kixelated]

@coderabbitai review

The last CodeRabbit pass covered d3093d4. Since then: rung names are no longer reused when the shared decode is rebuilt, the output track is finished by serve after both halves return rather than by the live path, and retirement freezes fetch intake in one pass. A fresh look at rs/moq-transcode/src/{ladder,rung}.rs would be welcome.

(written by Claude Opus 5)

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🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
set -euo pipefail

printf '%s\n' '=== PR branch and recent commits ==='
git status --short
git log --oneline --decorate -12

printf '%s\n' '=== Changed files after d3093d4 ==='
git diff --stat d3093d4..HEAD
git diff --name-status d3093d4..HEAD

printf '%s\n' '=== Target file outlines ==='
ast-grep outline rs/moq-transcode/src/ladder.rs --items all --view expanded
ast-grep outline rs/moq-transcode/src/rung.rs --items all --view expanded

printf '%s\n' '=== Post-d3093d4 target diff ==='
git diff --find-renames --find-copies --unified=80 d3093d4..HEAD -- \
  rs/moq-transcode/src/ladder.rs rs/moq-transcode/src/rung.rs

printf '%s\n' '=== Current ladder implementation ==='
cat -n rs/moq-transcode/src/ladder.rs

printf '%s\n' '=== Current rung implementation ==='
cat -n rs/moq-transcode/src/rung.rs

printf '%s\n' '=== Relevant tests and call sites ==='
rg -n -C 5 --glob '*.rs' \
  '(?:Ladder::|\.follow\(|\.retire\(|\bserve\(|\bfetch\(|spawn_fetch|queued|retir)' \
  rs/moq-transcode

Length of output: 159276

---

🏁 Script executed:

#!/bin/bash
set -euo pipefail

printf '%s\n' '=== Dynamic group-request implementation ==='
rg -n -C 12 --glob '*.rs' \
  '(?:struct Dynamic|impl Dynamic|fn requested_group|async fn requested_group|GroupRequest)' \
  rs/moq-net

printf '%s\n' '=== Track producer finish semantics ==='
rg -n -C 12 --glob '*.rs' \
  '(?:fn finish\(|pub fn finish\(|async fn finish\(|final sequence|final_sequence)' \
  rs/moq-net/src/model/track.rs rs/moq-net/src/model

printf '%s\n' '=== Targeted transcode test range ==='
sed -n '760,1120p' rs/moq-transcode/src/lib.rs

printf '%s\n' '=== Each post-review transcode commit ==='
for commit in 8951b307 ff00ddf6; do
  echo "=== $commit ==="
  git show --format=fuller --find-renames --find-copies --unified=50 "$commit" -- \
    rs/moq-transcode/src/ladder.rs rs/moq-transcode/src/rung.rs rs/moq-transcode/src/lib.rs
done

Length of output: 186638

---

@kixelated I found one blocking issue in the retirement drain.

Blocking — the “one pass” drain can still accept fetches after retirement.
In rs/moq-transcode/src/rung.rs Line 418, dynamic.requested_group() is before the immediately ready branch in a biased tokio::select!. requested_group() returns ready whenever the shared queue has an item. A client can add another fetch between iterations, so the request branch wins again. Under a continuous fetch stream, the loop can keep accepting post-retirement requests and never reach the spawn-and-join phase.

The moq_net::track::Dynamic implementation confirms that each requested_group() poll pops any currently queued request. The immediately ready branch only stops the loop when the queue is empty at the poll point. This does not create a fixed retirement-time snapshot.

Use an atomic queue-drain API, or otherwise close/gate dynamic fetch admission before draining the queue. Add a test that continues to enqueue distinct cache-miss groups after retirement and verifies that the rung still reaches its clean track end.

---

🧠 Learnings used

Learnt from: kixelated
Repo: moq-dev/moq PR: 3381
File: rs/moq-cli/src/play.rs:240-240
Timestamp: 2026-09-04T19:53:42.950Z
Learning: In the MoQ Hang broadcast model, timestamps belong to the broadcast timeline rather than an individual rendition track. Replacement transcode rungs and re-published importer renditions must preserve that timeline when they replace a retired rendition. A rendition that restarts timestamps at zero during an active broadcast is a timestamp-model violation that affects all players, including `js/watch`; it is not a `rs/moq-cli/src/play.rs` rendition-swap concern.

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Actionable comments posted: 1

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Inline comments:
In `@rs/moq-transcode/src/rung.rs`:
- Around line 418-419: Update the retirement flow around Dynamic and
requested_group() to close cache-miss admission before draining retirement
requests. Establish the retirement boundary first, then drain only requests
admitted before it so post-retirement misses cannot keep the drain alive or
delay completion.

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📒 Files selected for processing (5)

• doc/bin/cli.md
• quest/m0/README.md
• rs/moq-transcode/src/ladder.rs
• rs/moq-transcode/src/lib.rs
• rs/moq-transcode/src/rung.rs

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Reviewed commit: bb1cf0ed6d

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[kixelated]

@coderabbitai review

Two commits since your last pass on ff00ddf:

• bb1cf0e implements your admission finding: fetches owns the track::Dynamic and drops it after the one-pass cutoff, so admission is closed rather than merely bounded.
• 7f1d503 fixes an early exit in moq play that Codex found: a half that had not read the final catalog snapshot could be treated as done.

(written by Claude Opus 5)

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🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
set -euo pipefail

printf '%s\n' '=== PR state and target commits ==='
git status --short
git log --oneline --decorate -16
git show --stat --oneline bb1cf0e 7f1d503

printf '%s\n' '=== Target commit diffs ==='
git show --find-renames --find-copies --unified=100 bb1cf0e -- \
  rs/moq-transcode/src/rung.rs rs/moq-transcode/src/lib.rs rs/moq-transcode/src/ladder.rs
git show --find-renames --find-copies --unified=100 7f1d503 -- \
  rs/moq-cli/src/play.rs

printf '%s\n' '=== Current target outlines ==='
ast-grep outline rs/moq-transcode/src/rung.rs --items all --view expanded
ast-grep outline rs/moq-cli/src/play.rs --items all --view expanded

printf '%s\n' '=== Current focused implementations ==='
rg -n -C 16 --glob '*.rs' \
  '(?:async fn fetch|fn fetch|spawn_fetch|requested_group|Dynamic|async fn serve|fn serve|retir|finish\()' \
  rs/moq-transcode/src/rung.rs rs/moq-transcode/src/lib.rs rs/moq-transcode/src/ladder.rs
rg -n -C 16 --glob '*.rs' \
  '(?:async fn play|fn play|catalog|snapshot|select|JoinSet|FuturesUnordered|video|audio|ended|finished)' \
  rs/moq-cli/src/play.rs

printf '%s\n' '=== Tests changed and adjacent test context ==='
rg -n -C 12 --glob '*.rs' \
  '(?:retir|fetch|finish|catalog|snapshot|replacement|ended|play)' \
  rs/moq-transcode/src/lib.rs rs/moq-cli/src/play.rs

printf '%s\n' '=== Dynamic implementation and call sites ==='
rg -n -C 14 --glob '*.rs' \
  '(?:struct Dynamic|impl Dynamic|fn requested_group|async fn requested_group|GroupRequest|impl Drop for Dynamic)' \
  rs/moq-net

Length of output: 164877

---

🏁 Script executed:

#!/bin/bash
set -euo pipefail

printf '%s\n' '=== Exact commits ==='
git show --format=fuller --find-renames --find-copies --unified=35 bb1cf0e -- rs/moq-transcode/src/rung.rs rs/moq-transcode/src/lib.rs
git show --format=fuller --find-renames --find-copies --unified=35 7f1d503 -- rs/moq-cli/src/play.rs

printf '%s\n' '=== Rung retirement and fetch implementation ==='
rg -n -C 35 --glob '*.rs' \
  '^(pub\(crate\) )?async fn (serve|fetch)|^(async )?fn (serve|fetch)|struct Fetch|fetches|requested_group|retire|retired' \
  rs/moq-transcode/src/rung.rs

printf '%s\n' '=== Rung direct line ranges ==='
sed -n '1,520p' rs/moq-transcode/src/rung.rs

printf '%s\n' '=== Playback state machine and changed tests ==='
sed -n '220,485p' rs/moq-cli/src/play.rs
sed -n '1200,1300p' rs/moq-cli/src/play.rs

printf '%s\n' '=== Dynamic construction and ownership ==='
rg -n -C 8 --glob '*.rs' \
  '(?:\.dynamic\(\)|track::Dynamic|Dynamic)' \
  rs/moq-transcode/src

printf '%s\n' '=== Commit integrity ==='
git diff --check ff00ddf..HEAD
git diff --name-status ff00ddf..HEAD

Length of output: 83925

---

@kixelated I found one blocking issue.

Blocking — the new playback test cannot pass.
In rs/moq-cli/src/play.rs, a_final_snapshot_outlives_the_catalog reads the final snapshot for Kind::Video only. Kind::Audio is not playing, but its read value remains None. Therefore Playback::wants(Kind::Audio) remains true, Playback::pending() remains Some(...), and Playback::done() remains false. The assertion after playback.read(Kind::Video) will fail.

Read the final snapshot for Kind::Audio before the final assert!(playback.done()). Alternatively, initialize both halves as having read the first snapshot and then read both halves for the final snapshot.

The bb1cf0e change resolves the previous fetch-admission finding. fetches owns the only track::Dynamic, drains the current queue, and drops the handler before it waits for permits. This closes later cache-miss admission atomically with the handler state.

---

🧠 Learnings used

Learnt from: kixelated
Repo: moq-dev/moq PR: 3381
File: rs/moq-transcode/src/rung.rs:425-426
Timestamp: 2026-09-05T07:24:09.210Z
Learning: In `rs/moq-transcode/src/rung.rs`, `moq_net::track::Dynamic` admits cache-miss group requests for as long as its handler exists. During rung retirement, `fetches` must drain the currently queued requests and then drop `Dynamic` before waiting for concurrency permits or joining tasks. This prevents post-retirement requests from extending retirement indefinitely while allowing already-popped requests to finish.

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[kixelated]

Confirmed, and thank you: the test really could not pass, and the reason it looked green is worth recording.

moq play and everything in rs/moq-cli/src/play.rs sit behind the non-default play cargo feature. Neither just test nor CI compiles it, so none of this PR's play.rs tests had ever run: cargo nextest run -p moq-cli a_final_snapshot_outlives_the_catalog reported 0 tests run, 35 skipped. Running it properly (--no-default-features --features iroh,quinn,websocket,play) reproduced your analysis exactly: audio never read the final snapshot, so wants(Audio) stayed true, pending() stayed Some, and the closing done() was false.

Fixed in fbfa1e9 by reading the snapshot for both halves, which is what the selection pass does. Verified against the feature: 47 tests pass, and with done() reverted to ignoring an unread snapshot the test fails with exactly that assertion, so it discriminates.

Also confirming your read of bb1cf0e. That is the intent: drain the current queue, drop the handler to close admission, and only then wait on permits.

(written by Claude Opus 5)

[kixelated]

@codex review

Head is now fbfa1e9. Since your clean pass on 7f1d503 the only change is a test fix: a_final_snapshot_outlives_the_catalog now reads the final snapshot for both halves, the way the selection pass does. CodeRabbit caught that it could not pass as written, and the reason it looked green is that play.rs sits behind the non-default play feature, so neither just test nor CI ever compiled it.

(written by Claude Opus 5)

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Codex Review: Didn't find any major issues. 👍

Reviewed commit: fbfa1e9db2

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