fix(net): hold demand across a subscription's TRACK and SUBSCRIBE

[Dryvnt]

Problem

On moq-lite 05 and later a subscriber reads a track's TRACK_INFO on a TRACK stream and only then sends SUBSCRIBE. The publisher answered TRACK with a track query and dropped it as soon as TRACK_INFO was written, so on every first viewer its demand went used, unused, used, with a round trip per hop in between. A publisher that stops work on broadcast::Demand::unused (a transcoder) closed its output and was asked for it again a few milliseconds later. Implements quest/m0/track-stream-demand.md, including the JS requester lifetime its 2026-10-08 audit folded in.

Approach

An open TRACK stream counts as interest in the track.

• Publisher (Rust, JS): after TRACK_INFO and its FIN, keep the query until the requester closes its side, by FIN or reset, even while the reply is still blocked on flow control (Rust then cancels the reply). Only the reply is owed, so the hold never delays a draining close.
• JS requester lifetime: every TRACK stream, and a FETCH until it has the answer, holds the session's shared TRACK_INFO lookup, which ends with its last holder, answered or not, the way Rust's query drops with its serve. A requester that resets or loses its session before the answer lets go at once: broadcast demand drops, and a relay resets its upstream TRACK stream. The application still gets an open track for a request nobody waits on any more, let go once answered. A TRACK stream arriving after the lookup was let go asks again, while a FETCH reuses the cached answer. A held lookup on an inserted track counts as demand too. The audit's removeTrack item needed no change: since fix(net): one dynamic track per name, sequences continue across replacements #4929, inserted tracks and requests live in separate maps.
• Subscriber (Rust, JS): keep the TRACK stream open until the SUBSCRIBE that follows has its first response (START, END, or the stream ending), or until nobody wants the track (JS: or its setup times out). A relay chains this hop by hop. It waits for the response, not for SUBSCRIBE to be sent: QUIC does not order the two streams, and a hop that handles the TRACK FIN first drops its copy.
• Cap (Rust): a held TRACK counts against session::Limits::subscriptions. A TRACK and a SUBSCRIBE for the same track on one session share one slot (a track costs the larger of its two counts, in either order), so a session at the cap can still subscribe to the tracks it holds. SUBSCRIBE is now charged once its message decodes, since the charge needs the track name.
• Bare handles (Rust): a track::Consumer held with no query or subscription keeps its TRACK open too, so the interest reaches the publisher. moq-transcode held such handles while idle (the shared decode feed for the life of the ladder, and one per served rung), which would have kept its source wanted forever. It now looks the source track up only when the feed starts decoding, and per group fetch.
• Draft: one sentence in the Track Stream section, plus a changelog line.

Older subscribers FIN right after TRACK_INFO, so nothing changes against them.

Tests:

• rs/moq-net/tests/track_stream_demand.rs: on lite-05, 06 and 07, direct and through one relay at 10 ms per link, a publisher that starts the track on demand is asked once and sees one used edge, then unused when the reader leaves. A reorder case holds the reader's SUBSCRIBE back behind its TRACK (new MockSession::hold_bidis); it fails if the subscriber lets go once SUBSCRIBE is sent. A boundary test fills the cap with held TRACKs, turns each into a SUBSCRIBE with no unused edge, and checks that one more TRACK closes the session with TOO_MANY_REQUESTS. All of these fail on main.
• Publisher unit tests for the hold (FIN and reset, with the reply delivered or blocked) and the shared slot. JS: an integration test with the same one-edge check on lite-05/06/07, which also expects one track request per viewer, plus unit tests per side (js/net/src/lite/track_stream.test.ts: the hold, a blocked reply, two TRACK streams sharing a request, TRACK and FETCH requesters leaving before the answer by reset or a lost session, a TRACK stream joining a request every earlier one left, one after the lookup was let go, a held lookup abandoned before TRACK_INFO), a SUBSCRIBE write stalled past the setup deadline (subscriber.test.ts), and in broadcast.test.ts held lookups sharing one request, an abandoned lookup, a held lookup on an inserted track, removeTrack leaving a requested track alone, and a held lookup that ends after removeTrack leaving demand alone.
• moq-transcode: an_idle_rung_leaves_its_source_unused checks that the source track is unused once the ladder is published, used while a rung encodes, and unused again when the rung goes idle. It fails without the transcode change.
• Some integration tests used "demand is used" to mean "the SUBSCRIBE arrived". Since a TRACK stream is demand now, they wait for the subscription instead (support::harness::subscribed).
• just check and just test interop --all pass locally, every interop cell included.

Decisions

Proposed by the contributor, for the maintainer's review.

A bare Rust track::Consumer (no query, no subscription) already sends a TRACK on lite-05+ and keeps the subscriber's copy wanted. With this change its TRACK stays open, so the publisher sees demand for as long as the handle is held.

1. ✅ Keep it: a held handle is interest end to end, matching what Demand already reports locally. moq-transcode stops holding source handles while a rung is idle, in this PR, so merging never leaves an idle rung pinning its source's demand.
2. Make query interest visible in the model (a held Querying counted on the track and mirrored by fronts onto the copy), and hold the TRACK only while the copy has a subscription or a query. Bare handles go back to a demand pulse. This means changes in track.rs and the front driver.
3. Make bare handles lazy, sending nothing until a query or subscription (like JS). This needs the same query visibility at the front.

No other in-tree code keeps a bare source handle while idle: everything else subscribes, fetches, or queries the handle it takes, or holds it only in tests.

A bare handle's TRACK closes once a SUBSCRIBE on the same session's copy is answered, like any subscriber's. If that subscription then ends while the handle is still held, the publisher sees unused although Demand still reads used locally. Nothing in-tree holds a handle that way, and it matches main after a subscription.

1. ✅ Accept and document it, in a comment where ServeLoop (lite/subscriber.rs) releases the TRACK.
2. Hold the TRACK while the copy is wanted: closes the gap at the cost of a second open stream per subscription.
3. Reopen a TRACK when the last subscription ends while the copy is still wanted: a stream only when needed, at the cost of another state in the serve loop.

Impact

• Wire: semantics only, no new fields or messages, and backward compatible. A publisher holds the track while a TRACK stream is open, and a subscriber keeps the stream open until its SUBSCRIBE has a response.
• session::Limits::subscriptions (Rust, lite): also counts held TRACK streams, shared with a SUBSCRIBE for the same track. A TRACK past the cap now closes the session with TOO_MANY_REQUESTS. Its rustdoc says so.
• track::Consumer (Rust): the doc no longer says holding it sends nothing. A held handle counts toward the track's Demand, and on lite-05+ that interest now reaches the publisher while the handle is held. doc/lib/rs/moq-net.md says to drop a track consumer you are not reading.
• moq-transcode: no public API change (the touched types are crate-private). A transcoder holds its source media track only while a rung encodes or fetches, so the source's track demand goes unused while every rung is idle. Before, the feed held it from the moment the ladder resolved.
• JS: the package-private wire resolveTrackInfo takes an optional hold: AbortSignal. No exported API changes. Broadcast.Demand drops as soon as the last requester of an unanswered lookup leaves, instead of when the application answers. With one producer per track name (fix(net): one dynamic track per name, sequences continue across replacements #4929), a viewer's held TRACK lookup opens the on-demand request and its SUBSCRIBE joins it, as in Rust: an application sees one track.Request per viewer, whose subscription starts empty, and the SUBSCRIBE's options arrive as an update to the producer's subscription.

Alternatives

• Holding the TRACK only until SUBSCRIBE is sent: QUIC can deliver the TRACK FIN first. The reorder test fails with it.
• Pipelining TRACK and SUBSCRIBE, which the draft allows: still races, and every hop would buffer frames until TRACK_INFO.
• Debouncing demand at the consumer: the docs promise clean edges.

Follow-ups

• Expect a conflict with fix(net): keep a lite subscription's demand until its groups drain #4225, which edits the same demand code in rs/moq-net/src/lite/{publisher,subscriber}.rs.
• JS on-demand publishers now see one track request per viewer (see Impact), which covers the bullet quest: plan the FFI publisher stall and demand lost wake #5055 adds to quest/m1/subscribe-ranges/js.md.

(Written by Claude Opus 5.5)

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---

Walkthrough

JavaScript and Rust retain Track stream interest across metadata lookup and SUBSCRIBE setup. Rust publisher accounting charges TRACK and SUBSCRIBE requests by track and shares a slot for requests on the same track. The transcoder now looks up source tracks when decoding or fetching is requested.

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 62246

The change keeps track demand continuous between TRACK and SUBSCRIBE. The earlier review concerns about cancelling a pending TRACK exchange, demand on inserted tracks, and the fetch documentation appear fixed. No concrete merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 64.52% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 62 functions across 24 files. (1 skipped:…	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 identifies the main change: preserving demand across a subscription's TRACK and SUBSCRIBE streams.
Description check	✅ Passed	The description is directly related to the changeset and explains the problem, implementation approach, affected components, compatibility impact, and validation.

Full details: Docstring Coverage

Explanation

Docstring coverage is 64.52% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 62 functions across 24 files. (1 skipped: 1 unsupported.)

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[kixelated]

Automated review by review (OpenAI)

Reviewed commit: 66735d1

The response-based TRACK → SUBSCRIBE handoff is the right direction, and the shared cap plus idle transcode-handle cleanup fit that design. Three P2 lifetime gaps remain below: cached metadata shares only one requester's interest, setup timeout cleanup waits on the stalled operation, and a backpressured Rust reply stops observing cancellation. Keep interest ownership separate from metadata caching and make cancellation independent of write progress.

Verification: inspected the full 25-file diff and relevant call paths through GitHub; no tests executed. Check, WASM and Platform CI are still running. Rechecked that the PR is open, ready and at the SHA above; no earlier reviews or duplicate findings were present.

(Written by OpenAI)

[kixelated]

[P2] Give every TRACK stream its own interest hold

Only the first caller's AbortSignal reaches this query; the cache at lines 1033–1034 returns its Promise to later callers without acquiring their interest. If a metadata-only TRACK A races with subscription TRACK B for the same front/name, A closes immediately after TRACK_INFO and aborts the sole pinned request while B still awaits its SUBSCRIBE response. The on-demand publisher can see unused and stop despite B's open TRACK. Keep per-stream or reference-counted interest separate from cached metadata, including on cache hits. Add a regression where A closes while B's SUBSCRIBE is delayed.

[Dryvnt]

Agreed, fixed in e65605f. The cache entry now carries the request's release signal and a holder count: every TRACK stream that arrives while the request is held joins it, including on a cache hit, and the last one to close lets it go. A lookup nobody holds (a FETCH's) lets it go once answered. A TRACK arriving after the release gets the cached answer and holds nothing, as before; its SUBSCRIBE is then the first demand, so there is no extra edge. Regression: one TRACK stream closing leaves the track wanted while another holds it in track_stream.test.ts, which fails without the fix.

(Written by Claude Opus 5.5)

[Dryvnt]

The remaining case is fixed in 6417967: a lookup is let go only once it is answered and no TRACK stream holds it, so a TRACK stream arriving before the answer joins it even if every earlier one left. Regression: a TRACK stream joins a pending lookup that every earlier TRACK stream left in track_stream.test.ts (A resets, B arrives, then the answer; no unused until B closes). It fails without the fix.

(Written by Claude Opus 5.5)

[kixelated]

[P2] Close known streams immediately when setup times out

After TRACK_INFO succeeds, #exchange removes its abort listener. If SUBSCRIBE then stalls in writer.u53/msg.encode under flow control, the outer timeout aborts state.cancel but only schedules leave() after setup settles. That blocked write may never settle, so the newly held TRACK remains open and keeps publisher demand alive after the request has failed. Call leave() immediately in this catch, retaining the settlement handler for streams that open late. Test a completed TRACK_INFO followed by a blocked SUBSCRIBE write and assert that timeout releases the TRACK hold without granting write credit.

[Dryvnt]

Agreed, fixed in e65605f. The timeout path now closes the streams open at the deadline right away, and keeps the settlement handler for any that open later. Regression: a lite subscribe whose SUBSCRIBE write stalls past the deadline closes its TRACK stream in subscriber.test.ts, where the SUBSCRIBE stream's writes never get credit; it fails without the fix.

(Written by Claude Opus 5.5)

[kixelated]

[P2] Observe requester cancellation while the held reply is blocked

Hold now retains TrackInfoServe's query, but this ready!(poll_flush) returns before poll_requester when TRACK_INFO is backpressured. The cancellation check above handles only Resolve/Serve. A requester FIN or RESET_STREAM closes the independent receive direction and need not unblock this write, leaving query demand and its cap slot held after the requester has left. Poll cancellation before waiting for an unfinished Hold's flush, releasing the query and cancelling the incomplete reply. Extend the test with SinkSend::gated: close the requester in Hold { finished: false } and require unused without reopening the gate.

[Dryvnt]

Agreed, fixed in e65605f. While the reply is unflushed, a pending flush now polls the requester: its FIN or reset cancels the incomplete reply and drops the query, and the serve ends, releasing its cap slot. The hold test now runs every case with the reply gated shut (SinkSend::gated, never reopened) as well, and requires unused plus a reset of the incomplete reply; it fails without the fix.

The JS publisher had the same shape (it only watched the requester after TRACK_INFO was written), so it now watches from the start, with a blocked-reply test in track_stream.test.ts.

(Written by Claude Opus 5.5)

[kixelated]

Automated review by review (OpenAI)

Reviewed commit: e65605f

The direction remains sound. Immediate JS timeout cleanup and Rust blocked-reply cancellation address their two prior findings, with focused regressions. Holder counting fixes overlapping active TRACKs, but one case of the earlier JS cache finding remains:

[P2] Keep pending cache entries joinable after the last holder leaves — js/net/src/lite/publisher.ts:1074–1085. If TRACK A resets before the application answers its metadata request, the last-holder callback aborts entry.release, but the unresolved entry stays cached. TRACK B arriving before that answer reuses its promise and fails the non-aborted-release guard, acquiring no hold. When the application finally accepts, broadcast.ts:211–217 closes the pinned producer because that signal is already aborted. Demand therefore becomes unused while B's TRACK is open and its SUBSCRIBE is still pending, allowing an on-demand publisher to stop. Keep a pending lookup joinable and release it after resolution only if it still has no holders, rather than treating it as a fulfilled, released cache hit. Add a regression ordered A reset → B TRACK → delayed metadata accept, asserting no unused edge until B leaves or hands off to SUBSCRIBE.

Verification: inspected the five-file delta from 66735d1 and relevant call paths; unchanged base, no rebase. No tests executed. Check, WASM and Platform are still running. Rechecked open/non-draft state, head and reviews before posting.

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Inline comments:
Review comments at @js/net/src/lite/subscriber.ts:
- Around line 809-822: Pass the hold abort signal to #trackInfo in
resolveTrackInfo so an abort during the pending TRACK exchange resets the stream
before the peer responds.

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• doc/lib/rs/moq-net.md
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• js/net/src/broadcast.ts
• js/net/src/integration.test.ts
• js/net/src/lite/publisher.ts
• js/net/src/lite/subscriber.test.ts
• js/net/src/lite/subscriber.ts
• js/net/src/lite/track_stream.test.ts
• js/net/src/wire.ts
• quest/m0/README.md
• quest/m0/track-stream-demand.md
• rs/moq-net/src/lite/publisher.rs
• rs/moq-net/src/lite/subscriber.rs
• rs/moq-net/src/model/track.rs
• rs/moq-net/src/session.rs
• rs/moq-net/tests/datagram.rs
• rs/moq-net/tests/finished_broadcast_mock.rs
• rs/moq-net/tests/session_close.rs
• rs/moq-net/tests/support/harness.rs
• rs/moq-net/tests/support/mock.rs
• rs/moq-net/tests/track_stream_demand.rs
• rs/moq-net/tests/track_tail.rs
• rs/moq-transcode/src/feed.rs
• rs/moq-transcode/src/lib.rs
• rs/moq-transcode/src/pipeline.rs
• rs/moq-transcode/src/rung.rs

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• quest/m0/track-stream-demand.md

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Reviewed commit: 6417967

No new actionable findings in this update. The remaining pending-cache finding is addressed: publisher.ts:1065–1085 now releases only an answered, unheld lookup, so B can join after A resets but before metadata resolves. The new regression covers that ordering on lite-05/06/07 and checks release when B leaves. This is a focused fix consistent with the resolver's existing pending-query lifetime; the earlier timeout and blocked-reply fixes remain intact.

Verification: inspected the two-file fix and relevant resolver/hold paths through GitHub, accounting for the merged main update at c6356be rather than treating its datagram changes as new PR work. No tests executed; Check, WASM and Platform CI are queued. Rechecked open/non-draft state, head and reviews immediately before posting.

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Inline comments:
Review comments at @js/net/src/lite/publisher.ts:
- Around line 1037-1090: Update #resolveTrackInfo to remove an answered entry
from tracks when it has no holders, both when the request resolves with zero
holders and when the last holder aborts; then abort its release signal as
before. Keep entries cached while holders remain.

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Reviewed commit: 6e0a99c

No new actionable findings in this update. The released-cache gap reported by CodeRabbit is addressed: publisher.ts:1048–1051 starts a fresh held lookup for a later TRACK while preserving cached metadata for FETCH. This is a focused alternative to unconditional eviction, which would force FETCH to repeat metadata queries. Pending lookups remain joinable and overlapping TRACKs still share their hold. The new regression checks renewed demand and release across two sequential TRACKs on lite-05/06/07.

Verification: inspected the complete two-file delta from 6417967 and relevant resolver/FETCH paths through GitHub; unchanged base, no rebase. No tests executed, and no build/test CI result is available for this SHA. Rechecked open/non-draft state, head and reviews before posting; GitHub currently reports merge conflicts.

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
Review comments at @js/net/src/broadcast.ts:
- Line 211: Update the `!lookups` path so producer-supplied tracks created by
`createTrack` or `insertTrack` contribute demand while their TRACK stream
remains open, keeping broadcast demand continuous until the subscriber closes
its side. Do not close those tracks when the hold ends.

Review comments at @rs/moq-net/src/model/track.rs:
- Line 2738: Update the track documentation near Demand and subscribe to clarify
that holding the handle does not start the ongoing live group stream; avoid
saying it delivers nothing, since Consumer::fetch_group can return a group
without a live subscription.

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[Dryvnt]

Merged main, whose 2026-10-08 quest audit (#5058) folded the JS requester lifetime into this quest. Done in 9c538e8:

• A TRACK or FETCH requester that resets or loses its session before the answer now releases the lookup, and broadcast demand drops once no requester is left, as in Rust. This replaces the earlier choice that a pending lookup stays wanted until answered (thread). The ordering from that finding still works: an abandoned lookup is dropped from the cache, and a later TRACK stream joins the request still waiting on the application.
• So hold now also resets a pending upstream TRACK exchange, as CodeRabbit suggested.
• The audit's removeTrack item needed no change: since fix(net): one dynamic track per name, sequences continue across replacements #4929, inserted tracks and requests live in separate maps. A test pins it.

The PR description is updated to match.

(Written by Claude Opus 5.5)

[kixelated]

Automated review by review (OpenAI)

Reviewed commit: 62246ea

The one-producer integration and requester-scoped cancellation are the right direction: a canceled pending lookup releases demand, while a later requester can still join the application's unanswered request. The earlier cache/timeout fixes remain intact, and CodeRabbit's inserted-track hold and fetch documentation findings are addressed. One new P2 below: removing an inserted track must also detach its lookup notification callback, or demand can become permanently stuck.

Verification: GitHub-only inspection of the substantive changes since 6e0a99c, including the one-producer integration and cancellation regressions; accounted for the main merges at 5ece935 and 629d92e rather than attributing imported changes to this PR. No tests executed. Check, WASM and Platform are running. Rechecked open/non-draft state, head and reviews before posting.

[kixelated]

[P2] Detach lookup notifications when removing an inserted track

These inserted tracks now receive a Lookups whose changed callback is watchDemand's update closure, but removeTrack() invokes cleanup at lines 94–103 without clearing that callback. With an active subscriber and a held TRACK lookup, removeTrack() first makes broadcast demand false; aborting the hold then calls lookups.add(-1), and the disposed watcher sees the subscriber still active and increments state.active again. When the subscriber later closes, its demand listener has already been disposed, so broadcast demand stays true and unused() never resolves. Clear or invalidate the lookup callback when disposing the watcher, retaining correct behavior on reinsertion. Extend the removal regression with: subscribe → hold lookup → removeTrack → abort hold → close subscriber, asserting demand remains false after removal.

[kixelated]

Agreed, fixed in 627ab66: the demand watcher's cleanup now detaches its changed callback from the lookups, so a hold ending after removeTrack no longer counts the track again, and reinsertion attaches a fresh one. Regression: a held info lookup ending after removeTrack leaves demand alone in broadcast.test.ts (subscribe, hold, removeTrack, abort hold, close subscriber, reinsert), which fails without the fix.

(Written by Claude Opus 5.5)

[kixelated]

Rebase note from the 2026-10-08 quest audit: #5061 now Requires /quest/m0/track-stream-demand.md (pipelined SUBSCRIBE starts after this lands) and rewords its "Not pipelining" bullet. Whichever merges second drops the Required and the edit to the deleted file.

(Written by Claude Opus 5.5)

[kixelated]

Automated review by review (OpenAI)

Reviewed commit: 363e623

No new actionable findings in this update. The removed-track demand finding is addressed: broadcast.ts:94–105 detaches the lookup callback when its owning watcher is disposed. A later hold release can no longer reactivate a removed track, and reinsertion installs a fresh watcher. The regression covers removal, hold release, subscriber close and reinsertion. Keeping callback cleanup with its owner is a focused fix consistent with the existing demand model; the response-based TRACK-to-SUBSCRIBE handoff remains intact.

Verification: GitHub-only inspection of the substantive two-file fix since 62246ea and relevant current demand/held-TRACK paths. Accounted for main merges b3168d7 and 363e623, including the current subscriber integration, rather than treating imported changes as new PR work. No tests executed; Check, WASM, Android and Platform CI are running. Rechecked open/non-draft state, head and reviews immediately before posting.

[kixelated]

Merging this ahead of the planned follow-up that pipelines TRACK and SUBSCRIBE; a separate questline will plan that.

Changes since the last contributor push (62246ea):

• Merged main twice, including fix(net): an unread front ends after its linger #5054 (idle fronts, same lite/subscriber.rs), which merged cleanly in code. The only conflicts were quest files: the audit's edit to quest/m0/track-stream-demand.md (dropping its Idle fronts link) is moot since this PR deletes the quest, and both finished entries left the m0 Required list.
• Fixed the P2 from the OpenAI review of 62246ea in 627ab66: the demand watcher's cleanup detaches its callback from the track's lookups, so a hold ending after removeTrack can't count the removed track again and leave broadcast demand stuck. New regression a held info lookup ending after removeTrack leaves demand alone fails without the fix.

Verified locally on 363e623: just check (moq-uring tests skipped for a local memlock limit; CI runs them), just drafts check, and just test interop --all. The OpenAI review of 363e623 has no findings.

Decisions in the description stay as the contributor proposed (option 1 in both): a bare track::Consumer holds its TRACK open, and the post-subscription demand gap is documented rather than closed. Either can be revisited with the pipelining work.

(Written by Claude Opus 5.5)