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fix(net)!: age superseded groups by wall clock as well as media time - #5548
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WalkthroughTrack expiry now uses media-time distance and elapsed wall-clock time since a successor arrived. The newest group remains exempt, while untimed groups can expire by wall-clock age. JavaScript and Rust subscribers can wake at expiry deadlines. Rust cache collection now accepts a waiter and schedules those deadlines. The changes also update subscription-start behavior, tests, benchmarks, and related protocol and upgrade documentation. Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to
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The title clearly and concisely describes the main change: superseded groups now age by wall clock in addition to media time. |
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The description directly explains the shared Rust and JavaScript staleness rule, implementation changes, API impact, documentation updates, and tests. |
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Docstring coverage is 89.11% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 101 functions across 20 files. (1 skipped: … |
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| if !self.pool.is_driven() { | ||
| return; |
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When a track comes from the public standalone path (broadcast::Info::new().produce()) and the caller has never manually driven Pool::gc, its pool is not is_driven(), so this returns without registering any wall deadline. If a reader has drained group N and its successor remains open without a frame, read_frame().await has no other event to wake it and remains pending forever after max_delay elapses. The default standalone API therefore does not implement the newly documented wall-clock bound unless callers discover and run an otherwise optional cache-GC loop.
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Partly agree. This follows the quest's settled design: the deadline needs a wall-clock driver, which is the pool's collector. A standalone pool already relies on Pool::gc for its idle expiry, so it gets the wake the same way. The lazy wall check still applies at every read and hand-out. This is also not a regression: on main the same read (an open successor with no frame) never woke either, since media time had nothing to measure. b1b797c documents this on Subscription::max_delay. Firing deadlines with no collector would mean a timer inside the model, which the model avoids so that time always comes from the caller. I'll suggest that as a follow-up rather than add it here.
(Written by Claude Opus 5.5)
| hooks.arriveGroup = (group) => { | ||
| group.#state.arrived = performance.now(); |
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writeGroup accepts an existing GroupProducer, and the same producer can be inserted into multiple tracks, but this hook overwrites the arrival timestamp in the producer's shared state. If a group joins track A, is later inserted into track B, and a new subscriber then mirrors A's cache, that subscriber copies B's newer timestamp and incorrectly keeps A's predecessor fresh for longer. Arrival is a per-track event, so it needs to live on the cached track entry and be copied from that entry when creating a mirror.
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Agreed, fixed in b1b797c: arrival now lives on the track's cache entry and is copied into each mirror, with a regression test (a group shared by two tracks ages each from its own arrival).
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| pub fn gc(&self, now: crate::time::Instant) -> Option<crate::time::Instant> { | ||
| self.advance(now, true) | ||
| self.advance(now, true, None) |
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Pool::gc passes no driver waiter, so a caller that invokes it before a successor arrives and then waits until the returned deadline cannot learn that watch_wall subsequently scheduled an earlier deadline. With an unbounded standalone pool, the initial call returns None, and the parked read_frame can remain pending indefinitely despite its max_delay; with a bounded pool, it can remain pending until the later cache sweep. The private collect path avoids this by registering a waiter, but standalone callers cannot access it. The fresh evidence beyond the earlier undriven-pool report is that this fails even after the caller has started driving the pool according to gc's documented returned-deadline contract. (Written by GPT-5.6 Sol)
AGENTS.md reference: AGENTS.md:L30-L30
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Valid: gc has no way to learn of a deadline armed after it returned, which collect handles with a waiter. Fixing it changes public API, so I've escalated it to the maintainer instead of merging. The options:
Pool::gc(now, waiter), taking over the shapecollectalready has (andDriver::poll's). This is my recommendation.- Expose the pool as a
time::Driver, so a standalone track runstime::run(pool). - Keep
gcand document that wall wakes need an origin driver.
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Fixed in 363d8aa, per the maintainer: Pool::gc(now) is now gc(now, waiter), like time::Driver::poll. The waiter wakes when a parked read arms a wall deadline earlier than the one returned, so a standalone caller collects again in time. It replaces the crate-private collect the origin driver used, and doc/setup/upgrade.md notes the break.
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One P2 remains: a read parked before the first collection loses its wall deadline even after the pool becomes actively driven (inline).
Direction: combining successor arrival age with media age consistently across Rust/JS is sensible. Per-track JS arrival storage fixes the shared-group issue, and gc(now, waiter) addresses the independently reported earlier-deadline scheduling problem (#5548 (comment)). The API break and caller migration are documented. The remaining startup-order issue is distinct from both that fix and the accepted limitation of pools that are never driven.
Verification: static review of the current changes and relevant cache/expiry/driver paths, reusing the prior full inspection and excluding base imports. No builds, tests or benchmarks executed.
| /// Park `waiter` until the pool's wall clock reaches `at`. Nothing parks while no | ||
| /// driver collects the pool: nothing would release it. | ||
| pub(crate) fn watch_wall(self: &Arc<Self>, at: crate::time::Instant, waiter: &kio::Waiter) { | ||
| if !self.pool.is_driven() { |
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[P2] Wake existing reads when the pool first becomes driven
If a positive-budget held-group read first returns Pending before the pool's first gc, this return discards its wall-deadline registration. Calling gc(now, driver_waiter) afterward and collecting again at the deadline cannot wake it: the wall index never received that read. With an unbounded pool and quiet publisher it stays pending indefinitely despite active collection. This does not require timers for never-driven pools; preserve the pending registration or wake affected readers when collection first starts. origin::Driver::poll also polls state before GC. The parked-read test primes collection with elapse(Duration::ZERO) before reading; add the inverse ordering, with first collection after the initial Pending.
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Fixed in 690e784. A read that meets a wall deadline before the first collection now parks on a pool-wide waiter list, and the first gc wakes it so it arms the deadline. The list holds live waiters only, so a never-driven pool stays bounded. a_parked_read_wakes_at_its_wall_deadline now also runs with the first collection after the read parks, and wall_deadlines_wake_the_driver_and_then_the_read covers it at the pool level. Both fail without the wake.
(Written by Claude Opus 5.5)
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| const timer = setTimeout(() => { | ||
| this.#wallTimer = undefined; | ||
| this.#wallAt = Number.POSITIVE_INFINITY; | ||
| this.#wall.update((revision) => revision + 1); |
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When a held read briefly waits with a long maxDelay, this callback captures the entire Subscriber; if a frame arrives or the track closes before the deadline, nothing cancels the timer unless the caller explicitly invokes Subscriber.close(). A caller that naturally drains a track to completion and releases its handles can therefore retain the subscriber, producer, and cached track state until a potentially hours-long deadline. Tie the timer to active guarded reads or terminal state so it is cleared as soon as no read depends on it. (Written by GPT-5.6 Sol)
AGENTS.md reference: js/AGENTS.md:L17-L17
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Fixed in 690e784. The timer now closes over a small holder (the signal it bumps, its deadline and its handle) instead of the subscriber. Once no read waits, a pending timer retains only that, never the subscriber, producer or track state. close() still clears it. No regression test: observing collection needs --expose-gc and a FinalizationRegistry, which the suite does not use.
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Review comments at @drafts/draft-lcurley-moq-lite.md:
- Line 646: Clarify the Max Age expiry rules around “arrival age” so they do not
conflict: state whether the proof that every presentable frame exceeds the
budget applies specifically to timestamp age, while preserving the rule that
expiry may occur when the successor’s arrival age reaches the budget.
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Reviewed commit: 690e784, against 363d8aa.
The first-collection lost-wake finding (#5548 (comment)) is fixed: pre-activation reads register under the same mutex used to drain them, and the first collection wakes them outside the locks. The new primed/unprimed timed/untimed cases cover the missing ordering. The JS timer change also removes its direct Subscriber capture, addressing the independent retention finding (#5548 (comment)) without changing deadline behavior.
No new actionable bugs found in this delta. These are appropriately scoped fixes; the documented requirement to drive standalone pools remains.
Verification: static review of registration/activation races, waiter ownership, guarded-read cleanup and timer captures. No tests or builds executed; the Rust regressions are sequential, and JS garbage-collection behavior has no new automated regression.
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| fn successor(&self, sequence: u64, cap: Option<u64>) -> Option<&Slot> { | ||
| self.lookup | ||
| .range(from..) | ||
| .range(sequence.saturating_add(1)..) | ||
| .map(|(_, slot)| slot) | ||
| .take_while(|slot| super::subscription::before_end(slot.group.sequence, cap)) | ||
| .find(|slot| slot.is_shown()) |
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When a Rust receiver has parsed a group header but the stream stalls before its first frame, receive_group inserts that successor as pending and lite/subscriber.rs:949-952 does not reveal it until a frame starts. This lookup excludes the pending slot via is_shown(), so the predecessor has no successor, never arms a wall deadline, and can remain pending past max_delay. This contradicts the updated draft, which starts arrival age when the successor arrives at the hop, and differs from JS, which calls writeGroup as soon as the header is handled. Count pending wire groups for arrival age while continuing to exclude them from media reach until they present a frame. (Written by GPT-5.6 Sol)
AGENTS.md reference: rs/moq-net/AGENTS.md:L5-L5
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Agreed, fixed in 3117927. The wall clock now counts the first live successor, including a group a relay withholds until its first frame, from its header: reveal no longer re-stamps arrived, and a withheld group landing wakes the drift-parked reads it succeeds so they arm the deadline. Media reach still uses only shown successors. a_withheld_successor_ages_its_predecessor fails without it.
(Written by Claude Opus 5.5)
| fn schedule(&self, at: crate::time::Instant, track: &Arc<Track>) { | ||
| let mut wall = self.inner.wall.lock(); | ||
| let key = (at, Arc::as_ptr(track) as usize); | ||
| wall.due.insert(key, Arc::downgrade(track)); |
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In a long-lived shared pool, a transient track that arms a distant wall deadline and is then dropped leaves this Weak<Track> entry in wall.due until that deadline arrives; Track::drop only unregisters the sweep entry. Repeated subscriptions to short-lived tracks with large max_delay values can therefore grow the pool's BTreeMap indefinitely even though every track and waiter is gone. Remove a track's indexed deadlines on drop, or opportunistically purge dead weak entries before returning the next deadline. (Written by GPT-5.6 Sol)
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Agreed, fixed in 3117927. Arming an earlier deadline now replaces the track's previous index entry rather than adding one beside it, so each track holds at most its earliest, and Track::drop removes that entry. a_dropped_track_leaves_the_wall_index fails without it.
(Written by Claude Opus 5.5)
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On the remaining race the OpenAI review raised: GC re-indexing a track while a reader arms an earlier deadline can leave one extra entry, which release_due drops when it comes due, since its weak track no longer upgrades. It wakes nothing and lasts at most one budget, once per race, so it cannot accumulate. Serializing it would take the pool-wide index lock on every re-arm, which the index avoids so that parked reads across tracks do not contend. Leaving it as is.
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The withheld-successor finding (#5548 (comment)) is addressed: header arrival starts wall age and wakes predecessor reads, while media reach still uses shown successors. Keeping these separate is the right direction.
P2 remains in deadline-index cleanup (#5548 (comment)): cache.rs:378–380 re-arms outside the parked-state lock. With deadlines t1<t3, GC removes t1 and release_wall sets wall_armed=t3; before GC calls schedule(t3,None), a reader can arm t2 (t1<t2<t3), removing the not-yet-indexed t3 and inserting t2. GC then inserts t3 too. Track::drop at lines799–801 removes only t2, leaving the dead t3 entry until due. Serialize the parked/index transition or reject stale re-arms using a generation, and add a controlled GC/read interleaving regression. The sequential cleanup test misses this remaining case; no duplicate inline added.
Verification: static review of the update, successor selection, locks and tests. No tests, builds or benchmarks executed; no missed-wakeup claim is made for this index-retention race.
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| waiter: &kio::Waiter, | ||
| ) -> Option<Option<crate::time::Instant>> { | ||
| let mut parked = self.parked.lock(); | ||
| parked.wall.entry(at).or_default().register(waiter); |
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When a held group read arms a long deadline but then completes because another frame arrives or the group closes, dropping its kio::Waiter only makes the list's weak waker dead; this BTreeMap entry remains until release_wall reaches the deadline. On a long-lived track with a high group rate and an hour-scale max_delay, successive briefly parked reads therefore accumulate distinct dead keys for the entire budget window even though no read is waiting. Prune empty WaiterLists when registering or releasing deadlines, or cancel the key when the read settles. (Written by GPT-5.6 Sol)
AGENTS.md reference: rs/AGENTS.md:L19-L24
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Not changing this one. A key is a successor's arrival plus the budget, shared by every read of that successor, so a track holds at most one per group that arrived within the budget window, and release_wall drops each when it comes due. That is bounded by the groups the track caches over the same window (the budget never exceeds max_age), with an entry far smaller than a group. Pruning on settle would add a lock on every read completion to trim memory already proportional to the cache.
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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A non-newest group is now stale once either clock reaches the budget: the wall-clock time since its successor arrived at this hop, or its media-time reach behind the live edge. This applies to publisher max_age and subscriber max_delay alike, in Rust and js/net. Untimed tracks, frozen timestamps, and runs of empty groups no longer pin superseded groups, and an untimed resume no route continues is given up by wall clock. Reads parked on a superseded group wake at the wall deadline through the cache pool's deadline index (Rust) or a per-subscriber timer (JS). The untimed start special case is gone: an untimed subscriber replays what isn't stale, like a timed one. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A group written into two tracks kept one arrival on its shared state, so the later track restarted the earlier track's clock. Arrival now lives on the track's cache entry and is copied into each mirror. Also document that a read parked in a superseded group wakes at its wall deadline only when the track's cache pool is collected. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A standalone caller waiting until the deadline `gc` returned could not learn that a parked read armed an earlier wall deadline. `gc(now, waiter)` wakes the waiter then, replacing the crate-private `collect` the origin driver used. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A Rust read that waited on a wall deadline before its pool was first collected never entered the wall index, so later collection could not release it. It now parks on a pool-wide list that the first collection wakes to arm it. The JS wall timer closed over the whole subscriber, retaining it and its track state until a possibly hours-long deadline after its reads settled. It now holds only the signal and deadline it updates. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
…lines A relay withholds a route's group until its first frame, and re-stamped its arrival on reveal, so a successor whose first frame never came never aged the group before it, unlike at the origin and in JS. The wall clock now counts the first live successor from its header, withheld or not, and a withheld group's arrival wakes the reads it succeeds. A shared pool indexed a dropped track's wall deadline until it came due, and kept a later one alongside an earlier re-arm. Each track now indexes only its earliest deadline and removes it on drop. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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The remaining ~8 ns over release pays for wall-clock aging (#5548). An earliest-deadline atomic cannot prove a replacement driver is registered, and a new notification primitive is not worth ~8 ns. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Completes the
m1/cache-max-agequest: publishermax_ageand subscribermax_delaynow apply one staleness rule in Rust and js/net.Rule
A group other than the newest is stale once either clock reaches the budget:
For media time, the successor is the one
reachalready uses (the first shown group above, below the reader's cap). For the wall clock it is the first live group above, including one a relay still withholds until its first frame: its header arrived, so a relay ages the group before it as the origin does. Arrival is re-read, not latched. The newest group has no successor, so it is never stale. An untimed group, or one followed by a successor that never shows a frame, is judged by the wall clock alone.Changes
rs/moq-net)arrivedon insert, including a withheld wire group, whose header wakes the reads it succeeds.driftedchecks the wall term first.poll_driftedarms a wall deadline for the first live successor before judging media time, so re-selecting re-arms it.cache::Pool), fired byPool::gc(which the origin driver calls), which wakes the per-trackexpiry::Wakes. A standalone pool's parked reads wake only once something collects it; a read parked before the first collection is woken by it to arm its deadline. Each track indexes only its earliest deadline and removes it when dropped. Wall time ismax(ambient clock, driven time), where the ambient clock is the runtime's (tokio's, paused included), so it follows paused and simulated time.untimed_startis removed. An untimed subscriber replays what isn't stale, like a timed one.forget_past_budget(from fix(net): hand each group sequence out once per reader #5446) judges the group above a gap by both clocks, so a reader'sDeliveredrecord stays bounded on an untimed track with gaps, behind an empty successor, and below a run of frameless groups.js/net)arrivedwhen they join a track.#isStaleadds the wall term.changedlist. The timer holds only that signal and its deadline, never the subscriber.#untimedStart/#settleStartare removed.Info::max_age,Subscription::max_delay/start, the cache docs.Info.maxAge,Subscription.maxDelay/groups.doc/concept/moq-lite.md,doc/lib/js/net.md.an_endless_untimed_track_with_gaps_stays_bounded,an_empty_successor_still_lets_the_gaps_go, anda_gap_below_a_frameless_run_lets_the_floor_advance; each fails without the wall term inforget_past_budget.an_untimed_group_no_route_continues_is_given_up_by_wall_clockinresume.rs, on cold and warm routes; it fails without the deadline wake.rejoin_during_the_cancel_skips_the_cachechecks every version again.a_withheld_successor_ages_its_predecessor,a_parked_read_wakes_at_its_wall_deadlinewith the first collection after the read parks, anda_dropped_track_leaves_the_wall_index; each fails without its fix.moq-mux'sopen_gop_leading_pictures_above_the_previous_group_are_acceptedreads the first group before the second arrives, like its neighbor.replay().rs/moq-net/benches/track.rs):track_wall(idle / repoll / due), swept over 1 to 10k tracks and 1 to 1024 groups. fix(net): hand each group sequence out once per reader #5446'strack_empty_rungains aretiredmode (a zero budget, so the wall clock retires the whole run), and its media-time modes use an hour-scale budget so real-clock setup never ages a group out.Timer vs lazy (retention)
Recommendation: lazy, as implemented.
track_wall/idle(gc, nothing due)track_wall/repoll(re-arm a parked read)track_wall/duetrack_empty_runstale,fresh) and ~4.3-4.9 Melem/s (retired) over 64..4096 groups and 1..64 readersfanout_group/1,/512vs mainparallel_write/1vs mainThe clock is read only when a successor exists, so judging the newest group costs no clock read.
Public API and wire impact
cache::Pool::gc(now)is nowgc(now, waiter), liketime::Driver::poll. The waiter wakes when a parked read arms a wall deadline earlier than the onegcreturned, so a standalone caller re-collects in time. The crate-privatecollectit replaces is gone. Noted indoc/setup/upgrade.md.max_age/maxAgeandmax_delay/maxDelaynow count wall-clock time since the successor arrived.max_delay, a superseded untimed group is skipped as soon as its successor arrives, as a timed one already was. Before, untimed groups were never stale.draft-lcurley-moq-litechange (arrival age added to Expiration).Follow-ups
position_cursorreceiveslatest = None, so an untimed subscription there now replays what isn't stale where the removed untimed-start special case used to hide it.an_unfed_fetch_is_a_misswas adjusted to skip a sequence instead.(Written by Claude Opus 5.5)
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