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fix(net): track tail hardening - #4533
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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Cancel an IETF END_OF_TRACK stuck on stream credit with the request, count it once opened, abort a group whose first object is cut short, and hold the tail open while any stream is still being read. A group at or past the declared end aborts the track. The lite owed floor follows the demand, and tail gaps older than the grace fold away. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A group at or past the declared end aborts the track, the lite owed floor follows the demand with the groups below SUBSCRIBE_START accounted, and tail gaps older than the grace fold away. Finishes the track tail hardening quest. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A SUBSCRIBE_END below a received group, or a group at or past the declared end, aborts the track on lite-06 and later in both languages. lite-05 specified an inclusive end, and @moq/net 0.1.3 to 0.1.9 sent one, so there it only warns or drops that group. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Included review availability: This review used your included allowance. Your plan provides up to 4 included reviews per hour; 0 remain after this review. WalkthroughRust and JavaScript tail tracking now records timestamped gaps and active stream reads during settlement. IETF and moq-lite subscribers update group accounting and enforce track-end rules, with moq-lite behavior varying by draft version. IETF publisher handling of Priority: ⬇️ Low Merge Risk: 🟡 Moderate · up to IETF subscriptions can finish prematurely while a fill remains outstanding. Resolve the grace-expiry and fill-handoff interaction before merging. Security Architecture ReviewSecurity architecture risk: 🟡 Moderate · up to The changes improve stream-completion accounting, but the Rust completion wait appears able to retain an abandoned subscription when a publisher stalls an already-arrived stream. The exposure is within established subscriptions; no privilege escalation or cross-tenant access was established. Retained concerns
Security review detailsSecurity Blast Radius
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Full details: Docstring CoverageExplanation Docstring coverage is 77.78% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 63 functions across 12 files. (1 skipped: 1 unsupported.)
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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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| runs.push(run); | ||
| } else if (end < run.start) { | ||
| // Splitting a gap leaves both halves as late as it was. | ||
| if (!placed) runs.push(merged ?? { start, end, since: run.since }); |
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When a long-lived subscription lowers its floor after the existing run has aged beyond the grace, copying run.since makes newly requested holes look already expired. For example, after group 3's run is old, lowering the floor to 1 and receiving group 1 immediately folds missing group 2 into the surrounding runs, so a following FIN sees covers(1, 4) as true and closes before group 2 can arrive. Start aging that gap when demand expands, and make the equivalent correction in rs/moq-net/src/tail.rs; otherwise the new lowered-floor behavior can lose late groups.
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Agreed, fixed in 03bd421. Tail::demand / Tail.demand restarts the age of every gap a lowered floor reaches into, called from the lite update path in both languages. Unit tests with an injected clock cover the scenario.
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| await respond({ end: new SubscribeEnd(4, 1) }); | ||
| await fin(); | ||
| // Long enough for a subscription that owed nothing more to have ended. | ||
| await new Promise((resolve) => setTimeout(resolve, 50)); |
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This 50 ms sleep is the synchronization that determines whether the regression catches the old early-close behavior. On a loaded CI runner, a broken subscription may not finish teardown before the sleep returns, allowing groups 1 and 2 to arrive first and making the test pass spuriously. Advance mocked time and drain the relevant tasks deterministically instead of depending on scheduler timing.
AGENTS.md reference: AGENTS.md:L19-L19
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Declining. The mock transport is in-process with no timers, so the broken teardown is a microtask chain: every queued microtask drains before any timer fires, and a loaded runner only delays the timer, never the chain. The 50 ms is an event-loop drain, not a race. The age logic itself is now covered with an injected clock in tail.test.ts.
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A gap the demand newly asks for was aging from the run above it, so a floor lowered on a long-lived subscription could fold the new gap away at once and end the subscription before its groups arrived. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
…ardening # Conflicts: # quest/m1/README.md
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| reading: &mut Reading, | ||
| ) -> Result<Option<group::Producer>, Error> { | ||
| kio::wait(|waiter| { | ||
| reading.park(); |
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When an IETF fill completes, recv_fill installs Fill::Ready and then drops its Reading guard, while this parked subgroup does not call resume() until its separately scheduled task wakes. In that gap active can be zero with the full Stream Count received, so Settle may finish the track and remove the subscription before the subgroup claims the fill, losing the tail of the joined group. Transfer active ownership without a zero-count window and model-check this handoff interleaving.
AGENTS.md reference: rs/moq-net/AGENTS.md:L15-L15
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Agreed, the window was real. Fixed in the next push:
- Settle treats an outstanding fill as owed (
poll_settled), so a head that isReadybut unclaimed holds the end open. Nothing else does, since its tail is parked. A head that is never claimed falls back to the grace. claim_fillresumes itsReadingbefore it takes the head, so the fill turningDonenever lands while nothing is active.
A loom model can't reach this path: tests/loom.rs drives only the public model API, and Fill, Tail and claim_fill are private. an_unclaimed_fill_holds_the_end_open pins the interleaving deterministically instead. It parks the tail, finishes the fill, and asserts that Settle is still pending. It fails without the fix.
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| pub fn set_grace(&mut self, grace: Duration) { | ||
| self.grace = grace; |
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When a lite subscription increases max_age, set_grace only replaces the duration even though expire may already have permanently merged older gaps into accounted runs. For example, a gap can be folded while the grace is 100 ms, then an update can raise it to 1 s; if the publisher's FIN arrives before that delayed stream header, covers is already true and the subscription closes immediately instead of honoring the new grace, dropping the late group. The TypeScript tail has the same irreversible merge while reading grace dynamically, so increasing the grace must preserve or reconstruct such gaps in both implementations.
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Declining, as a deliberate trade-off. A gap folds only after it has outlived the grace in force at the time. At that point the tail has given up on its group, the same way the subscription's max age gives up on a stale group. Folding is what bounds the tail's memory. That was the quest's third open question, and keeping the exact gaps would undo that bound. The only case this leaves open is a group missing for longer than the grace that later becomes owed again or gets a longer grace, and the tail won't wait for it at teardown. The Tail docs in both languages now say a fold is final.
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A tail parked on its fill stops holding the end open, and the fill's own stream let go once it installed the head, so the subscription could settle and end before the tail woke to claim it. An outstanding fill is now owed, and the tail resumes before it takes the head. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Review comments at @rs/moq-net/src/ietf/subscriber.rs:
- Line 2955: Update Settle::poll and its completion predicate in run_subscribe
so a ready head with a parked tail blocks settlement even after grace expires,
while the timeout still applies when streams are missing. Extend the regression
test to cover grace expiring before the fill completes.
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rs/moq-net/src/ietf/subscriber.rs
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| fn poll_settled(settle: &mut Settle, waiter: &kio::Waiter, fill: &kio::Producer<Fill>, count: u64) -> Poll<()> { | ||
| // Read before the tail: Done is terminal, so the answer cannot go stale. | ||
| let filled = !fill.read().outstanding(); | ||
| settle.poll(waiter, |tail| filled && count > 0 && tail.streams() >= count) |
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🗄️ Data Integrity & Integration | 🟠 Major | ⚡ Quick win
Preserve the fill-to-tail handoff after grace expires.
Settle::poll in rs/moq-net/src/tail.rs (Lines 227–235) ignores this completion predicate after grace expires. If the fill takes longer than grace, its completion leaves Fill::Ready with no active reads while the tail remains parked. Settlement can then win before the tail resumes. run_subscribe finishes the track, and the remaining tail objects are discarded.
Make a ready head with a parked tail block settlement even after grace expires. Preserve the timeout for missing streams. Extend the regression test to cover grace expiring before the fill completes.
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Review comment at @rs/moq-net/src/ietf/subscriber.rs at line 2955:
Update Settle::poll and its completion predicate in run_subscribe so a ready
head with a parked tail blocks settlement even after grace expires, while the
timeout still applies when streams are missing. Extend the regression test to
cover grace expiring before the fill completes.
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| this.#runs = this.#runs.map((run) => { | ||
| const reached = below < end && start < run.start; | ||
| below = run.end; | ||
| return reached ? { ...run, since: now } : run; |
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When a floor is raised and later lowered again, this only retimestamps gaps that still exist between runs. If groups 1 and 3 previously arrived, missing group 2 aged past the grace, and #expire folded the runs, demand(1, 3) sees one continuous run and cannot recreate the hole. A following FIN therefore makes covers(1, end) true and closes the subscription before the newly requested group 2 can arrive. Preserve enough gap information or explicitly reopen the newly demanded span; the Rust Tail::demand has the same issue.
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Declining, as a deliberate trade-off. A gap folds only after it has outlived the grace in force at the time. At that point the tail has given up on its group, the same way the subscription's max age gives up on a stale group. Folding is what bounds the tail's memory. That was the quest's third open question, and keeping the exact gaps would undo that bound. The only case this leaves open is a group missing for longer than the grace that later becomes owed again or gets a longer grace, and the tail won't wait for it at teardown. The Tail docs in both languages now say a fold is final.
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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Merge summary. Changes during takeover:
Decisions:
Follow-ups are already planned as quests in this PR: (Written by Claude Opus 5.5) |
…ardening # Conflicts: # quest/m1/README.md
Closes the Codex findings left over from the track tail PRs (#4086 JS, #4116 Rust) and settles the quest's three open questions in both languages.
Bugs
ietf_leaving_cancels_a_blocked_end_of_track(mock gainswithhold_unis).openedonce its stream opens, like every group stream. JS already did this.an_unclaimed_fill_holds_the_end_open.Tailnow tracks active streams through aReadingguard, opened at the header on both lite and IETF, andSettlenever returns while one is being read. This matchesTail.settlein JS. An IETF stream parked inclaim_fillon another stream stops holding the end open, so a lost fill cannot wedge the subscription. Regression:the_grace_waits_for_a_stream_being_read.Math.max. JS dropped groups that an update newly asked for. Rust used SUBSCRIBE_START alone, so it also waited out the grace for groups that a raised floor no longer asked for. Regressions: JSa lowered floor owes the groups it newly asked for(fails onmain) anda floor below SUBSCRIBE_START owes nothing below it, plus Rustthe_owed_floor_follows_the_demand.Open questions
a_lost_datagram_never_delays_the_endlocks in the Rust side. The reader-visible difference is a model difference, listed below.ProtocolViolationin both languages, on IETF and on lite from lite-06 (see below). The quest said Rust already did this. A probe showed it didn't: Rust returnedClosedfor that stream and ended the track cleanly, the same as JS. Rust now mapscreate_group'sClosedto a track abort. Local finishes can't hit this, because the tail waits for every open reader before the subscriber finishes the track itself. JS checks the end the peer declared (SUBSCRIBE_END, or END_OF_TRACK recorded on the subscription). Tests in all four places.demand).accountfolds expired gaps, and so does settle on entry. The grace comes from the tail itself: lite keeps it in step with the subscription's max age (in Rust throughset_graceon create, update, and FIN; in JS through a getter), and IETF uses the fixed one second.Settlereads its grace from the tail, so the two can't disagree.Public API and wire impact
tailispub(crate)in Rust and@internalin JS. The JSTailconstructor now takes{ grace, now }, andsettledropped itsgraceargument.doc/concept/moq-lite.mdsubscription completion.lite-05 inclusive end
Two decisions are gated by version, following the maintainer's call. From lite-06 on, both languages abort the track with
ProtocolViolationwhen a SUBSCRIBE_END lands below a group already received, and when a group lands at or past the declared end. On lite-05 both only warn, or drop that one group, and the track still finishes clean. The reason is that lite-05 specified an inclusive end, and@moq/net0.1.3 to 0.1.9 sent one: #2333 fixed the JS publisher after those releases shipped.a_subscribe_end_below_a_received_group,a_group_past_the_declared_end_aborts_the_track) and in the JS lite tail tests.doc/concept/moq-lite.mdnow states the rule.Follow-ups
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