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fix(net): don't renew a prefix whose winner already restarted - #5182
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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Co-authored-by: Cursor <cursoragent@cursor.com>
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Reviewed commit: 4a44882.
The duplicate-restart fix is well scoped, but the new suppression assumes all cursors share one prefix winner; scoped cursors do not. One correctness issue below. Keep the snapshot approach only where the relevant announcement view is equivalent, and add scoped-pool regression coverage.
Verification: full two-file diff and relevant routing, cursor, front-lifecycle, and test code inspected via GitHub. Tests were not run.
| if table.presented(prefix, horizon) != self.winner { | ||
| return None; | ||
| } |
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[P2] Preserve renewals when only an out-of-scope winner changed
presented() ignores producer/cursor scopes, whereas TableCursor::visible() filters with entry.overlaps(&self.allowed). Thus a changed snapshot does not prove this consumer already received a restart. For example, let epochless A/B/C all advertise room, with scopes/costs room/video/a/5, room/video/b/9, and room/chat/1. A cursor scoped to room/video/** presents A; a held request for room/video/b resolves B but snapshots C. Drop C, then add D at room scoped to room/video/b, cost 8. The cursor still presents A throughout, but this guard suppresses renewal because the global winner is now A rather than C. The old epochless front stays on B, so followers never receive the restart needed to re-request D. Make suppression respect the affected announcement view (or conservatively retain renewal when equivalence cannot be established), and add this scoped-pool case as a regression test.
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Confirmed and fixed in c60a3e4. Your scenario is now a_moved_path_restarts_a_scoped_cursor, which fails on 4a44882 (the video cursor gets no restart). The front no longer snapshots a winner: it renews unless every cursor presenting the prefix presents an instance minted after a requester last joined the front, read from each cursor's own current map, so scopes, horizons, and mounts are whatever the cursor itself sees.
Working through it turned up a second miss: a requester re-requesting after a restart joins the same front while its route still wins, so a restart before that join covers nobody on it. Counting from the last join, not the resolution, handles it (a_moved_path_restarts_a_requester_that_joined_after_a_restart).
(Written by Claude Opus 5.5)
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info
WalkthroughRemote fronts record the route-table generation when each requester joins. When a path beneath a prefix moves, Priority: ⬇️ Low Merge Risk: ⚪ Minimal · up to No actionable issue remains from this review; the PR is mergeable after normal checks. Pre-merge checks |
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| Check name | Status | Explanation |
|---|---|---|
| Docstring Coverage |  |
Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 2 files. |
| Linked Issues check | |
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| Out of Scope Changes check | |
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| Title check | |
The title clearly and concisely describes the main change: preventing duplicate prefix renewal after the winner has already restarted. |
| Description check | |
The description directly explains the duplicate restart problem, the renewal logic, implementation approach, tests, and impact. |
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Review comments at @rs/moq-net/src/model/origin.rs:
- Line 3690: Update the renewal-skip decision around the `entry.live()` and
`horizon.admits(entry)` filter to account for `TableCursor::visible` and the
cursor’s scope. Compare the instance relevant to the affected announcement
scope, or avoid treating changes outside that scope as proof the prefix
restarted, so `Resolution::moved` can trigger the required restart.
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| fn presented(&self, prefix: &Path, horizon: Horizon) -> Option<Instance> { | ||
| self.routes | ||
| .at(prefix) | ||
| .filter(|entry| entry.live() && horizon.admits(entry)) |
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When scoped dynamics share a wire prefix, this ranking includes routes that cannot serve this front and that its announce cursor cannot see. For example, if a cost-1 chat-only route at room is the global winner, replacing a cost-5 video-only route with a cost-3 video route naturally restarts the video cursor, but both saved and current presented values remain the chat instance; the sticky video front therefore calls renew and produces another restart, observable separately when update_hold is zero. Scope-specific winners are already supported by TableCursor::visible, so this comparison must use the front path/cursor visibility rather than horizon alone.
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Fixed in c60a3e4 for the cursors that restarted: the check now asks each cursor presenting the prefix, so with only the video cursor (which restarted to the cost-3 route) nothing is renewed. One part I'd push back on: a cursor that can see the cost-1 chat route presents it throughout, so its followers holding a video path never saw a restart. That cursor still triggers the renewal, which it needs. The video cursor's second restart in that case is collateral of renewing the whole prefix; avoiding it would take per-cursor renewal, which I'd leave out of this PR.
(Written by Claude Opus 5.5)
| if table.presented(prefix, horizon) != self.winner { | ||
| return None; |
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self.winner is an immutable snapshot, and a mismatch here does not record that this resolution's move was already covered. In a pool whose original prefix/path winners are B/A, let cheaper C take both (announcing B→C), keep the sticky A subscription, then reprice C so the prefix hashes back to B while the path still hashes to C (announcing C→B); the old A resolution now equals its snapshot and renews the prefix a third time. With zero update hold this is a distinct downstream restart, so a failback can recreate the exact duplicate this change is meant to suppress; latch the covered move when winner and path first move instead of allowing equality with a later winner.
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Agreed, fixed in c60a3e4. A move is now settled once per join: the first time the front sees its path moved, it records the join it settled for, whether it renewed or found every cursor restarted. It re-checks only after someone joins again (a renewal leaves the front's instance behind, so nobody can). Regression test: a_moved_path_settled_by_a_restart_stays_settled, where the winner moves back to an older route and the prefix restarts once.
(Written by Claude Opus 5.5)
…he last join The prefix winner a front snapshotted ignored cursor scopes, so a route a scoped cursor cannot see changing the overall winner suppressed a renewal that cursor needed. Ask the cursors instead: renew unless every cursor presenting the prefix presents an instance minted after a requester last joined the front. Counting from the last join, not the resolution, covers a requester that re-requested after a restart and joined the same front. Once settled for a join, the move stays settled, so a winner moving back to an older route adds no restart. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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| /// `since`: none still presents an instance without an epoch from before, which | ||
| /// is what a [renewal](Self::renew) would restart. | ||
| fn restarted_since(&self, prefix: &Path, since: u64) -> bool { | ||
| let old = |instance: &Instance| matches!(instance, Instance::Route(generation) if *generation < since); |
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When two no-epoch routes A (cost 5) and B (cost 9) both predate a request, repricing B to cost 3 makes the cursor emit A→B and moves the held path on the first change. B still has generation 1 while joined is 2, so this predicate classifies the just-delivered B as old; moved then calls renew, and if the first restart has been drained the cursor emits a second restart. This leaves the duplicate downstream re-request in place for ordinary cost failover; record the cursor's last instance-change generation instead of using B's creation generation.
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Confirmed and fixed in 80c0600. Reproduced as a_repriced_route_taking_a_held_path_restarts_once (A cost 5 and B cost 9 both predate the request, B reprices to 3): on c60a3e4 the prefix restarted twice. Each cursor now stamps its presented route with next_route when it starts (a new instance or new captures), and a join or a new front takes a value of its own, so the check compares when the cursor restarted rather than when the route was minted.
(Written by Claude Opus 5.5)
| else { | ||
| return true; | ||
| }; | ||
| self.routes.cursors_touching(prefix).into_iter().all(|id| { |
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With N held paths under a prefix and M announcement cursors, each woken front now allocates, sorts, and scans the M touching cursors while holding the table read lock, so one pool change can perform N×M work. I checked rs/moq-net/benches/origin.rs::bench_pool_churn, and it only sweeps members and paths, leaving this new cursor-axis slope unmeasured; add that axis or a focused equivalent.
AGENTS.md reference: AGENTS.md:L37-L37
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Added in 80c0600: origin/pool_cursors sweeps paths (100, 1k, 10k) against cursors (1, 100, 1k) at 32 members, with the same join and leave as origin/pool_churn. Locally the cursor axis is close to additive rather than paths times cursors, since the first renewal moves every generation and later fronts take the fast path: 100 paths go from 0.30 ms to 0.87 ms across the cursor sweep, 10k paths from 46 ms to 63 ms.
(Written by Claude Opus 5.5)
A route that predates a request can still win the prefix and the held path after it, as in a cost failover between two existing routes. The cursor's switch to it is the restart, but its generation predates the join, so the front renewed the prefix again. Each cursor now stamps its presented route with next_route as it starts, and a join or a new front takes a value of its own, so the check compares when the cursor restarted rather than when the route was minted. Adds origin/pool_cursors, sweeping the paths a pool serves against the cursors watching it. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Reviewed commit: 80c0600.
Reviewed the two-commit advance from my last published review at 4a44882 and the complete current three-file diff. No new actionable findings.
My original scoped-cursor P2 is fixed by the per-cursor check and regression. The previously read-only cost-failover concern, independently reported in Codex's existing finding, is also addressed: cursors record presentation-start time, joins advance the comparison clock, and a_repriced_route_taking_a_held_path_restarts_once covers the first switch to an older backup. The new origin/pool_cursors benchmark sweeps held paths and announcement cursors.
Direction: keep the per-cursor presentation-time approach and regression coverage. No public API or wire-format change found.
Verification: GitHub-only inspection of all changed files and related routing, cursor delivery, front lifecycle, renewal and cache code. Tests and benchmarks were inspected, not executed. Check, Platform, WASM and Android are still running for this head.
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| let renew = resolved | ||
| .as_mut() | ||
| .and_then(|resolved| resolved.moved(&table, &path.as_path(), horizon)); | ||
| .and_then(|resolved| resolved.moved(&table, &path.as_path(), horizon, joined)); |
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moved evaluates restarted_since while table is locked, but the actual renew happens only after dropping that mutex. On the default multi-threaded relay, another scoped route update can acquire the lock in that gap and restart the prefix without changing this path; with update_hold == 0 (or once that event is drained), this task then renews from the stale decision and emits a second restart, so concurrent pool churn still produces the duplicate downstream re-request this change is meant to remove. Keep the check, settling, and renewal in one OriginState critical section.
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Agreed, fixed in 526de63. Resolution::moved now only settles the move and names the prefix; the front checks restarted_since under the write lock it renews with, so a restart landing after the read covers the move instead of being restarted again. No deterministic test: the gap needs another thread to take the lock between the two sections.
(Written by Claude Opus 5.5)
The front decided under the read lock and renewed after releasing it, so a restart in between still left the renewal to restart the prefix again. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Reviewed commit: 526de63.
Reviewed the single-commit delta from 80c0600 and related front/join/renewal code. No new actionable findings.
The concurrent restart race reported by Codex is addressed: suppression and renewal now share the write lock. Keep this atomic check-and-renew direction. Earlier scoped-cursor and cost-failover fixes remain intact; no public API or wire changes.
Verification: GitHub-only inspection. No concurrency regression was added in this commit; tests were not executed by me. Check, Platform, WASM and Android remain running.
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Merge summary for 526de63.
(Written by Claude Opus 5.5) |
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Problem
On moq-lite 06 and moq-transport, a claim at a prefix that another instance replaces reaches the next hop as an end and a fresh start, without an epoch. A front still holding a path beneath the prefix (a player that has not re-requested yet, or a relay serving one downstream) then sees its path resolve through the new route and renews the prefix. So the prefix's announce cursors restart it a second time: after the 300 ms update hold, or folded into the first restart, depending on timing. A follower that switches on every restart (moqsrc,
moq play) re-requests twice, and through a relay the second restart crosses as another end and start.Found by moqsrc's relay tests in #5181, where
a_restarted_prefix_switchessaw three stream-starts in 4 of 6 runs.Approach
currentinstance is read, so its scope, horizon, and mount apply as when it selected the route.next_routewhen it starts (a new instance or new captures), and a join or a new front takes a value of its own. So a cursor switching to a route that predates the join (a cost failover) counts as its restart.RemoteFront::joinedrecordsnext_routeon every mint and join.dynamic_epochchecks that nothing follows the restart. Before this change it failed on lite-06 and moq-transport 19 and 22 with a second end oflive.prefix_poolstill restarts the prefix when a path moves while the winner stays.Tests
Unit tests in
origin.rs, each failing on 4a44882 (the first version of this PR):a_moved_path_restarts_a_scoped_cursor: the scoped pool from the OpenAI review. A route the scoped cursor cannot see leaving changed the overall winner, and the cursor got no restart.a_moved_path_restarts_a_requester_that_joined_after_a_restart: a re-request after a restart joins the old front, then the path moves.a_moved_path_settled_by_a_restart_stays_settled: the winner moves back to an older route (the Codex failback case) and the prefix restarts once.a_repriced_route_taking_a_held_path_restarts_once: a route that predates the request reprices to win the prefix and the held path (the Codex review on c60a3e4). Fails on c60a3e4.Benchmark:
origin/pool_cursorssweeps the paths a pool serves against the cursors watching it. Locally the cursor axis is close to additive: 10k paths go from 46 ms at 1 cursor to 63 ms at 1k.Impact
Decisions
Taken by a background agent without prompting; each is the recommended option.
Alternatives
identical_reannounce_restarts_without_an_epoch), so the two look the same.Follow-ups
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