[kixelated]

Problem

A wire subscription's end (lite SUBSCRIBE_END, IETF END_OF_TRACK) can arrive before the group streams below it, since QUIC does not order streams. A @moq/net reader ended at the boundary with those groups still in flight, so a late lower group never reached it. Rust readers hold since #4225.

Approach

Mirror the Rust hold, judged by an arrival record rather than the cache (as the quest decided):

• js/net/src/ledger.ts: the JS Ledger, extracted from Tail like Rust's ledger.rs, plus GRACE (was TAIL_GRACE_MS) and grace(maxDelay). Tail composes it. An in-order arrival extends the last run in place.
• js/net/src/track.ts: a producer records every live group and datagram in a Ledger that eviction never touches. hooks.endTrack (the JS finish_at_pending) marks the tail pending. While a sequence between where the feed starts and the end has not arrived, in a gap younger than the subscription's grace, readers do not complete (owed on each sink). A clean close ends them regardless. An abort over a group the tail still owes errors readers; a gap already past the grace folds first, so it ends cleanly, as in Rust's commit_abort.
• Feed start, mirroring Rust start_at: new hooks.startTrack(producer, { start, requested }) from lite SUBSCRIBE_START, and from IETF SUBSCRIBE_OK's Largest (0 without one) for the live join, as Rust's IETF subscriber does. The START applies while the demand keeps the floor its SUBSCRIBE asked for; a moved floor stands on its own; without either, the first arrival stands in.
• A SUBSCRIBE_DROP range counts as accounted, so it owes nothing to a pending tail.
• js/net/src/lite/subscriber.ts: SUBSCRIBE_END goes through hooks.endTrack instead of finishAt. IETF already did, so it now holds too.
• Deletes quest/m1/js-pending-tail.md and its references.

Impact

• Public API: none. hooks.startTrack is package-internal. Wire: none.
• Behavior: a JS reader of a session-fed track waits at a hole below the declared end until the group arrives, the gap folds on a later arrival, or the session settles its tail and closes the track (bounded by the grace or the lite-07 stream count). Local tracks (finishAt) still end at the boundary with holes.
• Behavior: an abort over a group the pending tail still owes now surfaces the error instead of a clean truncated end.

Decisions (sub-agent picks, flagged for review)

• No JS set_tail_pending(false): both JS sessions close the producer once their tail settles, which ends held readers the same way Rust's clear plus finish() does. Recommend keeping it out until a session needs to end readers without closing.
• IETF floor: SUBSCRIBE_OK's Largest, else 0, matching Rust's start_at for a live join. Changed after review from "the requested floor, else the first arrival", which ended a reader early when a higher group landed before the lower ones. Against a strict publisher that drops the mid-group join stream, the reader holds for group Largest until the session's tail settles, as in Rust.
• Like Rust, no timer folds a gap: a held reader is released by an arrival, the session's settle, or an abort.

Validation

• New model tests in track.test.ts mirror the Rust ones (hole fills, close ends, abort errors short of a group, abort after a full tail, abort before/after the grace on a manual clock, evicted group, datagram, end after the highest group, START versus a moved floor, a dropped range with and without a later abort). New IETF session test: groups from SUBSCRIBE_OK's Largest landing after a higher group and END_OF_TRACK reach the reader. New lite session test: a lower group landing after SUBSCRIBE_END reaches the reader before its end. The existing "readers end before a missing group's grace" test now asserts the hold. With the hold disabled, the hold tests fail.
• @moq/net tests pass (2113), plus biome and tsc. just check stops at main's wasm-bindgen 0.2.129 CLI versus the 0.2.127 lock, fixed separately in fix(rs): match wasm-bindgen to the nix dev shell's 0.2.129 CLI #5911.
• just test interop --tail: all finite-tail lanes pass, including rust -> js-native-node/bun.
• bench/track.ts slope gate passes; publish cost stays flat across the retained sweep.

Follow-ups

• Make hooks.endTrack/hooks.startTrack public under the received-end shape (quest/m1/public-model/deferred.md, updated).
• Rust's owes still ignores drop_groups, so a dropped range holds a Rust reader until the session clears the pending tail; count drops there too for parity.

🤖 Generated with Claude Code

(Written by Claude Opus 5.5)

[kixelated]

Quest outcome: quest/m1/js-pending-tail.md is implemented here and deleted. JS readers now hold at a hole below a wire-declared end, judged by a per-track arrival ledger shared with the session Tail, matching Rust #4225 including the abort and grace rules. No public API or wire change. Three sub-agent picks are listed under Decisions in the description for maintainer review; none blocks merging. just check and just test interop --tail pass locally.

(Written by Claude Opus 5.5)

[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-19T06:50:49.610383Z	a032ccb	New commits

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[kixelated]

Maintainer decisions confirmed (2026-10-18): merging is approved, and the three agent-made decisions in the description are accepted as-is:

1. No hook to release held readers without closing the track; both JS sessions close the producer once the tail settles.
2. IETF counts from the requested start group, else the first arrival.
3. No timer releases a held reader, matching Rust; only an arrival, the session's settle, or an abort does.

(Written by Claude Opus 5.5)

[chatgpt-codex-connector]

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📥 Commits

Reviewing files that changed from the base of the PR and between d4834ba and a032ccb.

📒 Files selected for processing: 13

• js/net/src/ietf/subscriber.ts
• js/net/src/ietf/tail.test.ts
• js/net/src/internal.ts
• js/net/src/ledger.ts
• js/net/src/lite/subscriber.ts
• js/net/src/lite/tail.test.ts
• js/net/src/tail.test.ts
• js/net/src/tail.ts
• js/net/src/track.test.ts
• js/net/src/track.ts
• quest/m1/README.md
• quest/m1/js-pending-tail.md
• quest/m1/public-model/deferred.md

Walkthrough

The pull request adds a Ledger for sequence-range and gap-age tracking, then uses it for subscription tail accounting. Subscription start and end responses update tracking hooks. Producers record group and datagram arrivals, and readers wait for owed groups below a declared end until those groups arrive or the grace period expires. Tests cover late arrivals, requested floors, aborts, cache expiry, and datagrams.

Priority: ⬇️ Low

Merge Risk: 🔵 Low · up to d4834

A subscription with a dropped group can leave its reader waiting or report an avoidable abort error. This is a bounded edge case, but DROP should settle the pending tail.

Pre-merge checks: 5

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 3 functions across 9 files. (1 skipped: 1 …
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the main change: JS readers now wait for a track's pending tail.
Description check		The description directly explains the pending-tail problem, implementation approach, behavior changes, and validation for the changeset.

Full details: Docstring Coverage

Explanation

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 3 functions across 9 files. (1 skipped: 1 unsupported.)

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Inline comments:
Review comments at @js/net/src/track.ts:
- Around line 1376-1383: Update #drop to account the explicitly dropped range in
#arrivals and reevaluate pending tails with #owe after recording the drop; add
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📒 Files selected for processing: 12

• js/net/src/ietf/tail.test.ts
• js/net/src/internal.ts
• js/net/src/ledger.ts
• js/net/src/lite/subscriber.ts
• js/net/src/lite/tail.test.ts
• js/net/src/tail.test.ts
• js/net/src/tail.ts
• js/net/src/track.test.ts
• js/net/src/track.ts
• quest/m1/README.md
• quest/m1/js-pending-tail.md
• quest/m1/public-model/deferred.md

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• quest/m1/README.md
• quest/m1/js-pending-tail.md

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[kixelated]

Decision (2) changed in 5cb720b, after review: IETF no longer counts from the first arrival when no start was requested. The live join now declares its start at SUBSCRIBE_OK's Largest, or group 0 without one, exactly as Rust's IETF subscriber calls start_at. Without it, a higher group landing before the lower ones and END_OF_TRACK ended the reader with those groups still in flight (now a regression test in ietf/tail.test.ts). The cost, shared with Rust: against a strict publisher that drops the mid-group join stream, the reader holds for group Largest until the session's tail settles and closes the track.

Same push: a SUBSCRIBE_DROP range now counts as accounted for the pending tail, and the lite tail tests no longer use a timer to let a read park. Decisions (1) and (3) are unchanged.

(Written by Claude Opus 5.5)

[kixelated]

Merge summary: JS readers of a session-fed track now hold at a hole below a wire-declared end (lite SUBSCRIBE_END, IETF END_OF_TRACK) until the group arrives, its gap folds past the grace, or the session settles and closes the track, matching Rust #4225. No public API or wire change.

Review round (5cb720b): IETF now owes groups from SUBSCRIBE_OK's Largest (0 without one), as Rust's start_at does, which changes decision (2); a SUBSCRIBE_DROP range counts as accounted; the lite tail tests no longer use a timer. Decisions (1) and (3) stand as accepted. Main merged after #5911; CI is green and Codex reviewed a032ccb with no findings.

Follow-up: Rust's owes still ignores drop_groups, so count drops there too for parity.

(Written by Claude Opus 5.5)