[kixelated]

Problem

Since #6143, a SUBSCRIBE_DROP on moq-lite 05/06 ends the dropped group's readers with NOT_FOUND (JS: StreamError(NotFound, "dropped")). The most common lite-06 drop is a relay superseding a group a reader is still on. A snapshot reader such as @moq/json's Consumer or moq_json::snapshot::Consumer resyncs to the next group on OLD/EVICTED/TOO_FAR_BEHIND/GROUP_TOO_LARGE. It throws on NOT_FOUND, so a catalog subscription on a lite-06 session dies whenever the relay replaces a catalog group mid-read.

Repro (JS, upstream main): read group 0 with Consumer.latest(), apply the drop the subscriber decodes for SubscribeDrop { start: 0, end: 0 }, then write group 1. On lite-06 the read throws dropped. On lite-07, where the drop carries OLD, it yields group 1.

Approach

• SubscribeDrop::error (Rust) and SubscribeDrop.cause (JS) read a lite-05/06 drop as OLD. The rule from feat(lite): readers see SUBSCRIBE_DROP and cut groups end with their error #6143 is unchanged: an early DROP still settles the reader before the FIN. Only the error changes. OLD is a hole in both stacks, the same as a lite-07 publisher's OLD drop. A reader holding the group ends with it, and cursors don't hand the group to new readers.
• JS: a lite-07 drop code past u32 now reads invalid drop code: N instead of dropped: N. It is still INTERNAL.
• Rust: an origin front's in-flight track ends with Error::Cancel once its last reader leaves, instead of dropping its track::Producer unfinished. That path logged "track::Producer dropped without finish() or abort()" on every release and tore the track down as Error::Dropped.
• Docs: the draft changelog, doc/concept/moq-lite.md, and both MIGRATE.md entries for feat(lite): readers see SUBSCRIBE_DROP and cut groups end with their error #6143 now say OLD.

Audit of "dropped" errors

The goal was that no reader ends with a "dropped" error that doesn't name a real reason.

Peer-reachable, now fixed

• lite-05/06 SUBSCRIBE_DROP: NOT_FOUND / "dropped" becomes OLD (JS and Rust).

Peer-reachable, already a real reason

• JS resume.ts dropped(): Cancel, "no route serves the track".
• Rust session.rs: close(Error::Cancel, "dropped") when the last session handle is dropped without a close.

Local-only Error::Dropped
These are the producer-dropped-unfinished signal that rs/AGENTS.md already calls a bug in the holder.

To measure reachability, every Error::Dropped origin and fallback in moq-net's model was temporarily made to panic, and the whole moq-net suite was run. 1193 of 3221 tests hit one:

Site	Hits	What it is
track::Producer drop (track.rs teardown)	940	RAII teardown
group::Producer drop (commit_abort)	213	RAII teardown
Fill dropped unanswered	23	Deliberate bug signal, distinct from an explicit drop_groups (NOT_FOUND); pinned by fill_dropped_unanswered_names_the_rest_dropped
frame::Producer drop	14	RAII teardown
Staged drop	3	RAII teardown
A broadcast's last track handler dropped with requests queued	2	RAII teardown
Every abort.unwrap_or(Error::Dropped) fallback (track, group, bandwidth)	0	Never reached

With backtraces on the lite/ietf/session unit tests and the route-change, catalog-handover, auth and wide-sequence integration tests, every caller is either a test's own scaffolding dropping its producer, or the origin run_front in-flight release fixed above. No session handler reached Error::Dropped from a peer's input.

One local site stays as it is: the origin's orphaned-front path still drops in-flight tracks without a release. A reader still on its way must keep the copy, which an abort would end (an_orphaned_front_keeps_the_copy_for_a_pending_subscriber). (The JS Staged dispose this audit also kept is gone since #6747.)

Decisions

1. How loud an Error::Dropped should be. The maintainer asked whether it should panic.
   • ✅ (this PR) Keep the existing tracing::warn! plus Error::Dropped, and fix the library paths found. A debug_assert! or panic in the producer Drop impls would fail 1193 of 3221 moq-net tests today, because tests drop producers to end them, and it would also fire in debug builds of every application that ends a producer by dropping it.
   • Panic (or debug_assert!) on drop without finish()/abort(). This needs every such test and application path to finish or abort explicitly first, a large mechanical change across moq-net and its dependents, and a Drop that panics must skip while already unwinding.
   • debug_assert! only on the never-reached unwrap_or(Error::Dropped) fallbacks. They are unreached in the suite but not proven impossible, so this PR leaves them.
2. What a lite-05/06 drop reads as.
   • ✅ OLD, as the maintainer decided: superseded, so a reader moves on.
   • Keep NOT_FOUND and add it to the JSON consumers' resync list. This was rejected because "dropped" should never reach a reader.

Impact

• Behavior: on lite-05/06, a reader of a group named in SUBSCRIBE_DROP ends with Error::Stream(StreamError::Old) / StreamCode.Old instead of NotFound. A cursor no longer hands such a group to a new reader (a hole, as with a lite-07 OLD drop). No wire change.
• Public API: none.

Alternatives

• Teach every snapshot and window consumer to skip NOT_FOUND. That would hide the meaningless code instead of fixing it, and leave every other reader with "dropped".

Checks

• New or updated tests that fail without the fix: JS tail.test.ts "a SUBSCRIBE_DROP ends a group still arriving, before the FIN" (lite-05/06 lanes now expect OLD, and the track goes on to the next group), and Rust a_subscribe_drop_ends_an_arriving_group_with_its_code (same, all three versions).
• JS snapshot.test.ts "snapshot consumer skips a group the publisher dropped mid-read". It ends the group with an OLD close, since refactor(net)!: keep session-only model items private #6747 made Track.Producer.dropGroups internal. Rust's a_lost_group_waits_for_its_replacement already covers Stream(Old).
• moq-net origin, resume, route-change and catalog-handover tests: 425 passed.
• just ci: passed (check and test for rs and js, drafts, markdown).
• just test interop --all: all checks passed, including the lite-05, lite-06 and lite-07-wip drop lanes in both directions.

Follow-ups

• If decision 1 goes to a hard panic, that is its own PR converting test and application drops to explicit finish()/abort().

(Written by Claude Opus 5.5)

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📝 Code Review	✅ Completed 2026-10-26T18:55:00.036404Z	70aa17b	New commits

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[kixelated]

Automated review by review (OpenAI)

Reviewed commit: 3caa228

No actionable bugs found in the changed paths. Mapping legacy SUBSCRIBE_DROP to OLD in both stacks is a focused compatibility fix: existing snapshot gap handling can resynchronize without broadening NOT_FOUND handling or changing the wire format. The held-group/next-group regressions cover the intended continuity, while lite-07 retains its explicit error code. The origin cleanup uses Cancel only after the ended front's track becomes unused, preserving the separate orphaned-front path.

Verification: static inspection of the diff, drop consumers, snapshot resync, and origin/release lifecycle. I did not run builds, tests, interop, or independently reproduce the reported audit counts.

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No actionable comments were generated in the recent review.

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Reviewing files that changed from the base of the PR and between 3caa228 and 47da76f.

Files selected for processing: 1

• doc/concept/moq-lite.md

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Walkthrough

The JavaScript and Rust Lite implementations now map Lite 05 and Lite 06 dropped-group errors to OLD. Lite 07 code handling remains in place. Tests cover continued reads after a group is dropped and pending snapshot reads receiving a newer group. Migration and protocol documentation was updated. Origin handling now releases the route and cancels the accepted producer when an in-flight track becomes unused.

Priority: Normal

Merge Risk: Minimal · up to 47da7

The documented compatibility behavior is consistent with the reported implementation changes, with no remaining merge-blocking concern identified.

Pre-merge checks: 5

Check name	Status	Explanation
Title check		The title clearly identifies the main change: lite-05/06 SUBSCRIBE_DROP readers now end with Old.
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 5 functions across 6 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.
Description check		The description is specific and directly related to the changeset. It explains the problem with SUBSCRIBE_DROP on moq-lite 05/06 returning NOT_FOUND, and why readers of a dropped group should instead …

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 5 functions across 6 files. (1 skipped: 1 unsupported.)

Full details: Description check

Explanation

The description is specific and directly related to the changeset. It explains the problem with SUBSCRIBE_DROP on moq-lite 05/06 returning NOT_FOUND, and why readers of a dropped group should instead get OLD. It also covers the JS invalid drop code message change, the Rust origin in-flight release change (Error::Cancel instead of an unfinished producer drop), and the doc updates. It lists the tests added or updated and the checks that were run, so a reviewer can see what changed and why.

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Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @doc/concept/moq-lite.md:
- Around line 274-277: Update the compatibility scope in the `SUBSCRIBE_DROP`
documentation: state that publishers explicitly report groups that will not
arrive starting with Lite 03, and that the application-specific `OLD` handling
applies to Lite 03 through 06.

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Reviewing files that changed from the base of the PR and between 17be545 and 3caa228.

Files selected for processing: 10

• doc/concept/moq-lite.md
• drafts/draft-lcurley-moq-lite.md
• js/json/src/snapshot/snapshot.test.ts
• js/net/MIGRATE.md
• js/net/src/lite/subscribe.ts
• js/net/src/lite/tail.test.ts
• rs/moq-net/MIGRATE.md
• rs/moq-net/src/lite/subscribe.rs
• rs/moq-net/src/lite/subscriber.rs
• rs/moq-net/src/model/origin.rs

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[kixelated]

Merge summary: 47da76f rewords doc/concept/moq-lite.md so the OLD reading covers every pre-07 SUBSCRIBE_DROP, since both stacks also decode one on lite-03/04 as OLD. The CodeRabbit suggestion to say publishers report drops from lite-03 was not taken: our publishers send SUBSCRIBE_DROP only on lite-05 and later. With #6819 closed as superseded, the lite-06 JSON snapshot resync is covered at both seams: tail.test.ts pins that a lite-05/06 drop ends a mid-read group with OLD and the track continues, and snapshot.test.ts pins that the snapshot consumer resyncs past an OLD mid-read.

(Written by Claude Opus 5.5)

[kixelated]

The merge queue dropped this because #6747 made Track.Producer.dropGroups package-internal, and the new json snapshot test still called it (TS2339 in @moq/json check). 70aa17b merges main and ends the reader's group with first.close(new NetError.Stream(StreamCode.Old)) instead, the public idiom the neighbouring json test already uses. just ci passes locally (9026 tests, 0 failed). Ready to re-queue.

(Written by Claude Opus 5.5)

[kixelated]

Requeueing after the failed_checks dequeue at 47da76f.

Cause: a semantic conflict with #6747, which made JS Track.Producer.dropGroups internal (now hooks.dropTrackGroups). This PR's new @moq/json snapshot test called the public dropGroups, so the merged check js failed with TS2339. @moq/json cannot reach @moq/net internals.

Fix (70aa17b): main merged in, and the test now ends the group the reader is on with first.close(StreamError(Old)). The reader sees the same OLD a SUBSCRIBE_DROP gives it, so the test still covers the consumer skipping to the next snapshot. The PR body's audit no longer lists the removed JS Staged dispose.

Verified on this content: just check (9026 Rust tests, JS check and tests), just test interop --all (including the lite-05/06/07-wip drop lanes in both directions), and quest check. Codex's thumbs up covers this head.

(Written by Claude Opus 5.5)