feat(moq-net): stitch a draft-20 fill into the group it joined - #3325
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Included review availability: Your plan provides up to 4 included reviews per hour; 2 remain after this review. WalkthroughThe change adds draft-20 Merge Risk: ⚪ Minimal · up to This change enables draft-20 current-group joins by stitching fetched group heads to live tails; contradictory or unsolicited streams are rejected, and no actionable merge-blocking risk remains after normal checks and review. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches✨ Simplify code
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Actionable comments posted: 2
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Inline comments:
In `@rs/moq-net/src/ietf/subscriber.rs`:
- Around line 242-249: Update Fill::retire to record newer sequence values while
in Fill::Serving, preserve that value when recv_fill transitions Serving into
Fill::Active, and initialize seen to None in the SUBSCRIBE_OK Serving
construction.
- Line 1819: Update the Some(0) branch in the tail creation logic to reject the
stream when the same sequence already has a waiting head with Fill::Ready,
instead of creating another producer. Preserve the existing create(track)
behavior when no conflicting head exists.
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In `@rs/moq-net/src/ietf/subscriber.rs`:
- Around line 1865-1876: Track the sequence of any group delivered whole and
carry that marker into the fill-settlement path. Update the relevant fill state
and settle logic around `run_fill_objects` so that when a `Fill::Ready` head
matches the recorded whole-group sequence, it is refused or released instead of
creating or retaining another producer; preserve normal behavior for nonmatching
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A moqt-20 subscription starts at the next object, so joining a live track part way through a group drops that group's head. The draft's answer is a fill: a fetch stream carrying the objects published before the subscription started. We served fills as a publisher but never requested one, so against a strict publisher a join degraded to the next group boundary. Request the canonical current-group join, a Next Object filter plus a StartGroup=1 FILL_PARAMETERS, and stitch the two streams back into one group. The fill stream creates the group from its first object's absolute IDs and hands the producer over on FIN; the subscription's subgroup stream peeks its first Object ID and appends the tail once the head lands. Fetch streams decode through a shared FetchObject codec the publisher encodes with too, so both directions agree byte for byte. The rest is what a head does when no tail claims it. An unclaimed head waits for the subscription to end, which already publishes it, and nothing shorter is safe to infer: a later group arriving looks like proof no tail is coming, but streams are independent and the tail's own can still be behind it. Finishing the head on that guess drops the tail when it lands, so a publisher that ends a group inside the fill leaves it unfinished until the subscription is, stalling a consumer draining in order rather than losing frames it was sent. Four cases settle it earlier, each on evidence rather than a guess: - An empty tail stream claims the head and finishes the group. - A head that does not meet the tail is refused, both halves go, and the head is published as the prefix it is. - A whole group arriving for a sequence the fill already headed means the publisher contradicted its own fill; the head is published and the duplicate stream dropped, since the model holds one producer per group. - SUBSCRIBE_OK without LARGEST_OBJECT says the track has no content, so no fetch stream is owed and the fill settles immediately. A stream with FIRST_OBJECT clear is still dropped when no fill is outstanding (#3308); that check moves below the alias lookup, where the subscription's fill is known, so the head we asked for is not refused with everything else. The fill's stream and the subgroup peek both race the subscription going away, since aborting a track does not close a group producer. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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| &mut self, | ||
| stream: &mut Reader<S::RecvStream, Version>, | ||
| track: &mut track::Producer, | ||
| timescale: Option<Timescale>, | ||
| head: &mut Option<(u64, u64, group::Producer)>, |
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run_fill_objects is introduced with four non-receiver arguments, including a mutable out-parameter whose cleanup invariant is separately owned by run_fill. This crosses the repository's documented threshold for replacing an argument bundle with a struct, and keeping the decoder state split across parameters makes future fields and error cleanup easier to desynchronize. Move the fill-decoding context into a small state object instead of expanding this signature. (Written by GPT-5.6 Sol)
AGENTS.md reference: AGENTS.md:L156-L160
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Not addressed here: this arrived after auto-merge fired (PR merged 23:32:10Z as 5eea9e3, review posted against b25db5b). The finding is fair and matches the repo's Refactor As You Go rule. Concretely there are two smells: run_fill_objects takes four non-receiver args, and its out-parameter is an anonymous (u64, u64, group::Producer) that mirrors Fill::Ready's fields, with the abort-on-error invariant owned by run_fill. A named Head { sequence, next, producer } shared with Fill::Ready, or a small fill-decoder state that owns its own cleanup, would fix both. Tracking as follow-up rather than leaving a TODO in the code.
🤖 Addressed by Claude Code
`broadcast_moq_transport_20_current_group_join` arrived from main with #3325 and was never reconciled when it merged into dev: the crate is `moq_tokio` rather than `moq_native`, configs moved to `listen::Config` / `connect::Config`, origins come from `origin::spawn(Hop::random())`, and `announce::Update` carries `{prefix, route, active}` instead of a broadcast handle. Nothing had recompiled moq-tokio's tests since, because `just check` only builds the crates a branch touches plus their dependents, and no PR in between touched moq-net. This one does (a doc comment), which is what surfaced it. Ported onto the same pattern `broadcast_test` in this file already uses, so the test asserts what it always did: the head arrives on the fill fetch stream, the tail on the subscription's own subgroup stream, and both land in one group in order and exactly once. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Summary
A
moqt-20subscription starts at the next object, so joining a live track part way through a group drops that group's head. The draft's answer is a fill: a fetch stream carrying the objects published before the subscription started.Root cause: we served fills as a publisher but never requested one. The subscriber asked for
Relative(1)and refused any incoming fetch stream, so against a strict publisher (one that delivers nothing published before the subscription) the mid-group stream was dropped and the join degraded to the next group boundary.StartGroup=1FILL_PARAMETERS.FetchObject/FetchSubgroupcodec that the publisher'swrite_fill_objectencodes through too, so both directions agree byte for byte.What ends an unclaimed head
This is the part that took the most iteration, so it is worth stating plainly. An unclaimed head waits for the subscription to end, which already publishes it. Nothing shorter is safe to infer.
An earlier revision retired a head as soon as a later group arrived, on the theory that the publisher had moved on. That is wrong: subgroup streams are independent, so a later group's stream can overtake the tail's own. Finishing the head on that guess drops the tail's frames when it lands. Credit to the Codex review for pushing on this until the inference itself was the thing questioned rather than patched.
The trade is explicit: a publisher that ends a group inside the fill and opens no tail stream leaves that group unfinished until the subscription ends, which stalls a consumer draining in order rather than losing frames it was sent. Tightening that bound without guessing is follow-up work.
Four cases still settle a head early, each on evidence rather than inference:
SUBSCRIBE_OKwithoutLARGEST_OBJECTsays the track has no content, so no fetch stream is owed and the fill settles immediately.Interaction with #3308
A stream with
FIRST_OBJECTclear is still dropped when no fill is outstanding. That check moves below the alias lookup, where the subscription's fill is known, so the one head we explicitly asked for is not refused along with everything else. Behavior is unchanged wherever a fill was never requested or has already settled.Public API changes
None.
coding,ietf,lite, andmodelare all private modules inmoq-net, so thesubscribe_filter->subscribe_joinrename and the newFetchObjectcodec are crate-internal. Targetsmainaccordingly.Test plan
just checkandjust testgreen: 3049 Rust tests passed, all JS packages 0 fail.ietf::subscriber::stitch_tests: head+tail stitch, empty tail finishes the head, tail without a fill is dropped, head that misses the tail is refused, unsolicited fill is refused, whole group for a headed sequence refused, and the reverse arrival order.ietf::fetchcodec round-trips, including End-of-Range markers and the subgroup spellings;FetchObjectadded to the IETF fuzz dispatch with a seed.an_empty_track_settles_the_fill/a_track_with_content_still_awaits_its_fill.moq-native::broadcast broadcast_moq_transport_20_current_group_join, verified as a real regression test: reverting the fill request makes it fail withread_frame timed out.The publisher's existing pinned fill tests pass unchanged, which is what confirms the shared codec is byte-compatible with the previous hand-rolled encoding.
Cross-package sync
Skipping the
js/netrow deliberately.js/net/src/ietf/subscriber.tsdocuments the identical gap after #3297 landed the JS publisher half. This changes join behavior using a wire feature both sides already encode and decode, so the wire is unchanged and the JS mirror can land separately;doc/concept/layer/moq-lite.mdis scoped to say the browser subscriber still takes the degradation.No draft update:
FILL_PARAMETERSismoq-transportdraft-20, not ourmoq-litewire.🤖 Generated with Claude Code