fix(relay): keep only finished groups warm when a track goes idle

[kixelated]

Problem

Found while chasing the Smoke "audio/video sync: after rejoin" flake (player-side fix in #3973).

When the last reader leaves a relayed track, the origin front parks it: it copies the cached groups to a warm local track, then drops the source copy. That cancels the upstream subscription and resets the group still in flight. The warm copy had adopted that open group, so the logical track's resume boundary pointed mid-group ((18, N)) into a head that was now aborted.

On rejoin the relay re-spliced from that boundary and sent upstream SUBSCRIBE { Group Start: 18, Frame Start: N }. It only got the tail, and nobody could read the head, so group 18 never reached the returning reader. The player got 17 (warm) and then 19.

The cache itself is not at fault. create_group already evicts an aborted incarnation (claim_sequence), the same way writeGroup does in JS.

The repro needs lite-06 in the browser because pre-lite-06 peers widen a mid-group boundary to frame 0. In Rust-to-Rust relaying the group is lost on lite-05 as well.

Approach

warm_copy now adopts only finished groups. An open group is left to the re-splice, which resumes at its first frame and delivers it whole.

Regression test rs/moq-net/tests/rejoin.rs (mock transport, lite-05 and lite-06): leave mid-group, the publisher cuts the group, rejoin, and group 1 must come back with every frame. It fails on both versions without the fix.

Impact

• No public API change. Adds a crate-private group::Producer::is_finished.
• No wire change. A rejoin now sends Group Start = <open group>, Frame Start = 0 instead of a mid-group frame.
• A returning reader re-fetches the open group from upstream instead of reading it from the warm copy.

Alternatives

• Make track::resume_position ignore an aborted group's resume_frame. That would break failover, where readers still hold the aborted head and want only the tail.

Follow-ups

• A lite-06 join after an idle stretch still gets the latest group, even when it's stale in wall-clock time. That is intended: Max Age is measured from the latest group, and a subscriber that doesn't want a group again sets Group Start to last+1. No draft change.

🤖 Generated with Claude Code

(written by Claude Opus 5.5)

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📥 Commits

Reviewing files that changed from the base of the PR and between db73903 and a7e7d9d.

📒 Files selected for processing (4)

• doc/lib/c/index.md
• rs/moq-net/src/model/group.rs
• rs/moq-net/src/model/origin.rs
• rs/moq-net/tests/rejoin.rs

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-24T01:28:13.506286Z	a7e7d9d	New commits

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[kixelated]

Merged (squash, 5056a96).

• Merged origin/main into the branch (36 commits, no conflicts, none touching the changed files).
• Locally: cargo test -p moq-net --test rejoin passes, all 1188 moq-net lib tests pass, clippy (--all-targets -D warnings) and rustfmt are clean.
• CI green on a7e7d9d (Check, Test, WASM, Android).
• Reviews: Codex gave a thumbs up with no findings. CodeRabbit was rate-limited.
• The doc/lib/c/index.md edit belongs here: it updates the demand note to say only finished groups stay warm.

(Written by Claude)

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Generated by Claude Code