feat(net): cancel a pending fetchGroup with an AbortSignal

[kixelated]

Problem

A @moq/net caller could not abandon one pending fetchGroup() without closing the track or session (the follow-up #4357 left). Implements quest/m1/js-fetch-cancel.md.

Approach

• FetchGroupOptions.signal?: AbortSignal, threaded through broadcast.ts (local) and lite/subscriber.ts (wire). The IETF path still rejects fetchGroup and is unchanged.
• An already-aborted signal rejects with its reason before subscribing or opening any stream.
• On abort, the caller's mirror is closed and its promise rejects with the signal's reason. Coalesced callers keep sharing the one FETCH stream.
• The shared stream is cancelled only once every sharer has left. After acceptance the existing demand-driven pump already did this; setup now watches demand too (untilAbandoned), so a fetch abandoned during TRACK_INFO never sends its FETCH, and one abandoned while waiting for acceptance resets the stream with CANCELLED. To make that watch valid from the start, the first caller's mirror is reserved before the fetch starts.
• Aborting after the promise resolves does nothing; close the group instead (same as connect({ signal })).
• Small util/abort.ts helper (untilAborted) shared by both paths.
• Tests: already-aborted, one of two sharers aborting (the other still gets the group, stream not reset), last sharer aborting during TRACK_INFO and during FETCH, plus the local path. All four lite tests fail without the fix.
• Documented in doc/lib/js/net.md.

Impact

• Public API: additive FetchGroupOptions.signal in @moq/net.
• Wire: none new. Abandoning a pending fetch uses the existing reset path with CANCELLED.

Alternatives

• The signal could also close the resolved group, like fetch() does for a body. Not done: the quest scopes it to a pending fetch, and connect({ signal }) already ignores an abort after it resolves.

Follow-ups

• None required. Possible: the lite publisher could pass a signal when the peer resets the FETCH it is serving.

(Written by Claude Opus 5.5)

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 4a0c450 and 02f4172.

📒 Files selected for processing (9)

• doc/lib/js/net.md
• js/net/src/broadcast.test.ts
• js/net/src/broadcast.ts
• js/net/src/lite/subscriber.test.ts
• js/net/src/lite/subscriber.ts
• js/net/src/track.ts
• js/net/src/util/abort.ts
• quest/m1/README.md
• quest/m1/js-fetch-cancel.md

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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 2026-09-29T22:53:23.574362Z	02f4172	Draft marked ready

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[kixelated]

Automated review by review (OpenAI)

Reviewed head: 02f4172

Direction: useful and proportionate. Reserving each caller's mirror before setup, then abandoning the shared fetch only after the final caller leaves, is the right ownership model. A per-caller AbortSignal should not close the track or cancel another caller's fetch.

No actionable defect found in the changed local/lite fetch paths, abort helper, or setup-abandonment cleanup. The tests cover pre-abort, one surviving sharer, and final-sharer cancellation during TRACK_INFO and FETCH. Keeping cancellation scoped to setup, with callers closing an already returned group themselves, is a clear API boundary.

Verification: static diff and surrounding subscriber/broadcast/track cleanup review; no Bun tests executed.

(Written by OpenAI)