alkssh/tasks/implementation/w4-tty-coverage.md

id, name, status, depends_on, scope, risk, impact, level

id	name	status	depends_on	scope	risk	impact	level
w4-tty-coverage	Wave 4: tty coverage minors (wave-3 gate booked item 3)	completed	w4-channel-not-held-rows	narrow	low	component	implementation

Description

Close the wave-3 gate's booked finding item 3 (review-wave-3 §Booked findings, "minors, one ticket's worth"):

• The channel_close-fired per-channel cancel path is unexercised on the wire; only the teardown signal is. Land the wire row: a channel closing (client-side close on a served tty row) fires the per-channel cancel path (the cap release + the per-channel state teardown observed, the disjoint release sites the gate verified per row, now wire-witnessed for this path).
• The Closed/Break no-exit-chunk close shape has no wire row, so land the row(s): a session ending via the Closed/Break close shape emits no exit chunk (the derivation contract's close-shape class, the absent-report posture pinned, not just the happy paths).
• The tty stdout write_all carries no deadline, recorded as a known limit only (module docs): backpressure is bounded, liveness is covered by the inactivity timeout; no code change (the gate's recorded disposition, since a deadline here is a behavior change, not a coverage minor).

Depends on w4-channel-not-held-rows to keep tests/tty_mapping.rs single-writer (that task's rows land first; these land into the settled file).

Acceptance Criteria

• Wire row: the channel_close-fired per-channel cancel path (cap release + state teardown observed on the wire)
• Wire row(s): the Closed/Break no-exit-chunk close shape
• The stdout write_all no-deadline limit recorded in module docs (no code change)
• cargo test (+ per-feature + --all-features), clippy ×2 (-D warnings), fmt, cargo doc --no-deps pass

References

• tasks/implementation/review-wave-3.md §Booked findings item 3
• docs/architecture/mapping.md §The exit derivation contract (the close-shape classes; the PTY-path rule-5 limit, the docs-posture template), src/tty_map.rs, src/channel_cap.rs (the release sites the rows witness)
• tests/tty_mapping.rs (the per-build matrix the rows join)

Notes

• Test + module-docs only, as planned, hence no production code change; the shapes held on the wire exactly as documented.
• channel_close-fired cancel path, namely a_client_close_on_a_served_row_fires_the_per_channel_cancel_and_releases_the_cap (tests/tty_mapping.rs, cfg session): a served pipe row under a cap of 1; a second open is refused (AdministrativelyProhibited, the admission held), the client then closes the served channel, and the row witnesses (a) the per-channel state teardown, with the shim's kill-on-drop witness fires (the bridge surface's cancel → the serving task aborts the alktty session), polled before unmount so it is the channel_close site, not the teardown; (b) no exit report fabricated on the wire (plain close / channel end); (c) the serving path's ended lifecycle event; (d) the cap release, with the next open admits and routes (release site 1, disjoint from the bridge-failure and arrival sites).
• Closed/Break no-exit-chunk close shape, namely a_bridge_end_without_an_exit_chunk_reports_nothing_and_closes_plain (same file): the shape is constructed deterministically through the real path: the shim's stdout carries a payload beyond alktty's own MAX_CHUNK_LEN wire bound; alktty's ChunkWriter rejects it locally, its drainer shuts the write half down, and the bridge's next read is EOF (BridgeFrame::Closed) with the scripted exit (Some(0)) never forwarded. The row pins: the in-bounds chunk rode first, the end is a plain close, no exit-status/exit-signal ever rides (the absent-report posture, neither 0 nor 255), the session truly served (an allocation happened, so this is the close shape, not a decline), and the ended lifecycle event landed.
• Break's classification row, which pins clean_ends_classify_closed_and_oversized_lengths_classify_break (src/tty_map.rs test mod): Break is unreachable on the wire through alktty's own validated ChunkWriter (an oversized chunk fails local validation → the drainer break → the read sees EOF, exactly the Closed row's construction; a non-EOF duplex read error has no wire-constructible producer at this pin, and the serving-path body of Closed/Break is one arm). So the family's serving outcome is wire-witnessed by the Closed row, and the frame-level Break classification (bounds-checked, never an oversized read) is pinned at the frame layer by the unit row, recorded here as the reason one wire row + one unit row cover the "row(s)".
• The stdout write_all no-deadline limit, recorded in src/tty_map.rs's module docs (§the translation bridge): the gate's recorded disposition verbatim; backpressure bounded, liveness covered by the inactivity timeout, a deadline would be a behavior change, not a coverage minor. No code change.
• A session_with_limits / session_config(limits) driver variant was added to the binary (the cap row's ceiling of 1); session_with delegates unchanged.

Summary

The wave-3 gate's booked item 3 is closed: the channel_close-fired per-channel cancel path is now wire-witnessed (a served pipe row under a cap of 1: the refused over-cap open pins the admission, the client close pins the cancel, with the kill-on-drop witness firing before any teardown, no exit report is fabricated, the ended lifecycle event lands, and the succeeding re-open pins the cap release from the established-channel-close release site); the Closed/Break no-exit-chunk close shape landed its wire row (constructed through the real path via an oversized shim chunk dying at alktty's own ChunkWriter validation: EOF without an exit chunk → plain close, nothing fabricated, the scripted exit never forwarded) plus the frame-level Break classification row (Break is wire-unreachable through alktty's validated writer at this pin, as recorded); and the stdout write_all no-deadline limit is recorded in the module docs per the gate's disposition. No production code change. Verified: cargo test (default, session, session,git, git, tunnel, sftp, --all-features, with the tty rows' 5×-repeat flake check included), clippy ×2 (-D warnings), cargo fmt --check, cargo doc --no-deps 0 warnings, taskgraph validate ✓ (35 tasks).