---
id: w3-tty-session
name: "Wave 3: session → alktty (translation bridge + exit derivation)"
status: completed
depends_on: [w3-git-mapping]
scope: moderate
risk: medium
impact: phase
level: implementation
---

## Description

Land the session branch, the biggest single-row surface of the spine
(mapping.md's `session` rows): pty/shell/pipe-exec become a real
serving path through the published alktty seam, as a **translation
bridge, not a byte pump** (POC-3 §D.1). Everything POC-3 stage D
proved, re-landed as contract:

- **The per-channel state + the store-and-take reuse**: the channel
  store `w3-git-mapping` lands (`channels.remove(&id)` →
  `into_stream()` in the spawned serving task) gains the session
  rows' per-channel state, the PTY params a `pty_request` delivered
  (consumed by the later `shell_request`/`exec_request` on the same
  channel, since `ssh -t host "cmd"` sends pty-then-exec and must
  serve in PTY-mode; a `shell_request` without a preceding pty serves
  **pipe-mode** `tty = None`, and whether such a shell serves or
  declines is for the task to decide and record).
- **Negotiation, built locally, never wire-derived** (OQ-08: the
  serve-config is the whole truth): `pty_request` args →
  `TerminalParamsWire{term, cols, rows, pixel_w/h, modes}` (SSH
  ttymodes → the reserved modes JSON, which is alktty OQ-44's
  reservation, deferred there); `shell_request` → `NegotiateRequest{carriage:
  "raw", cmd: shell_argv (the serve-config shell, verbatim), cwd:
  session_cwd, env: session_env, backend: the seam selector}` →
  **`drive_session_pre_negotiated(writer, reader, req, backends,
  ownership, identity)`** spawned; `exec_request` (pipe row) → `cmd =
  wrapper ++ [cmd]` (the OQ-08 argv append with vanilla quoting
  semantics; **never naively split**), `tty = None` pipe mode.
  **Unset-declines** (OQ-08): `shell_argv` unset → `shell_request`
  declines; `exec_wrapper_argv` unset → the pipe row declines (never
  a fallback). Decide the wire shape per row (capability decline,
  channel stays usable, vs the serving-path failure+close shape) and
  record.
- **The translation bridge**, two small adapters around a duplex the
  serving task drives (the POC-3 §D.1 shape): SSH `data`/EOF →
  `Chunk::stdin` (decide the EOF translation: the zero-length
  stdin-chunk sentinel vs alktty's `ControlMessage::Eof` CTRL_IN;
  verify what alktty's ChunkReader/backend actually consumes and
  record); stdout chunks → SSH `data`; stderr chunks →
  `extended_data(1)` (PTY mode merges upstream, where the backend's
  stderr stream is `None`, so the translation is universal with no
  mode-switch); the exit chunk (`STREAM_CTRL_OUT`) → the derivation
  contract. No unbounded buffering in either adapter (convention 9).
- **Control rows**: `window_change_request` → `ControlMessage::
  Resize` CTRL_IN; `signal` → `ControlMessage::Signal` (alktty's
  uppercase-name vocabulary, with the name accepted verbatim; decide
  the decline shape for names alktty's backend semantics reject and
  record). Mechanics: the requests arrive on the packet loop after
  the serving task owns the wire, so reach them into the spawned
  bridge through a per-channel control surface (a bounded mpsc; decide
  plus record; a silent unbounded proxy is a bounded-resources bug).
- **The exit derivation contract** (mapping.md rules 1–6,
  normative): the alktty exit chunk (`{"type":"exit","code":i32}`,
  exit-chunk-is-last) is the sole fidelity input, never widened:
  `code ≥ 0` → `exit-status(N)`; negative `−N`, N ≥ 2 →
  `exit-signal(name)` via the **server-side Unix signum→name table**
  (crate-local; not derived from alktty's `signal_from_name` inverse
  unless that is public and sound; decide the table's form and
  record; `core_dumped` **never asserted**, the bool stays false);
  `−1` → `exit-status(255)`, never a signal claim (rule 3, the
  SIGHUP collision accepted symmetrically per the mapping.md
  rationale); non-Unix adds no other negatives (rule 4); the PTY-path
  rule-5 fidelity limit documented in the module docs (signal-
  terminated PTY sessions arrive `exit-status(1)`, portable-pty's
  exit surface, upstream); unmapped negatives → 255 (rule 6). Sent
  **before** close, from the spawned task.
- **`env_request` stays declined** (OQ-09, unchanged from wave 2);
  `session_cwd`/`session_env` ride `NegotiateRequest` only.
- **Contract rows through the harness, per build**, with the wave-2
  stub rows (`pty_row_stub`/`shell_row_stub`/
  `pipe_row_stub`) replaced; the decline matrix's unset-field rows
  (feature on + handle unset → the rows decline; featured+set serves
  through the alkssh-side **TtyBackend shim double**, since alktty's
  `MockBackend`/`testing.rs` are `pub(crate)` + `cfg(test)` and so
  never importable; alkssh hand-rolls the shim, a backend that
  records `TtyParams`, emits scripted stdout/stderr/exit chunks and
  consumes stdin/resizes/signals, per the `TtyHandle` field shapes). The
  identity-landing item (the parked wave-2 note) asserted through the
  row that observes the identity at `drive_session_pre_negotiated`.
- **The real-tool smoke**: one OpenSSH pty session through the
  harness against the shim (or against a real PTY via a dev-dep
  `alktty` `local` composition; decide which posture and record; the
  real bash/`stty size` fidelity was POC stage D's, wave 5 re-lands it
  as e2e): the smoke owns the wire rows a plain rustsh-client row
  can't pin (echo paths, `stty size`-grade fidelity are wave-5's
  matrix; here the pty lifecycle rows only).

## Acceptance Criteria

- [x] pty/shell/pipe-exec rows serve through
      `drive_session_pre_negotiated` (spawned, handler fast); the
      store-and-take + per-channel PTY state reused from the git
      task's handoff; pipe-less shell decided + recorded
- [x] `pty_request` → `TerminalParamsWire` (ttymodes → the reserved
      JSON), `shell_request` → shell argv, exec → `wrapper ++ [cmd]`,
      cwd/env session-side; unset-declines on both OQ-08 rows with
      recorded wire shapes
- [x] The bridge translated per row (stdin chunks + EOF shape
      verified against alktty; stdout→data; stderr→`extended_data(1)`;
      resize/signal CTRL_IN reach the serving path through a bounded
      per-channel surface)
- [x] The derivation contract rules 1–6 in code with the signum→name
      table; `−1`→255; `core_dumped` never; PTY-path rule-5 documented;
      exit-status before close from the spawned task, pinned by rows
- [x] The decline matrix per build green through the harness (unset-
      field rows included); identity ride asserted; the `env_request`
      decline unchanged; skeleton stub rows migrated
- [x] The real-tool pty smoke row (posture decided + recorded)
- [x] `cargo test` (+ per-feature + `--all-features` rows), clippy ×2
      (`-D warnings`), fmt, `cargo doc --no-deps` pass

## References

- docs/architecture/mapping.md (the `session` channel-type row,
  request-level rows, §Exit chunk → SSH exit reporting, rules 1–6)
- docs/architecture/decisions/006-mapping-bridge-and-dispatch.md
  (decision 4, spawn-and-return: status before close from the spawned
  task), docs/architecture/decisions/003-feature-graph.md
- docs/architecture/auth.md §Server direction step 4 (the identity
  ride), §Dispositions (OQ-03)
- docs/research/poc-3-mapping-bakeoff.md §Stage D (the bridge shape,
  control rows, the signal/exit fidelity findings, §D.3 mechanics)
- docs/research/reverse-runner.md (the pipe-mode pid-only signal
  caveat, the fidelity note)
- alktty 0.8.1: `adapter.rs:234-241` (`drive_session_pre_negotiated`),
  `negotiation.rs` (`NegotiateRequest`/`TerminalParamsWire`),
  `wire.rs` (chunks, readers/writers), `control.rs` (`ControlMessage`),
  `backend.rs` (`TtyHandle`/`TtyParams`/`TtyBackend` shapes)

## Notes

### Decisions recorded (implementation, 2026-10-28)

- **The module** (`src/tty_map.rs`, feature `session`): the session
  rows live in one module following the `git_map` pattern, covering
  the bridge, the locally-built negotiation, the derivation contract,
  and
  the serving path. `serde_json` (the OQ-44 modes JSON + the exit
  chunk/error-frame reads) and `bytes` (the chunk plane) joined as
  session-gated optional deps at alktty's own pins.

- **The negotiation** (locally built, never wire-derived; OQ-08):
  `pty_request` args → `TerminalParamsWire` with SSH ttymodes → the
  **reserved `{}` modes JSON** (alktty OQ-44's reservation, deferred
  there; the map is honest and nothing is carried). Shell argv rides the
  serve-config **verbatim**; exec rides `wrapper ++ [cmd]` (never
  naively split); cwd/env come from `session_cwd`/`session_env` only.
  `backend_params` stays empty (no backend-specific selector fields on
  this door; the seam selector is the `backend` string).
  **Pipe-less shell decided: a `shell_request` without a preceding
  pty SERVES pipe-mode (`tty = None`)**, because vanilla sshd serves
  a shell without a terminal too (`ssh -T` is legitimate vanilla).

- **Unset-declines (OQ-08), wire shape decided + recorded**: the
  **serving-path shape (`channel_failure` then close, from a spawned
  task)** for `shell_argv`-unset (`shell-unconfigured`),
  `exec_wrapper_argv`-unset (`wrapper-unconfigured`), and the
  `session_seam`-unset rows (`seam-unconfigured`, the git row's
  recorded shape); the pty row checks the seam first (a doomed
  allocation is declined before its params are stored (decline first,
  alloc never). Field checks precede the seam check. The unvanilla
  guards keep the git row's shapes: `channel-not-held` → inline
  request-level failure; `no-identity` → the spawned serving-path
  decline. A routed request answers `channel_success` inline (the
  POC-3 §A.5 shape).

- **The translation bridge** (the POC-3 §D.1 shape, two adapters
  around a duplex pair): SSH `data` → `Chunk::stdin` through a bounded
  mpsc (bound 64) feeding a framing task; SSH channel EOF → the
  **zero-length stdin-chunk sentinel**, decided and verified against
  alktty 0.8.1's `pump_client_to_backend`: a zero-length stdin chunk
  shuts the backend's stdin, identical in effect to
  `ControlMessage::Eof`, and rides the data path the SSH EOF already
  chose (no extra control frame). stdout chunks → SSH `data`; stderr
  chunks → `extended_data(1)`; the exit chunk → the derivation
  contract. All buffers bounded (convention 9): mpsc 64, duplexes
  64 KiB, frames bounds-checked against alktty's `MAX_CHUNK_LEN`.

- **The negotiation-error frame is a serving-path decline class**
  (decided + recorded): alktty re-runs validation/allocation/ownership
  inline even on the pre-negotiated path and answers failures with a
  length-prefixed error frame (first byte `0x00`, which is alktty's
  ADR-052 §5 disambiguation invariant). The bridge reads it via a 4-byte
  first-window disambiguation and maps it to `negotiation-failed` +
  **`exit-status(255)` + close, the one place this door claims 255**:
  the session never started, nothing ran, so 255 is the honest
  could-not-start semantic (contrast the git row, whose service runs
  and honestly reports `1`; `255` is never fabricated for a real
  outcome). A bridge read ending **without** an exit chunk reports
  nothing (sole-fidelity-input posture: absence is neither 0 nor 255),
  and simply closes.

- **The control rows**: `window_change_request` →
  `ControlMessage::Resize`, `signal` → `ControlMessage::Signal`,
  forwarded through the **per-channel `BridgeSurface`** (handler
  state while the row serves): a bounded mpsc whose `try_send` is
  never awaited on the packet loop, so a full/closed surface drops
  the control event (fire-and-forget rows carry no SSH reply to decline
  with; bounded resources, convention 9). **Signal names are accepted
  verbatim, no decline shape**: the backend owns the rejection
  semantics, nothing on the wire can answer. A pre-serve resize
  updates the stored pty params instead (the final size wins,
  matching vanilla sshd's ordering).

- **The derivation contract** (mapping.md rules 1–6, in code + unit
  rows + wire rows): `derive_exit`, where `code ≥ 0` →
  `exit-status(N)`;
  negative `−N`, N ≥ 2 → `exit-signal(name)` via the crate-local
  [`SIGNAL_NAMES`] signum→name table (1–31, server-side work,
  deliberately not alktty's `signal_from_name` inverse); `−1` →
  `exit-status(255)` (rule 3, the SIGHUP collision accepted
  symmetrically); `core_dumped` **never asserted** (the bool stays
  false on every exit-signal row); unmapped negatives (and
  `i32::MIN`'s checked-abs failure) → 255 (rule 6); the PTY-path
  rule-5 fidelity limit documented in the module docs. Report sent
  **before** close from the spawned task; names ride russh's `Sig`
  (known variants verbatim, `Sig::Custom` for the rest; RFC 4254
  §6.10).

- **Cancellation** (bounded resources + alktty's kill-on-drop): the
  serving path watches two signals: the per-channel cancel (fired by
  `channel_close` via the surface's watch) and the connection
  teardown (fired by the handler's `Drop`; the russh session ended
  however it ended). On either: the serving task **aborts the alktty
  session task**, dropping its not-yet-resolved `exit_code` future,
  letting alktty's ADR-056 kill-on-drop contract clean up the
  cancelled session, then closes the channel and ends. Witnessed live by the
  shim's kill log row.

- **The contract tests** (`tests/tty_mapping.rs`, session-gated;
  `tests/common/tty_doubles.rs`): the shim double (hand-rolled per
  the `TtyHandle` field shapes, because alktty's `MockBackend` is
  `pub(crate)` and never importable) records TtyParams, consumes
  stdin/EOF/resizes/signals, emits scripted chunks and exit codes,
  and carries the kill-on-drop witness; the ownership witness double
  observes the identity at the seam. The wave-2 stub rows migrated:
  `dispatch_core.rs`'s session-build rows now pin the
  unset-decline reasons (`seam-unconfigured` / `wrapper-unconfigured`,
  same serving-path wire shape); `observe.rs`'s two session rows
  updated (and a one-line pre-existing clippy lint in its git-build
  cap row fixed). `env_request` decline unchanged (OQ-09).

- **The identity ride** (the parked wave-2 item, discharged): asserted
  through alktty's ADR-050 ownership check, with the shim's
  `resource_id` guaranteeing the check evaluates, and the ownership
  double records the resolved identity; the deny shape also pins the
  negotiation-error wire shape.

- **Real-tool smoke posture (recorded)**: one OpenSSH `-tt` pty
  session (`ssh -tt … smoke-rider@127.0.0.1` with piped stdin) through
  a TCP-mounted adapter against a **real PTY via a dev-dep alktty
  `local` composition**, the decided posture: a shim row cannot show
  what vanilla OpenSSH really sends, and POC-3 stage D proved exactly
  this composition. The dev-dep `features = ["local"]` unification
  applies to the test graph only (the published lib never enables it;
  `src/` never names a local shape). Requires ssh + sh; absent
  tooling is skipped **with a printed reason**, never silently
  green.
  The smoke pins the pty lifecycle (serve, stdin ride, clean exit →
  exit-status(0) before close); the `stty size`-grade fidelity matrix
  is wave 5's.

- **Verification**: `cargo test` (default), `cargo test --features
  session` (incl. the smoke + shim rows), `cargo test --features git`,
  `cargo test --features tunnel`, `cargo test --all-features`;
  `cargo clippy --all-targets -- -D warnings` ×5 (default / session /
  git / tunnel / all-features); `cargo fmt --check`; `cargo doc
  --no-deps` (default + session + all-features); all green.