feat: channels-path establisher migration (alkcall 0.5.0 / ADR-049) — bump to 0.2.0
Adopt alkcall 0.5.0's channel-open establishment phase (ADR-049 — review 006 E-01 + N-1) and migrate the channels-path semantic failures per its §5 sequencing (alktty ADR-010). - `register_openable` registers `channels/tty/sub` with an establisher (`register_openable_with_establisher`): full `NegotiateRequest` parse of schema-valid `input`, `carriage == "raw"`, non-empty `cmd`, backend lookup, and the ADR-050 ownership check run before the open reply; rejections are `channel:open_failed` with `details.reason` (`unknown_resource` / `handler_error` / `timeout`) — no phantom channel (the SSH contract holds consumer-visibly) - `backend.allocate` deliberately stays in the pump handler: `Establishment` is payloadless so the `TtyHandle` cannot cross the establisher→handler boundary, and re-allocating would violate ADR-005's kill-on-Drop contract — `allocate_failed` remains the one in-band failure class on the channels path (pinned by test) - `TtySessionError::ChannelsOpen` carries alkcall's typed `ChannelOpenError` (`#[from]`) instead of a flattened `String` — the N-1 fix at alktty's layer (breaking) - channels-path semantic failures change shape from `NegotiationRejected` in-band frames to `channel:open_failed` call errors (breaking); the direct-ALPN path is unchanged - `tty_open_spec()` gains a `description` (review 006 E-02) and an ErrorDefinition for `channel:open_failed` (ADR-016 — disclosed via services/schema) - alkcall = "0.5.0"; version 0.2.0; ADR-010 + ADR-009 amendment + tty-adapter.md + CHANGELOG Verification: cargo test (112 lib + integration), cargo test --all-features (136), clippy --all-targets -D warnings (host + wasm), fmt --check, cargo doc --no-deps clean; wasm32-unknown-unknown check confirms the default crate stays wasm-clean.
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# ADR-010: Channels-Path Establishment Failures Move into an Establisher
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## Status
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Accepted (2026-09-06). Migrates the channels-path semantic-failure
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surface per alkcall 0.5.0 / ADR-049 (review 006 E-01 + N-1) — the
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sequencing ADR-049 §5 names ("alktty migration — channels-path
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semantic failures move into an establisher; the direct-ALPN in-band
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error frame is retained — two transports, two contracts").
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Prerequisites: alkcall 0.5.0 (`ChannelCore::register_openable_with_establisher`,
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`channel:open_failed`, typed `ChannelOpenError`).
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Amends ADR-009's R4 amendment: the in-band error-frame path on the
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channels path shrinks to one failure class (`allocate_failed`).
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## Context
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ADR-009 made the open op's `input` the negotiation and moved semantic
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validation into the producer-side handler. Because alkcall 0.4.x's
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open wrapper could not fail after allocation (review 006 E-01), the
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semantic-failure classes had nowhere to go but the channel stream:
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the handler wrote a `0x00`-prefixed negotiation error frame
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(`malformed_negotiation`, `unknown_backend`, `allocate_failed`,
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ownership-denial `forbidden`) on a channel the open op had already
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reported as succeeding. The consumer observed success-then-frame —
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a per-crate workaround ADR-049 explicitly retires:
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> alktty's per-crate in-band error vocabulary is retired on the
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> channels path; every future ALPN crate gets the establishment reply
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> for free.
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alkcall 0.5.0 implements the resolution: the wrapper awaits a bounded
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establisher (`OpenEstablisher` — `Fn(Value, AuthContext) -> BoxFuture<
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Result<Establishment, EstablishmentError>>`) between channel
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allocation and the open reply; on rejection it tears the channel down
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and replies `channel:open_failed` with `details: { reason, message }`
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(reason ∈ `dial_failed` / `unknown_resource` / `resource_shortage` /
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`handler_error` / `timeout`). The SSH contract holds
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consumer-visibly: a failed open never returns a `channel_id`.
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`ChannelClient::open_channel` carries the `CallError` verbatim in a
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typed `ChannelOpenError` (review 006 N-1), so the reason is
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branchable end-to-end.
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## Decision
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### 1. The channels path registers an establisher
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`register_openable` (alktty's helper, signature unchanged) now calls
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`ChannelCore::register_openable_with_establisher` with
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`make_tty_establisher` — the establishment phase runs the semantic
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validation that used to be post-open error frames:
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- Full `NegotiateRequest` parse of the registry-schema-validated
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`input` (the schema stays deliberately partial — the opaque ADR-053
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backend params pass through; the establisher is the typed gate).
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- `carriage == "raw"`, non-empty `cmd`.
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- Backend lookup against the adapter's backend map.
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- The ADR-050 ownership check (moved from the pump handler's
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`validate_and_allocate`; the scope gate stays the registry's —
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pre-allocation, unchanged).
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Rejections map into ADR-049's fixed reason vocabulary (no new codes —
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the vocabulary is alkcall's one-way wire surface):
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| TTY failure class | `EstablishmentError` variant | `details.reason` |
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|---|---|---|
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| `NegotiateRequest` parse failure | `HandlerError` | `handler_error` |
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| `carriage != "raw"`, empty `cmd` | `HandlerError` | `handler_error` |
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| unknown backend | `UnknownResource` | `unknown_resource` |
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| ownership denial | `HandlerError` | `handler_error` |
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| establishment deadline | (wrapper) | `timeout` |
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`dial_failed` and `resource_shortage` have no TTY producer path (TTY
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dials nothing at open time; `allocate` capacity failures are the
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in-band class below) — the spec's `ErrorDefinition` for
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`channel:open_failed` declares only the three reachable reasons.
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`HandlerError` covers both the malformed-negotiation semantics (the
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request shape is wrong) and the ACL-outcome ownership denial (an
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authorization failure, not a dial failure).
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### 2. Backend allocation stays in the pump handler — `allocate_failed` stays in-band
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The one failure class that cannot migrate: `backend.allocate`. Two
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reasons, both structural:
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1. **`Establishment` is payloadless** ("reserved for a channel plan").
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There is no sound way to hand the allocated `TtyHandle` from the
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establisher to the pump handler: the establisher and handler are
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separate closures registered once per connection (no per-open key
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is shared between them), and an `Arc<Mutex<Option<TtyHandle>>>`
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stash would need exactly that key to be race-free.
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2. **Double-`allocate` is unsound under ADR-005's kill-on-`Drop`**
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contract: re-allocating in the handler after an establisher-side
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allocate would spawn the session target twice (the first handle's
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kill-guard would fire on its drop — a session killed before it
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started), and leaking the establisher-side handle would leak the
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live target.
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So `backend.allocate` runs where it always did — inside
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`drive_session_pre_negotiated`'s `validate_and_allocate` — and
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`allocate_failed` remains a post-open `0x00`-prefixed negotiation
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error frame, surfaced consumer-side as
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[`TtySessionError::NegotiationRejected`] via the same peek the direct
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path uses. A pinned test (`allocate_failure_still_arrives_in_band_on_
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channels_path`) guards this boundary. Revisit when alkcall gives
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`Establishment` a payload (a channel plan carrying the handle would
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let `allocate_failed` join the call-error surface).
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The pump handler's parse arm (`malformed_negotiation` frame) and
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`validate_and_allocate`'s other error frames are now defense-in-depth:
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the establisher rejects those classes first, so on the registered
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path only `allocate_failed` is reachable. The arms stay (a
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no-establisher registration still compiles against alkcall's API and
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must not silently EOF), and the R4 seam test pins the handler-side
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frame.
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### 3. The direct-ALPN path is unchanged
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`TtyAdapter::handle` / `drive_session` keep the wire-frame
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negotiation (ADR-001) and the full in-band error-frame vocabulary
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(`unknown_backend`, `malformed_negotiation`, `allocate_failed`,
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`forbidden`). Two transports, two contracts — the direct path has no
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open op to fail, and its `0x00`-peek disambiguation is the ADR-001
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wire-stable contract. The `enforce_scope` split from ADR-009 is
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unchanged (`true` direct, `false` channels — the registry's
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`AccessControl` is the channels scope gate).
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### 4. Spec enrichment (review 006 E-02 + ADR-016)
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`tty_open_spec()` gains:
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- `description` — disclosed by `services/list` (the open op describes
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itself; the produced resource set stays OQ-40's deferred
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`channel/resources/subscribe` shape).
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- An `ErrorDefinition` for `channel:open_failed` with the
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`details.reason` enum — so `services/schema` discloses the
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establishment-failure contract per ADR-016 (ADR-049 §3: "ALPN
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crates' open-op specs gain matching `ErrorDefinition` entries so
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`services/schema` discloses the failure contract").
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### 5. Consumer surface
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`TtySession::open_via_channels` failures after this ADR:
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| Failure | Where | Consumer-visible as |
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|---|---|---|
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| schema-invalid params | registry gate | `ChannelsOpen(CallFailed{ INVALID_INPUT })` |
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| unparseable params (local fail-fast) | pre-open local parse | `InvalidParams` |
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| malformed negotiation (schema-valid) | establisher | `ChannelsOpen(CallFailed{ channel:open_failed, reason: handler_error })` |
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| unknown backend | establisher | `ChannelsOpen(CallFailed{ channel:open_failed, reason: unknown_resource })` |
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| ownership denial | establisher | `ChannelsOpen(CallFailed{ channel:open_failed, reason: handler_error })` |
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| establishment timeout | wrapper bound | `ChannelsOpen(CallFailed{ channel:open_failed, reason: timeout })` |
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| ACL denial (scope) | registry gate | `ChannelsOpen(CallFailed{ FORBIDDEN })` |
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| channel cap | wrapper `check_open` | `ChannelsOpen(CallFailed{ channel:too_many_channels })` |
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| **allocate failure** | pump handler, post-open | **`NegotiationRejected{ "allocate_failed" }`** |
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`TtySessionError::ChannelsOpen` now carries alkcall's typed
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`ChannelOpenError` verbatim (`#[from]`) instead of a flattened
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`String` — the N-1 fix applied at alktty's layer, so consumers branch
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on `establishment_reason()` without unwrapping strings.
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## Consequences
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**Positive:**
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- The phantom-channel workaround is retired: a semantically invalid
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open no longer allocates → succeeds → in-band-fails. Ledger, policy
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count, and manager state balance on every rejection.
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- Retry policy / UX can branch: `unknown_resource` (bad config — don't
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retry), `timeout` (maybe retry), `allocate_failed` (capacity —
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still distinguishable in-band).
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- The consumer's `open_via_channels` failure is a typed call error —
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no peeking at the data stream for anything but `allocate_failed`.
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**Negative:**
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- **Breaking (0.2.0):** channels-path semantic failures change shape
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(`NegotiationRejected{unknown_backend}` → `ChannelsOpen(CallFailed{
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channel:open_failed})`), and `ChannelsOpen`'s payload changed. The
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direct path's failure surface is unchanged.
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- The `channel:open_failed` reason codes the establisher emits join
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the wire-stable error set (ADR-049's one-way door).
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- `allocate_failed` remains a second, in-band failure shape on the
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channels path — one residual asymmetry, pinned and documented until
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`Establishment` gains a payload.
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## Door type
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**One-way (wire-visible).** The establisher mapping fixes
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`details.reason` values for TTY's channels path — consumers branch on
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`unknown_resource` / `handler_error` / `timeout`, so re-mapping a
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failure class between reasons is a peer-breaking change. The
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establisher's internal validation order is a two-way door
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(implementation detail behind the fixed surface). ADR-009's
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"two-way for now" negotiation-location decision is unaffected — this
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ADR moves failure *reporting*, not the negotiation *location*.
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## References
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- alkcall ADR-049 (the establisher, `channel:open_failed`, the typed
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client error — the upstream capability this ADR adopts)
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- alkcall review 006 (E-01 the establishment gap; N-1 the typed
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client error; E-02 the discovery enrichment this ADR's spec
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enrichment adopts)
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- ADR-009 (the open op's `input` is the negotiation; the R4
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amendment's in-band error frames — shrunk to `allocate_failed` by
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this ADR)
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- ADR-005 (kill-on-`Drop` — why establisher-side allocation is
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unsound)
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- ADR-001 (the §5 `0x00` disambiguation — retained for the direct
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path and the channels `allocate_failed` frame)
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- ADR-002 (backend selection/allocation ownership)
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- `src/channels.rs` — `make_tty_establisher`, `make_tty_open_handler`,
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`tty_open_spec`
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- `src/session.rs` — `TtySessionError::ChannelsOpen`, `open_via_channels`
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- `src/testing.rs` — the pinned `allocate_failed`-stays-in-band test
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