alktty/docs/architecture/decisions/002-ttybackend-trait-and-ttyhandle.md

# ADR-002: TtyBackend Trait and TtyHandle — the Backend Inversion Point

## Status

Accepted (ported from alknet ADR-053 2026-08-17; cross-references
renumbered to alktty's ADR range — ADR-052→001, ADR-053→002,
ADR-054→003, ADR-055→004, ADR-056→005, ADR-057→006, ADR-077→007,
ADR-093→008. Alknet ADRs referenced by alknet number (003, 007, 017,
050) are not ported into alktty's ADR range because they are not
tty-specific; the alknet originals at
`/workspace/@alkdev/alknet/docs/architecture/decisions/` remain
authoritative.)

## Context

alktty's wire format (ADR-001) is backend-agnostic — the chunk codec
pumps bytes and JSON control messages without knowing whether the
backend is a docker container, an SSH session channel, or a local
process. The question this ADR answers: **what is the seam between the
wire-format adapter and the backends?**

The alknet-tty research identified the `TtyBackend` trait as the
inversion point. The guiding insight:

> A terminal session is not an SSH concern, or a Docker concern — it is
> a terminal concern. SSH and Docker are just two backends that can
> allocate a PTY.

The trait is what makes that insight load-bearing: alktty defines the
trait and the wire-format adapter; the backend crates (alknet-docker,
alknet-ssh, alktty's `local` feature module) implement the trait. This
preserves alknet ADR-003's no-handler-depends-on-another-handler rule:
alktty depends on alkcall; backend crates depend on alktty for the
trait; alktty does not depend on any backend (and, per ADR-006, does
not depend on alkcall's internal wire types either — the negotiation
framing is self-contained).

### What the local-PTY POC discovered about the trait shape

The Phase 0 POC was built *before* this ADR specifically to discover
constraints the trait sketch would have missed by reading docs alone.
Two requirements fell out of it (recorded as REQ-TTY-01 and REQ-TTY-02
in the alknet findings doc):

- **REQ-TTY-01: backends are not required to be natively async.**
  `portable_pty` is a blocking `std::io::{Read, Write}` API with a
  blocking `Child::wait()`. The POC bridges it to async via three
  dedicated std threads (reader, writer, waiter) feeding tokio
  mpsc/oneshot channels — the same pattern wezterm (portable_pty's
  primary consumer) uses. The trait's adapter-facing types
  (`AsyncWrite`, `Stream<Item = Bytes>`, `BoxFuture`) are the
  *adapter's* contract; a backend may expose blocking handles
  internally and bridge them. The bridging pattern is a documented,
  supported implementation strategy, not a workaround.

- **`exit_code` is a `Future` the adapter awaits, not a method on
  `TtyHandle`.** A `oneshot::Receiver<i32>` (or any
  `BoxFuture<'static, i32>`) lets the adapter `select` between exit and
  stream-close without coupling to the handle's other fields. The local
  backend's waiter thread produces exactly this shape for free.

The POC's `LocalPty` struct is the reference implementation of what a
backend produces: `stdout: mpsc::Receiver<Bytes>`,
`stdin: mpsc::Sender<StdinCmd>`, `control: PtyControl` (Clone),
`exit_code: oneshot::Receiver<i32>`. The trait shape below generalizes
this.

### What the local-PTY POC did not resolve

The POC used a separate cloneable `PtyControl` struct for resize/signal,
not a trait object. The research noted this worked cleanly because the
control-chunk dispatcher needs to be `Clone` to hand off to the spawned
pump task. Phase 1 confirms the `control` field as a separate
`TtyControlHandle` newtype — a concrete `#[derive(Clone)]` struct
wrapping `Arc<dyn TtyControl + Send + Sync>` (the trait is NOT `Clone`;
`Clone` is not object-safe — see OQ-43). The newtype carries the
`Clone`-ability; the trait stays object-safe.

## Decision

### 1. `TtyBackend` trait

```rust
#[async_trait]
pub trait TtyBackend: Send + Sync {
    /// Allocate a terminal/process session and return the handles the
    /// adapter pumps. The `backend` field of the negotiation frame
    /// (ADR-001) selects which registered backend's `allocate` is called.
    async fn allocate(&self, params: &TtyParams) -> Result<TtyHandle, TtyError>;

    /// The pre-existing resource this session targets, for ownership
    /// checks (alknet ADR-050). `None` = no pre-existing resource (the session
    /// creates its own — local process, SSH channel). `Some((kind, id))`
    /// = the session targets an existing resource the caller must own
    /// (e.g., DockerTtyBackend returns `Some(("container", id))`). The
    /// adapter calls this at negotiation to gate access; the backend
    /// extracts the id from its own `backend_params`. Default `None`
    /// (most backends create their own resource).
    fn resource_id(&self, _params: &TtyParams) -> Option<(&'static str, String)> { None }
}
```

The adapter holds a `HashMap<String, Arc<dyn TtyBackend>>` keyed by the
negotiation frame's `backend` string (`"local"`, `"docker"`, `"ssh"`).
The assembly layer registers backends at startup; the adapter dispatches
by the `backend` field. A backend is the *thing that allocates a
session*; the wire-format pump is backend-agnostic.

### 2. `TtyParams` — the allocation request

```rust
pub struct TtyParams {
    /// Terminal parameters. `None` = pipe mode (no PTY — the runner case,
    /// ADR-003). `Some` = allocate a PTY with these dimensions.
    pub terminal: Option<TerminalParams>,
    /// Command vector (argv[0] + args).
    pub cmd: Vec<String>,
    /// Working directory (backend-specific; None = inherit/default).
    pub cwd: Option<PathBuf>,
    /// Environment variables (backend-specific; empty = inherit).
    pub env: HashMap<String, String>,
    /// Backend-specific selector fields from the negotiation frame,
    /// unparsed. The adapter passes the JSON object through verbatim; the
    /// backend deserializes its own strongly-typed params struct from it.
    /// alktty has zero knowledge of any backend's params shape. See
    /// §"Backend params are opaque" below.
    pub backend_params: serde_json::Map<String, serde_json::Value>,
}

pub struct TerminalParams {
    pub term: Option<String>,      // e.g., "xterm-256color"; None = backend default
    pub cols: u16,
    pub rows: u16,
    pub pixel_width: u16,
    pub pixel_height: u16,
    pub modes: serde_json::Value,  // reserved — OQ-44; backends MUST ignore content in v1
}
```

`terminal: None` is the pipe/runner case — no PTY, separate
stdout/stderr (ADR-003). `terminal: Some` is the PTY case —
stdout/stderr merged into the single stdout stream
(`TtyHandle.stderr` is `None`), real terminal semantics (resize, signal
delivery to process group).

### Backend params are opaque

`backend_params` is a `serde_json::Map<String, serde_json::Value>`, not
a typed enum. The adapter passes the negotiation frame's
backend-specific fields through verbatim; the backend deserializes its
own strongly-typed params struct. alktty has zero knowledge of any
backend's params shape — not docker's `container`, not an SSH host
selector, not anything.

Each backend defines its own params struct:

```rust
// in alknet-docker
#[derive(Deserialize)]
struct DockerBackendParams { container: String }

// in alknet-ssh
#[derive(Deserialize)]
struct SshBackendParams { /* host selector if multi-host; else empty */ }

// in alktty's local feature module
// no backend-specific params — backend_params is empty
```

And deserializes from `params.backend_params` inside `allocate()`:

```rust
impl TtyBackend for DockerTtyBackend {
    async fn allocate(&self, params: &TtyParams) -> Result<TtyHandle, TtyError> {
        let p: DockerBackendParams = serde_json::from_value(
            serde_json::Value::Object(params.backend_params.clone())
        ).map_err(|e| TtyError::Backend { message: e.to_string() })?;
        // use p.container ...
    }
}
```

This is a complete inversion: the *trait* is inverted (backends
implement, alktty doesn't depend on them) and the *params* are inverted
(backends define their own typed shape, alktty doesn't carry it). A new
backend crate requires zero changes to alktty — no enum variant to add,
no forward-reference type to place, no dependency edge.

**Why not a typed enum.** The earlier draft of this ADR defined
`BackendParams` as a `#[non_exhaustive]` enum with `Local`, `Docker {
container }`, and `Ssh { channel: SshChannelRef }` variants. Three
problems:

1. **Rust enums are closed.** `#[non_exhaustive]` prevents *consumers*
   from matching exhaustively, but only the *defining crate* (alktty)
   can add variants. A backend crate cannot add a variant; every new
   backend requires modifying alktty. The inversion is only partial.
2. **`SshChannelRef` was an output, not an input.** The SSH channel is
   what `allocate()` *opens* (`session.channel_open_session()` →
   `pty_request` → `shell_request`). It doesn't exist until the backend
   creates it; the client doesn't send one.
3. **Dependency contradiction.** `SshChannelRef` "wraps a russh
   `ChannelId` and session reference." If it lives in alknet-ssh,
   alktty depends on alknet-ssh (violates the inversion). If it lives in
   alktty, alktty pulls in russh types (same violation). Opaque params
   dissolve the contradiction — there is no `SshChannelRef` type in
   alktty at all.

The earlier draft rejected `serde_json::Value` because "it loses type
safety and forces the adapter to parse backend-specific JSON it
shouldn't interpret." The first concern doesn't apply (each backend has
its own strongly-typed struct via serde; type safety moves from alktty
to the backend where it belongs). The second was already inconsistent
with the adapter, which hardcoded extraction of docker's `container`
field for the ownership check — the adapter *was* parsing
backend-specific JSON. The opaque approach removes that: the adapter
delegates the resource-id extraction to the backend via `resource_id()`
below.

### 3. `TtyHandle` — what a backend produces

```rust
pub struct TtyHandle {
    /// Stdin writer — bytes the adapter pumps from client stdin chunks.
    /// `tokio::io::AsyncWrite` (the tokio flavor, not the `futures::io`
    /// one — they are incompatible traits; the tokio stack is the
    /// adapter's runtime).
    pub stdin: Box<dyn tokio::io::AsyncWrite + Send + Unpin>,
    /// Stdout stream — bytes the adapter pumps to client stdout chunks.
    /// Ends when the backend's stdout reaches EOF (process exited,
    /// container output stream ended, SSH channel closed).
    /// `futures_core::Stream<Item = bytes::Bytes>` (re-exported by
    /// `tokio_stream::StreamExt` for extension methods).
    pub stdout: Pin<Box<dyn futures_core::Stream<Item = bytes::Bytes> + Send>>,
    /// Stderr stream — `None` for PTY backends (stdout/stderr merged
    /// into `stdout`). `Some` for pipe backends (separate streams).
    pub stderr: Option<Pin<Box<dyn futures_core::Stream<Item = bytes::Bytes> + Send>>>,
    /// Exit code — a `Future` the adapter awaits. Resolves when the
    /// process/container/SSH exec exits. The adapter sends the result
    /// as the `{"type":"exit","code":N}` control chunk (ADR-004) and
    /// closes the stream. This is `BoxFuture`, not a method on
    /// `TtyHandle`, so the adapter can `select` between exit and
    /// stream-close without coupling to the other fields. (REQ-TTY-01.)
    pub exit_code: BoxFuture<'static, Result<i32, TtyError>>,
    /// Control handle (resize, signal) — `Clone` so the adapter can
    /// hand it to the spawned control-chunk dispatcher. `None` only
    /// when the backend genuinely has no control path (e.g., a pipe
    /// backend with no PTY — signal still works via `kill(pid, sig)`,
    /// but resize is a no-op). See OQ-43 for the `TtyControlHandle`
    /// newtype rationale.
    pub control: Option<TtyControlHandle>,
}

pub trait TtyControl: Send + Sync {
    /// Resize the terminal. Maps to SSH `window-change`, docker exec
    /// resize, or `ioctl(TIOCSWINSZ)` on a local PTY. No-op for pipe
    /// backends without a PTY (the adapter still calls it; the backend
    /// ignores).
    fn resize(&self, cols: u16, rows: u16, pixel_width: u16, pixel_height: u16);

    /// Forward a signal by name. Best-effort delivery to the foreground
    /// process group (see tty-local.md REQ-TTY-02). Unknown names fall
    /// back to the backend's default kill.
    fn signal(&self, name: &str);
}

/// The `Clone`-able handle to a backend's control path. The `TtyControl`
/// trait is NOT `Clone` (`Clone` is not object-safe — `fn clone(&self) ->
/// Self` returns `Self`, which forbids `dyn` dispatch); the `Clone`-ability
/// lives on this concrete newtype, which holds the trait object behind an
/// `Arc`. The adapter clones the `Arc` to hand a handle to the spawned
/// control-chunk dispatcher. See OQ-43.
#[derive(Clone)]
pub struct TtyControlHandle(Arc<dyn TtyControl + Send + Sync>);
```

### 4. Backends are not required to be natively async (REQ-TTY-01)

The trait's adapter-facing types (`AsyncWrite`, `Stream<Item = Bytes>`,
`BoxFuture`, the `TtyControl` trait object) are the **adapter's
contract**. A backend may expose blocking handles internally and bridge
them to these async-facing types. The bridging pattern — blocking
`std::io` on dedicated std threads or `tokio::task::spawn_blocking`,
feeding tokio mpsc/oneshot channels — is a **documented, supported
implementation strategy**, not a workaround.

The local backend (ADR-003) uses this pattern: `portable_pty` is a
blocking API, and the backend's `allocate()` spawns reader/writer/waiter
threads that feed `mpsc::Receiver<Bytes>` (stdout), `mpsc::Sender<StdinCmd>`
(stdin wrapped as `AsyncWrite`), and `oneshot::Receiver<i32>` (exit).
The adapter consumes the bridged async-facing types and is unaware of
the threading. See `tty-local.md` for the bridge details.

### 5. Backend registration and the assembly layer

The `TtyAdapter` does not know the set of backends at compile time — it
holds a `HashMap<String, Arc<dyn TtyBackend>>` populated at
construction. The assembly layer (the CLI binary) constructs backends
with their dependencies (a `DockerTtyBackend` wraps a `bollard::Docker`
client; an `SshTtyBackend` wraps an SSH session; a `LocalTtyBackend`
takes no deps) and registers them:

```rust
let mut backends = HashMap::new();
backends.insert("local".into(), Arc::new(LocalTtyBackend::new()) as Arc<dyn TtyBackend>);
backends.insert("docker".into(), Arc::new(DockerTtyBackend::new(docker_client)) as _);
backends.insert("ssh".into(), Arc::new(SshTtyBackend::new(ssh_session)) as _);
let tty_adapter = TtyAdapter::new(Arc::new(backends));
```

A deployment that doesn't want docker registers only `local`. A browser
terminal endpoint that proxies to remote docker/ssh registers `docker`
and/or `ssh` backends. The adapter is backend-agnostic; the assembly
layer chooses what's available.

## Consequences

**Positive:**

- The wire-format adapter is backend-agnostic and testable with a mock
  backend (in-memory pipes). The build order (ADR-001 wire format +
  mock backend first, real backends last) follows directly.
- alktty stays dependency-light: no bollard, no russh, no `portable_pty`
  in the default crate. The heavy deps live in the backend crates /
  the `local` feature. This is the same inversion as
  `OperationAdapter` (alknet ADR-017): the trait lives where the types
  live; the implementations live where their transport dependencies
  live.
- Blocking-API backends (portable_pty) are first-class — the trait
  accommodates them by making the adapter-facing types the contract and
  the bridging pattern a documented strategy. No re-spec required when
  a future backend is also blocking.
- `exit_code` as a `Future` (not a method on the handle) lets the
  adapter `select` between exit and stream-close — the load-bearing
  composition the session lifecycle needs (the exit chunk is sent after
  the child is reaped, then the stream closes — ADR-004).

**Negative:**

- **Backend params are opaque (`serde_json::Map`), not a typed enum.**
  Each backend deserializes its own params struct; the adapter passes
  the JSON through verbatim. The cost is one serde deserialize per
  `allocate()` call (negligible — allocation is once per session, not on
  the hot path). The benefit is a complete inversion: alktty has zero
  knowledge of any backend's params shape, and a new backend crate
  requires zero changes to alktty (no enum variant, no forward-reference
  type). See §"Backend params are opaque" for the full rationale and
  why the typed-enum alternative was rejected (Rust enums are closed;
  the earlier `SshChannelRef` variant was an output modeled as an input
  and created a dependency contradiction).
- **`TtyControl` is not `Clone`; the `TtyControlHandle` newtype is.**
  `Clone` is not object-safe (`fn clone(&self) -> Self` returns `Self`,
  which forbids `dyn` dispatch), so `Box<dyn TtyControl + Clone>` does
  not compile. The design splits the concerns: the `TtyControl` trait
  stays object-safe (`Send + Sync`, no `Clone`); the `TtyControlHandle`
  newtype (a concrete struct holding `Arc<dyn TtyControl + Send +
  Sync>`) implements `Clone` by cloning the `Arc`. This is the cost of
  the POC-discovered constraint that the control-chunk dispatcher needs
  to be `Clone` to hand off to the spawned pump task. See OQ-43 for the
  confirmation and the concrete newtype approach.
- A backend that produces neither a PTY nor a process (a hypothetical
  "recorded session replay" backend) would have a no-op `TtyControl`
  and a synthetic `exit_code`. The trait accommodates it but the
  `TtyParams` shape (`cmd` is `Vec<String>`, `terminal` is `Option`)
  assumes command-spawning. A non-command backend would supply an empty
  `cmd` and synthesize one internally. Not a current use case; the
  trait shape doesn't preclude it but doesn't optimize for it.

## Door type

**One-way.** The `TtyBackend` trait method `allocate()`, the
`TtyHandle` field set, and the `TtyControl` trait are the API surface
every backend crate implements and the adapter consumes. Changing them
after backends exist is a rewrite across crates. `backend_params` as an
opaque `serde_json::Map` is part of the one-way `TtyParams` shape — the
*carrier* is fixed (opaque JSON), but the *contents* are
backend-defined and require no alktty change for new backends. The
`resource_id()` default method is additive (a new method with a default
impl doesn't break existing implementors); its return type
`Option<(&'static str, String)>` is one-way.

## Assumptions

1. **The `TtyControl` trait is kept object-safe by NOT putting `Clone`
   on it; the `TtyControlHandle` newtype holds the trait object behind
   an `Arc` and implements `Clone` by cloning the `Arc`.** The POC used
   a concrete `PtyControl` struct (inherently `Clone` — it held
   `Arc<Mutex<...>>` fields). The newtype generalizes the POC's shape so
   a backend produces its own control type via
   `TtyControlHandle::new(Arc::new(MyControl))` without the adapter
   knowing the concrete shape. `Clone` cannot live on the trait itself
   (it is not object-safe); the newtype is the seam. OQ-43 confirms.

2. **Backends produce a single session per `allocate()` call.** The
   adapter calls `allocate()` once per accepted bidi stream (one session
   per stream — ADR-001). A backend that multiplexed multiple sessions
   over one `allocate()` would not fit the trait; no such backend is
   contemplated.

3. **The adapter, not the backend, owns the exit-chunk ordering.** The
   backend resolves `exit_code`; the adapter awaits it, sends the exit
   control chunk, and closes the stream (ADR-004). The backend does not
   write to the wire — it produces handles; the adapter pumps. This
   keeps the wire-format logic in one place (the adapter) and the
   backend focused on its allocation target (docker, ssh, local
   process).

## References

- alknet ADR-003 + Amendments 1 & 2 — no-handler-depends-on-another-
  handler; alktty depends on alkcall (no alkcall-internal-wire-types per
  Am. 2 / ADR-006); backends depend on alktty for the trait
- [ADR-006](006-negotiation-framing-self-contained.md) — alktty does not
  depend on alkcall's internal wire types (self-contained negotiation
  framing)
- alknet ADR-007 — `Connection`, `SendStream`, `RecvStream` (the adapter
  receives a `Connection`, accepts bidi streams, pumps per-session)
- [ADR-001](001-wire-format-and-two-carriage.md) — the wire format this
  trait's backends feed
- [ADR-003](003-local-backend-placement.md) — the local backend's
  module placement (folded into alktty behind a `local` feature)
- [ADR-004](004-exit-code-on-control-chunk.md) — the exit chunk ordering
  this trait's `exit_code` field feeds into
- [ADR-005](005-backend-cleanup-on-session-cancel.md) — the cancel-
  cleanup contract on this trait (`exit_code` future's `Drop`-on-cancel
  kills the session target)
- alknet ADR-017 — the adapter-location-map pattern (trait where types
  live, implementation where deps live) this ADR follows
- alknet ADR-050 — the ownership model the `resource_id()` default
  declares against
- OQ-43 — `TtyControl` as `Clone` trait object (resolved: confirmed)
- OQ-44 — terminal modes (deferred(scope): not needed for current scope)
- Spec: [tty-backend.md](../tty-backend.md)
- Port origin: alknet ADR-053 at
  `/workspace/@alkdev/alknet/docs/architecture/decisions/053-ttybackend-trait-and-ttyhandle.md`