alknet/tasks/tty-local/backend-impl.md

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

id	name	status	depends_on	scope	risk	impact	level
tty-local/backend-impl	Implement LocalTtyBackend (TtyBackend) branching on terminal Some/None	completed	tty-local/pty-mode tty-local/pipe-mode	narrow	low	component	implementation

Description

Implement LocalTtyBackend in src/backend.rs: the TtyBackend implementation that branches on TtyParams.terminal to dispatch to PTY mode (task tty-local/pty-mode) or pipe mode (task tty-local/pipe-mode).

LocalTtyBackend

pub struct LocalTtyBackend;

impl LocalTtyBackend {
    pub fn new() -> Self;
}

#[async_trait]
impl TtyBackend for LocalTtyBackend {
    async fn allocate(&self, params: &TtyParams) -> Result<TtyHandle, TtyError> {
        match &params.terminal {
            Some(terminal) => pty::allocate_pty(
                terminal.clone(),
                params.cmd.clone(),
                params.cwd.clone(),
                params.env.clone(),
            ).await,
            None => pipe::allocate_pipe(
                params.cmd.clone(),
                params.cwd.clone(),
                params.env.clone(),
            ).await,
        }
    }

    fn resource_id(&self, _params: &TtyParams) -> Option<(&'static str, String)> {
        None  // local backend creates its own resource (process); no pre-existing resource
    }
}

LocalTtyBackend takes no constructor dependencies (unlike DockerTtyBackend which wraps a bollard::Docker client). The portable_pty system is process-global. The assembly layer constructs one LocalTtyBackend and registers it as "local".

Validation

allocate() validates params.cmd is non-empty (the adapter already checks this at negotiation, but the backend should fail gracefully if called directly with an empty cmd — return TtyError::AllocFailed). The backend_params map is ignored by the local backend (it has no backend-specific selector fields); if a caller passes unexpected backend_params, they are silently ignored (the local backend doesn't define a typed params struct).

Default trait methods

resource_id() returns None (the local backend creates its own resource — a process — so there's no pre-existing resource for the ownership check). The default impl on TtyBackend already returns None, so this can be omitted unless an explicit impl is preferred for clarity.

Tests

• PTY dispatch: allocate with terminal: Some → returns a TtyHandle with stderr: None and a PtyControl.
• Pipe dispatch: allocate with terminal: None → returns a TtyHandle with stderr: Some and a PipeControl.
• Empty cmd: allocate with empty cmd → TtyError::AllocFailed.
• resource_id: returns None.
• These can be lightweight (the heavy tests are in pty-mode and pipe-mode); this task verifies the dispatch wiring.

Acceptance Criteria

• LocalTtyBackend struct with new() constructor (no deps)
• impl TtyBackend for LocalTtyBackend with allocate() branching on params.terminal
• terminal: Some → dispatches to pty::allocate_pty
• terminal: None → dispatches to pipe::allocate_pipe
• resource_id() returns None (or uses the default)
• Empty cmd → TtyError::AllocFailed
• LocalTtyBackend re-exported from lib.rs (pub use backend::LocalTtyBackend)
• Unit tests: PTY dispatch, pipe dispatch, empty cmd, resource_id
• cargo test -p alknet-tty-local succeeds
• cargo clippy -p alknet-tty-local succeeds with no warnings

References

• docs/architecture/crates/tty/tty-local.md — §"PTY Mode", §"Pipe Mode", §"LocalTtyBackend takes no constructor dependencies"
• docs/architecture/crates/tty/tty-backend.md — TtyBackend trait, resource_id()
• docs/architecture/decisions/054-local-tty-backend-sibling-crate.md — ADR-054 (per-session PTY vs pipe choice)

Notes

This is the wiring task — the heavy lifting is in pty-mode and pipe-mode. The branch on TtyParams.terminal is the per-session choice (ADR-054): one LocalTtyBackend serves both terminal and runner use cases. The local backend ignores backend_params (no backend-specific selector fields); a docker/SSH backend would deserialize its own typed params from it.

Summary

To be filled on completion