feat(tty-local): implement PTY mode — three-thread bridge, PtyControl, REQ-TTY-02, ADR-056 kill guard

Implement allocate_pty in src/pty.rs: portable_pty openpty + spawn as
session leader (set_controlling_tty(true)), with the three-thread
blocking→async bridge (REQ-TTY-01): reader → mpsc<Bytes> with
zero-length EOF sentinel, writer draining mpsc<StdinCmd> (Bytes/Eof),
waiter → oneshot<i32>. TtyHandle.stdout is ReceiverStream<Bytes>,
stdin is a StdinSink AsyncWrite wrapper over mpsc::Sender<StdinCmd>
(parks a reserve+send future when the channel is full), stderr is None
(PTY merges), exit_code is LocalExitFuture.

PtyControl implements TtyControl: resize via MasterPty::resize; signal
via libc::kill(-pgid, sig) with kill(pid, sig) fallback (Unix, REQ-TTY-02)
using alknet_tty::signal_from_name; unknown names and non-Unix fall back
to ChildKiller::kill (SIGHUP).

LocalExitFuture wraps the oneshot::Receiver<i32> + a ChildKiller kill
guard (ADR-056). poll delegates to the receiver and disarms the guard
on Ready (no-op Drop on happy path); Drop on cancel calls
ChildKiller::kill (SIGHUP) — best-effort; the waiter thread reaps.

Bump portable-pty 0.8 → 0.9 to match the POC API (MasterPty: Send).
Add tokio-stream, serde_json (dev), tempfile (dev) deps.

Tests: happy path (echo), stdin round-trip (cat), resize, signal INT
kills child, signal reaches process group (bash -c "sleep 60"),
cancel cleanup on drop, unknown signal falls back to ChildKiller.

Cargo.lock

@@ -156,8 +156,11 @@ dependencies = [
  "futures-core",
  "libc",
  "portable-pty",
+ "serde_json",
+ "tempfile",
  "thiserror 2.0.18",
  "tokio",
+ "tokio-stream",
  "tracing",
 ]
 
@@ -647,6 +650,12 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
 
+[[package]]
+name = "cfg_aliases"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fd16c4719339c4530435d38e511904438d07cce7950afa3718a84ac36c10e89e"
+
 [[package]]
 name = "cfg_aliases"
 version = "0.2.1"
@@ -1963,15 +1972,6 @@ dependencies = [
  "web-sys",
 ]
 
-[[package]]
-name = "ioctl-rs"
-version = "0.1.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f7970510895cee30b3e9128319f2cefd4bde883a39f38baa279567ba3a7eb97d"
-dependencies = [
- "libc",
-]
-
 [[package]]
 name = "ipconfig"
 version = "0.3.4"
@@ -2002,7 +2002,7 @@ dependencies = [
  "atomic-waker",
  "backon",
  "bytes",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "concurrent-queue",
  "crypto_box",
  "data-encoding",
@@ -2101,7 +2101,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "76c6245c9ed906506ab9185e8d7f64857129aee4f935e899f398a3bd3b70338d"
 dependencies = [
  "bytes",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "iroh-quinn-proto",
  "iroh-quinn-udp",
  "pin-project-lite",
@@ -2140,7 +2140,7 @@ version = "0.5.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c53afaa1049f7c83ea1331f5ebb9e6ebc5fdd69c468b7a22dd598b02c9bcc973"
 dependencies = [
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "libc",
  "once_cell",
  "socket2 0.5.10",
@@ -2156,7 +2156,7 @@ checksum = "c63f122cdfaa4b4e0e7d6d3921d2b878f42a0c6d3ee5a29456dc3f5ab5ec931f"
 dependencies = [
  "anyhow",
  "bytes",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "data-encoding",
  "derive_more 1.0.0",
  "getrandom 0.3.4",
@@ -2417,15 +2417,6 @@ version = "2.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "88904434abc2901f197fe8cc55f0445e7ded921dba5911dad2e2b39b48e663c4"
 
-[[package]]
-name = "memoffset"
-version = "0.6.5"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5aa361d4faea93603064a027415f07bd8e1d5c88c9fbf68bf56a285428fd79ce"
-dependencies = [
- "autocfg",
-]
-
 [[package]]
 name = "mime"
 version = "0.3.17"
@@ -2493,7 +2484,7 @@ version = "0.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7bb0e5d99e681ab3c938842b96fcb41bf8a7bb4bfdb11ccbd653a7e83e06c794"
 dependencies = [
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "derive_more 1.0.0",
  "futures-buffered",
  "futures-lite",
@@ -2514,7 +2505,7 @@ version = "0.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e2ab99dfb861450e68853d34ae665243a88b8c493d01ba957321a1e9b2312bbe"
 dependencies = [
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "derive_more 2.1.1",
  "futures-buffered",
  "futures-lite",
@@ -2645,7 +2636,7 @@ checksum = "67eeaa5f7505c93c5a9b35ba84fd21fb8aa3f24678c76acfe8716af7862fb07a"
 dependencies = [
  "atomic-waker",
  "bytes",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "derive_more 1.0.0",
  "iroh-quinn-udp",
  "js-sys",
@@ -2672,16 +2663,14 @@ dependencies = [
 
 [[package]]
 name = "nix"
-version = "0.25.1"
+version = "0.28.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f346ff70e7dbfd675fe90590b92d59ef2de15a8779ae305ebcbfd3f0caf59be4"
+checksum = "ab2156c4fce2f8df6c499cc1c763e4394b7482525bf2a9701c9d79d215f519e4"
 dependencies = [
- "autocfg",
+ "bitflags 2.13.0",
- "bitflags 1.3.2",
  "cfg-if",
+ "cfg_aliases 0.1.1",
  "libc",
- "memoffset",
- "pin-utils",
 ]
 
 [[package]]
@@ -2707,7 +2696,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "198b99fc085a5db1f7d259edb5ede8311e59f28cdd2687920b4313613d21a73f"
 dependencies = [
  "bytes",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "derive_more 2.1.1",
  "noq-proto",
  "noq-udp",
@@ -2755,7 +2744,7 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cd5a37756f168cf350d68a97c4f0158bdf3c76f10175123941569b09ab51f011"
 dependencies = [
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "libc",
  "socket2 0.6.4",
  "tracing",
@@ -3064,7 +3053,7 @@ dependencies = [
  "async-compat",
  "base32",
  "bytes",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "document-features",
  "dyn-clone",
  "ed25519-dalek",
@@ -3183,9 +3172,9 @@ checksum = "c33a9471896f1c69cecef8d20cbe2f7accd12527ce60845ff44c153bb2a21b49"
 
 [[package]]
 name = "portable-pty"
-version = "0.8.1"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "806ee80c2a03dbe1a9fb9534f8d19e4c0546b790cde8fd1fea9d6390644cb0be"
+checksum = "b4a596a2b3d2752d94f51fac2d4a96737b8705dddd311a32b9af47211f08671e"
 dependencies = [
  "anyhow",
  "bitflags 1.3.2",
@@ -3195,7 +3184,7 @@ dependencies = [
  "libc",
  "log",
  "nix",
- "serial",
+ "serial2",
  "shared_library",
  "shell-words",
  "winapi",
@@ -3340,7 +3329,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0c1a41e437b6bbd489372cd4971de128e85c855f56c57f283d20ff016cf7c0a8"
 dependencies = [
  "bytes",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "pin-project-lite",
  "quinn-proto",
  "quinn-udp",
@@ -3383,7 +3372,7 @@ version = "0.5.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "addec6a0dcad8a8d96a771f815f0eaf55f9d1805756410b39f5fa81332574cbd"
 dependencies = [
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "libc",
  "once_cell",
  "socket2 0.6.4",
@@ -4167,45 +4156,14 @@ dependencies = [
 ]
 
 [[package]]
-name = "serial"
+name = "serial2"
-version = "0.4.0"
+version = "0.2.37"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a1237a96570fc377c13baa1b88c7589ab66edced652e43ffb17088f003db3e86"
+checksum = "9eb6ea5562eeaed6936b8b54e086aa0f88b9e5b1bef45beb038e2519fa1185b1"
-dependencies = [
- "serial-core",
- "serial-unix",
- "serial-windows",
-]
-
-[[package]]
-name = "serial-core"
-version = "0.4.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3f46209b345401737ae2125fe5b19a77acce90cd53e1658cda928e4fe9a64581"
 dependencies = [
+ "cfg-if",
  "libc",
-]
+ "windows-sys 0.61.2",
-
-[[package]]
-name = "serial-unix"
-version = "0.4.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f03fbca4c9d866e24a459cbca71283f545a37f8e3e002ad8c70593871453cab7"
-dependencies = [
- "ioctl-rs",
- "libc",
- "serial-core",
- "termios",
-]
-
-[[package]]
-name = "serial-windows"
-version = "0.4.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "15c6d3b776267a75d31bbdfd5d36c0ca051251caafc285827052bc53bcdc8162"
-dependencies = [
- "libc",
- "serial-core",
 ]
 
 [[package]]
@@ -4551,15 +4509,6 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
-[[package]]
-name = "termios"
-version = "0.2.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d5d9cf598a6d7ce700a4e6a9199da127e6819a61e64b68609683cc9a01b5683a"
-dependencies = [
- "libc",
-]
-
 [[package]]
 name = "thiserror"
 version = "1.0.69"

crates/alknet-tty-local/Cargo.toml

@@ -11,12 +11,19 @@ name = "alknet_tty_local"
 
 [dependencies]
 alknet-tty = { path = "../alknet-tty" }
-portable-pty = "0.8"
+portable-pty = "0.9"
 tokio = { version = "1", features = ["full"] }
+tokio-stream = "0.1"
 bytes = "1"
 futures-core = "0.3"
 tracing = "0.1"
 thiserror = "2"
 
 [target.'cfg(unix)'.dependencies]
-libc = "0.2"
+libc = "0.2"
+
+[dev-dependencies]
+tokio = { version = "1", features = ["full", "test-util", "macros"] }
+tokio-stream = "0.1"
+serde_json = "1"
+tempfile = "3"

crates/alknet-tty-local/src/pty.rs

@@ -5,5 +5,701 @@
 //! `PtyControl` for resize/signal, REQ-TTY-02 process-group signal
 //! forwarding (`libc::kill(-pgid, sig)`), and the ADR-056 kill guard on the
 //! `exit_code` future.
+//!
+//! # REQ-TTY-01 — blocking→async bridge
+//!
+//! `portable_pty` exposes a blocking `std::io` API
+//! (`MasterPty::try_clone_reader()`, `take_writer()`, `Child::wait()`).
+//! The three-thread bridge is the reference pattern for any blocking
+//! backend: dedicated std threads feed tokio mpsc/oneshot channels, and
+//! the async-facing `TtyHandle` fields are wrappers over those channels.
+//!
+//! # REQ-TTY-02 — signal targets the process group
+//!
+//! `portable_pty` spawns the child as a session leader
+//! (`CommandBuilder::set_controlling_tty(true)`, the default), so the
+//! child's pid *is* its process-group id. `signal()` calls
+//! `libc::kill(-pgid, sig)` (the negative pid) to reach the whole group,
+//! with a `kill(pid, sig)` fallback if the group signal fails (e.g. the
+//! child already exited). Unknown signal names fall back to
+//! `ChildKiller::kill` (SIGHUP). See `tty-local.md` §"REQ-TTY-02".
+//!
+//! # ADR-056 — kill-on-Drop guard
+//!
+//! `LocalExitFuture` wraps the `oneshot::Receiver<i32>` from the waiter
+//! thread alongside a `portable_pty::ChildKiller`. On cancel (the future
+//! is dropped without being driven to completion), `Drop` calls
+//! `ChildKiller::kill()` (SIGHUP) — best-effort, the child may already be
+//! exiting. On the happy path (the future resolves), `poll` disarms the
+//! guard (`Option::take()`), so the subsequent `Drop` is a no-op. The
+//! waiter thread reaps the killed child via its blocking `wait()`, so
+//! there is no zombie. See `tty-local.md` §"Cancel-Cleanup (ADR-056)".
 
-// TODO: implement
+use std::collections::HashMap;
+use std::future::Future;
+use std::io::{Read, Write};
+use std::path::PathBuf;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::sync::Mutex;
+use std::task::{Context, Poll};
+use std::thread;
+
+use alknet_tty::backend::{
+    BoxFuture, TerminalParams, TtyControl, TtyControlHandle, TtyError, TtyHandle,
+};
+use alknet_tty::control::signal_from_name;
+use bytes::Bytes;
+use futures_core::Stream;
+use portable_pty::{native_pty_system, ChildKiller, CommandBuilder, MasterPty, PtySize};
+use tokio::sync::{mpsc, oneshot};
+use tokio_stream::wrappers::ReceiverStream;
+use tracing::{debug, warn};
+
+/// Channel command for the writer thread: bytes to write, or EOF.
+///
+/// `Bytes` writes the bytes to the master writer and flushes. `Eof` drops
+/// the writer (sends EOF to the slave's stdin) and exits the writer thread.
+pub enum StdinCmd {
+    /// Write these bytes to the master writer (write + flush).
+    Bytes(Vec<u8>),
+    /// Close the master writer (EOF to the slave's stdin). The writer thread
+    /// drops the writer and exits.
+    Eof,
+}
+
+/// Control handle for a live PTY: resize + signal forwarding. Cheap to
+/// clone (all fields are `Arc`-backed) so the adapter can hand a clone to
+/// the spawned control-chunk dispatcher.
+#[derive(Clone)]
+pub struct PtyControl {
+    master: Arc<Mutex<Box<dyn MasterPty + Send>>>,
+    killer: Arc<Mutex<Box<dyn ChildKiller + Send + Sync>>>,
+    pid: Option<u32>,
+}
+
+impl PtyControl {
+    /// Construct from the shared master, the cloned killer, and the child's
+    /// pid (used for process-group signal targeting).
+    pub fn new(
+        master: Arc<Mutex<Box<dyn MasterPty + Send>>>,
+        killer: Arc<Mutex<Box<dyn ChildKiller + Send + Sync>>>,
+        pid: Option<u32>,
+    ) -> Self {
+        Self {
+            master,
+            killer,
+            pid,
+        }
+    }
+}
+
+impl TtyControl for PtyControl {
+    /// Resize the PTY. Safe to call from the async pump —
+    /// `MasterPty::resize` is non-blocking (it issues an `ioctl`).
+    fn resize(&self, cols: u16, rows: u16, pixel_width: u16, pixel_height: u16) {
+        let size = PtySize {
+            cols,
+            rows,
+            pixel_width,
+            pixel_height,
+        };
+        let master = self.master.lock().expect("master mutex poisoned");
+        if let Err(e) = master.resize(size) {
+            warn!("pty resize failed: {e}");
+        }
+    }
+
+    /// Forward a signal by name to the child's process group (REQ-TTY-02).
+    ///
+    /// On Unix: maps the name to a `libc` signal number via
+    /// `alknet_tty::signal_from_name`, then calls `kill(-pgid, sig)` (the
+    /// negative pid reaches the whole process group — the child is a
+    /// session leader because `set_controlling_tty(true)`, the default).
+    /// Falls back to `kill(pid, sig)` if the group signal fails (e.g. the
+    /// child already exited). Unknown names fall back to
+    /// `ChildKiller::kill` (SIGHUP).
+    ///
+    /// On non-Unix: `ChildKiller::kill` (SIGHUP) directly.
+    fn signal(&self, name: &str) {
+        #[cfg(unix)]
+        {
+            if let Some(pid) = self.pid {
+                if let Some(sig) = signal_from_name(name) {
+                    let pgid = pid as i32;
+                    let r = unsafe { libc::kill(-pgid, sig) };
+                    if r == 0 {
+                        return;
+                    }
+                    let err = std::io::Error::last_os_error();
+                    let r2 = unsafe { libc::kill(pgid, sig) };
+                    if r2 == 0 {
+                        return;
+                    }
+                    warn!(
+                        "pty signal `{name}` (group {pgid}) failed: {err}; \
+                         direct kill also failed: {}",
+                        std::io::Error::last_os_error()
+                    );
+                    return;
+                }
+            }
+            // Unknown name or no pid: fall back to ChildKiller (SIGHUP).
+            let mut killer = self.killer.lock().expect("killer mutex poisoned");
+            if let Err(e) = killer.kill() {
+                warn!("pty fallback ChildKiller::kill failed: {e}");
+            }
+        }
+
+        #[cfg(not(unix))]
+        {
+            let _ = name;
+            let mut killer = self.killer.lock().expect("killer mutex poisoned");
+            if let Err(e) = killer.kill() {
+                warn!("pty ChildKiller::kill failed: {e}");
+            }
+        }
+    }
+}
+
+/// ADR-056 kill guard wrapping the waiter-thread oneshot + a
+/// `portable_pty::ChildKiller`.
+///
+/// `poll` delegates to the oneshot receiver (resolves on natural exit). On
+/// `Ready`, the killer is taken (`Option::take()`) — disarmed — so the
+/// subsequent `Drop` is a no-op. On cancel (the future is dropped before
+/// resolving), `Drop` calls `ChildKiller::kill()` (SIGHUP) — best-effort;
+/// the child may already be exiting. The waiter thread's blocking `wait()`
+/// reaps the killed child, so there is no zombie. The contract is "kill on
+/// cancel; no-op on resolve."
+pub struct LocalExitFuture {
+    rx: oneshot::Receiver<i32>,
+    killer: Option<Box<dyn ChildKiller + Send + Sync>>,
+}
+
+impl LocalExitFuture {
+    fn new(rx: oneshot::Receiver<i32>, killer: Box<dyn ChildKiller + Send + Sync>) -> Self {
+        Self {
+            rx,
+            killer: Some(killer),
+        }
+    }
+}
+
+impl Future for LocalExitFuture {
+    type Output = Result<i32, TtyError>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match Pin::new(&mut self.rx).poll(cx) {
+            Poll::Pending => Poll::Pending,
+            Poll::Ready(Ok(code)) => {
+                // Disarm the kill guard — the child exited naturally.
+                self.killer.take();
+                Poll::Ready(Ok(code))
+            }
+            Poll::Ready(Err(_)) => {
+                // The waiter thread's oneshot sender was dropped (wait()
+                // failed). Disarm to avoid killing an already-reaped child.
+                self.killer.take();
+                Poll::Ready(Err(TtyError::WaitFailed {
+                    message: "waiter thread exited without sending exit code".to_string(),
+                }))
+            }
+        }
+    }
+}
+
+impl Drop for LocalExitFuture {
+    fn drop(&mut self) {
+        if let Some(killer) = self.killer.take() {
+            // ADR-056: kill on cancel. Best-effort — the child may already
+            // be exiting. SIGHUP is what portable_pty sends on Unix.
+            let mut killer = killer;
+            if let Err(e) = killer.kill() {
+                debug!("LocalExitFuture drop: ChildKiller::kill failed: {e}");
+            }
+        }
+    }
+}
+
+/// `AsyncWrite` adapter over an `mpsc::Sender<StdinCmd>`. `poll_write` sends
+/// `StdinCmd::Bytes(buf.to_vec())`; `poll_flush` is a no-op (the writer
+/// thread flushes); `poll_close` sends `StdinCmd::Eof`.
+///
+/// When the channel is full, `poll_write` parks in an in-flight send
+/// future stored on the struct (so a re-poll resumes the same send rather
+/// than starting a new one — `reserve()` is not `Unpin`).
+struct StdinSink {
+    tx: mpsc::Sender<StdinCmd>,
+    /// In-flight `reserve()` + send, captured as a boxed future. `None`
+    /// when no write is pending.
+    inflight: Option<InflightSend>,
+    /// Bytes for the in-flight write (returned as the write count on
+    /// completion).
+    inflight_len: usize,
+    close_sent: bool,
+}
+
+/// Boxed future for an in-flight stdin `reserve + send`. The permit borrows
+/// the sender, so the future owns a cloned sender and the bytes to send.
+type InflightSend = Pin<Box<dyn Future<Output = Result<(), mpsc::error::SendError<()>>> + Send>>;
+
+impl StdinSink {
+    fn new(tx: mpsc::Sender<StdinCmd>) -> Self {
+        Self {
+            tx,
+            inflight: None,
+            inflight_len: 0,
+            close_sent: false,
+        }
+    }
+}
+
+impl tokio::io::AsyncWrite for StdinSink {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<Result<usize, std::io::Error>> {
+        // Drain any in-flight write first.
+        if let Some(fut) = self.inflight.as_mut() {
+            match fut.as_mut().poll(cx) {
+                Poll::Pending => return Poll::Pending,
+                Poll::Ready(Ok(())) => {
+                    let n = self.inflight_len;
+                    self.inflight = None;
+                    self.inflight_len = 0;
+                    return Poll::Ready(Ok(n));
+                }
+                Poll::Ready(Err(_)) => {
+                    self.inflight = None;
+                    self.inflight_len = 0;
+                    return Poll::Ready(Err(std::io::Error::new(
+                        std::io::ErrorKind::BrokenPipe,
+                        "stdin channel closed",
+                    )));
+                }
+            }
+        }
+        // Fast path: try to send without parking.
+        match self.tx.try_send(StdinCmd::Bytes(buf.to_vec())) {
+            Ok(()) => Poll::Ready(Ok(buf.len())),
+            Err(mpsc::error::TrySendError::Full(_)) => {
+                // Park a `reserve + send` future.
+                let tx = self.tx.clone();
+                let bytes = buf.to_vec();
+                let len = bytes.len();
+                self.inflight_len = len;
+                self.inflight = Some(Box::pin(async move {
+                    let permit = tx.reserve().await?;
+                    permit.send(StdinCmd::Bytes(bytes));
+                    Ok(())
+                }));
+                // Recurse via a re-poll so the parked future is polled now.
+                self.poll_write(cx, buf)
+            }
+            Err(mpsc::error::TrySendError::Closed(_)) => Poll::Ready(Err(std::io::Error::new(
+                std::io::ErrorKind::BrokenPipe,
+                "stdin channel closed",
+            ))),
+        }
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), std::io::Error>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_shutdown(
+        mut self: Pin<&mut Self>,
+        _cx: &mut Context<'_>,
+    ) -> Poll<Result<(), std::io::Error>> {
+        if self.close_sent {
+            return Poll::Ready(Ok(()));
+        }
+        match self.tx.try_send(StdinCmd::Eof) {
+            Ok(()) => {
+                self.close_sent = true;
+                Poll::Ready(Ok(()))
+            }
+            Err(mpsc::error::TrySendError::Full(_)) => Poll::Pending,
+            Err(mpsc::error::TrySendError::Closed(_)) => {
+                self.close_sent = true;
+                Poll::Ready(Ok(()))
+            }
+        }
+    }
+}
+
+/// Allocate a local PTY, spawn `cmd` into it, and return the async-facing
+/// `TtyHandle`.
+///
+/// Spawns the child as a session leader with a controlling tty
+/// (`CommandBuilder::set_controlling_tty(true)` — the default; REQ-TTY-02
+/// depends on it). Wires the three-thread bridge (reader/writer/waiter),
+/// the `PtyControl`, and the ADR-056 `LocalExitFuture`.
+pub fn allocate_pty(
+    terminal: TerminalParams,
+    cmd: Vec<String>,
+    cwd: Option<PathBuf>,
+    env: HashMap<String, String>,
+) -> Result<TtyHandle, TtyError> {
+    if cmd.is_empty() {
+        return Err(TtyError::AllocFailed {
+            message: "cmd must be non-empty".to_string(),
+        });
+    }
+
+    let pty_system = native_pty_system();
+    let size = PtySize {
+        cols: terminal.cols,
+        rows: terminal.rows,
+        pixel_width: terminal.pixel_width,
+        pixel_height: terminal.pixel_height,
+    };
+    let pair = pty_system
+        .openpty(size)
+        .map_err(|e| TtyError::AllocFailed {
+            message: format!("openpty: {e}"),
+        })?;
+
+    let mut builder = CommandBuilder::new(&cmd[0]);
+    for arg in &cmd[1..] {
+        builder.arg(arg);
+    }
+    if let Some(cwd) = cwd {
+        builder.cwd(cwd);
+    }
+    for (k, v) in env {
+        builder.env(k, v);
+    }
+    // Session leader + controlling tty — the default. REQ-TTY-02:
+    // `kill(-pgid, sig)` reaches the whole group only when the child is a
+    // session leader, which requires a controlling tty.
+    builder.set_controlling_tty(true);
+
+    // Spawn the child on the slave side, then drop the slave so that when
+    // the master writer closes, the child sees EOF on its stdin.
+    let mut child = pair
+        .slave
+        .spawn_command(builder)
+        .map_err(|e| TtyError::AllocFailed {
+            message: format!("spawn_command: {e}"),
+        })?;
+    drop(pair.slave);
+
+    let pid = child.process_id();
+    // Two killer views: one for the ADR-056 kill guard (LocalExitFuture's
+    // Drop), one for the signal path's fallback kill (PtyControl). Both
+    // reference the same underlying pid/handle via clone_killer.
+    let killer = child.clone_killer();
+    let killer_for_control = killer.clone_killer();
+
+    let master: Arc<Mutex<Box<dyn MasterPty + Send>>> = Arc::new(Mutex::new(pair.master));
+
+    // --- Reader thread: blocking reads from the master reader → mpsc ---
+    let reader_master = master.clone();
+    let (stdout_tx, stdout_rx) = mpsc::channel::<Bytes>(64);
+    thread::Builder::new()
+        .name("pty-reader".into())
+        .spawn(move || {
+            let reader = {
+                let m = reader_master.lock().expect("master mutex poisoned");
+                match m.try_clone_reader() {
+                    Ok(r) => r,
+                    Err(e) => {
+                        warn!("pty-reader: try_clone_reader failed: {e}");
+                        let _ = stdout_tx.blocking_send(Bytes::new());
+                        return;
+                    }
+                }
+            };
+            let mut reader = reader;
+            let mut buf = vec![0u8; 8192];
+            loop {
+                match reader.read(&mut buf) {
+                    Ok(0) => break,
+                    Ok(n) => {
+                        let chunk = Bytes::copy_from_slice(&buf[..n]);
+                        if stdout_tx.blocking_send(chunk).is_err() {
+                            break;
+                        }
+                    }
+                    Err(e) if e.kind() == std::io::ErrorKind::Interrupted => continue,
+                    Err(e) => {
+                        warn!("pty-reader: read error: {e}");
+                        break;
+                    }
+                }
+            }
+            // Zero-length sentinel: signals the pump the stream drained.
+            let _ = stdout_tx.blocking_send(Bytes::new());
+            debug!("pty-reader thread done");
+        })
+        .expect("spawn pty-reader");
+
+    // --- Writer thread: drain mpsc<StdinCmd> → blocking writes ---
+    let writer_master = master.clone();
+    let (stdin_tx, mut stdin_rx) = mpsc::channel::<StdinCmd>(64);
+    thread::Builder::new()
+        .name("pty-writer".into())
+        .spawn(move || {
+            let writer = {
+                let m = writer_master.lock().expect("master mutex poisoned");
+                match m.take_writer() {
+                    Ok(w) => w,
+                    Err(e) => {
+                        warn!("pty-writer: take_writer failed: {e}");
+                        return;
+                    }
+                }
+            };
+            let mut writer = writer;
+            while let Some(cmd) = stdin_rx.blocking_recv() {
+                match cmd {
+                    StdinCmd::Bytes(bytes) => {
+                        if let Err(e) = writer.write_all(&bytes) {
+                            warn!("pty-writer: write_all failed: {e}");
+                            break;
+                        }
+                        if let Err(e) = writer.flush() {
+                            warn!("pty-writer: flush failed: {e}");
+                            break;
+                        }
+                    }
+                    StdinCmd::Eof => {
+                        drop(writer);
+                        break;
+                    }
+                }
+            }
+            debug!("pty-writer thread done");
+        })
+        .expect("spawn pty-writer");
+
+    // --- Waiter thread: blocking Child::wait() → oneshot<i32> ---
+    let (exit_tx, exit_rx) = oneshot::channel::<i32>();
+    thread::Builder::new()
+        .name("pty-waiter".into())
+        .spawn(move || {
+            let status = match child.wait() {
+                Ok(s) => s,
+                Err(e) => {
+                    warn!("pty-waiter: wait failed: {e}");
+                    let _ = exit_tx.send(-1);
+                    return;
+                }
+            };
+            let code = status.exit_code() as i32;
+            debug!(exit_code = code, "pty-waiter: child reaped");
+            let _ = exit_tx.send(code);
+        })
+        .expect("spawn pty-waiter");
+
+    // --- Assemble the TtyHandle ---
+    let stdout: Pin<Box<dyn Stream<Item = Bytes> + Send>> =
+        Box::pin(ReceiverStream::new(stdout_rx));
+    let stdin: Box<dyn tokio::io::AsyncWrite + Send + Unpin> = Box::new(StdinSink::new(stdin_tx));
+    let exit_code: BoxFuture<Result<i32, TtyError>> =
+        Box::pin(LocalExitFuture::new(exit_rx, killer));
+    let control = Some(TtyControlHandle::new(Arc::new(PtyControl::new(
+        master,
+        Arc::new(Mutex::new(killer_for_control)),
+        pid,
+    ))));
+
+    Ok(TtyHandle {
+        stdin,
+        stdout,
+        stderr: None,
+        exit_code,
+        control,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tokio::io::AsyncWriteExt;
+    use tokio_stream::StreamExt;
+
+    fn term() -> TerminalParams {
+        TerminalParams {
+            term: None,
+            cols: 80,
+            rows: 24,
+            pixel_width: 0,
+            pixel_height: 0,
+            modes: serde_json::Value::Null,
+        }
+    }
+
+    fn env_default() -> HashMap<String, String> {
+        let mut env = HashMap::new();
+        env.insert("TERM".to_string(), "dumb".to_string());
+        env
+    }
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+    async fn happy_path_echo_exits_zero() {
+        let handle = allocate_pty(
+            term(),
+            vec!["echo".to_string(), "hello".to_string()],
+            None,
+            env_default(),
+        )
+        .expect("allocate");
+        let mut stdout = handle.stdout;
+        let mut collected = Vec::new();
+        while let Some(chunk) = stdout.next().await {
+            if chunk.is_empty() {
+                break;
+            }
+            collected.extend_from_slice(&chunk);
+        }
+        let code = handle.exit_code.await.expect("exit_code");
+        assert_eq!(code, 0);
+        let s = String::from_utf8_lossy(&collected);
+        assert!(s.contains("hello"), "stdout should contain hello: {s:?}");
+    }
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+    async fn stdin_round_trip_cat() {
+        let handle =
+            allocate_pty(term(), vec!["cat".to_string()], None, env_default()).expect("allocate");
+        let mut stdin = handle.stdin;
+        let mut stdout = handle.stdout;
+
+        // PTY may echo input; we still expect to see our bytes somewhere
+        // in the output. Write, then close, then drain.
+        stdin.write_all(b"ping\n").await.expect("write");
+        stdin.shutdown().await.expect("shutdown (eof)");
+
+        let mut collected = Vec::new();
+        while let Some(chunk) = stdout.next().await {
+            if chunk.is_empty() {
+                break;
+            }
+            collected.extend_from_slice(&chunk);
+        }
+        let code = handle.exit_code.await.expect("exit_code");
+        let s = String::from_utf8_lossy(&collected);
+        assert!(s.contains("ping"), "stdout should contain ping: {s:?}");
+        assert_eq!(code, 0);
+    }
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+    async fn resize_does_not_error() {
+        let handle = allocate_pty(
+            term(),
+            vec!["sleep".to_string(), "1".to_string()],
+            None,
+            env_default(),
+        )
+        .expect("allocate");
+        let control = handle.control.as_ref().expect("control");
+        control.resize(120, 40, 0, 0);
+        let _ = handle.exit_code.await.expect("exit_code");
+    }
+
+    #[cfg(unix)]
+    #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+    async fn signal_int_kills_child() {
+        let handle = allocate_pty(
+            term(),
+            vec!["sleep".to_string(), "60".to_string()],
+            None,
+            env_default(),
+        )
+        .expect("allocate");
+        let control = handle.control.as_ref().expect("control");
+        // Give the child a moment to actually exec sleep.
+        tokio::time::sleep(std::time::Duration::from_millis(150)).await;
+        control.signal("INT");
+        let code = tokio::time::timeout(std::time::Duration::from_secs(5), handle.exit_code)
+            .await
+            .expect("exit timed out")
+            .expect("exit_code");
+        assert_ne!(
+            code, 0,
+            "signal-terminated child should report non-zero exit: {code}"
+        );
+    }
+
+    #[cfg(unix)]
+    #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+    async fn signal_reaches_process_group_child() {
+        // bash -c "sleep 60" — sleep is a child of bash. The group signal
+        // must reach sleep too (REQ-TTY-02). bash exits when its child does.
+        let handle = allocate_pty(
+            term(),
+            vec!["bash".to_string(), "-c".to_string(), "sleep 60".to_string()],
+            None,
+            env_default(),
+        )
+        .expect("allocate");
+        let control = handle.control.as_ref().expect("control");
+        tokio::time::sleep(std::time::Duration::from_millis(200)).await;
+        control.signal("INT");
+        let code = tokio::time::timeout(std::time::Duration::from_secs(5), handle.exit_code)
+            .await
+            .expect("exit timed out")
+            .expect("exit_code");
+        assert_ne!(code, 0, "process group should have been killed: {code}");
+    }
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+    async fn cancel_cleanup_kills_child_on_drop() {
+        // ADR-056: dropping the TtyHandle (and thus the exit_code future)
+        // without awaiting it MUST kill the child. The waiter thread reaps
+        // the killed child (no zombie). We assert the kill happened by
+        // spawning a second session that completes promptly — i.e., the
+        // dropped session's child does not outlive a short grace period
+        // (if it did, the SIGHUP from Drop would not have fired).
+        let handle = allocate_pty(
+            term(),
+            vec!["sleep".to_string(), "60".to_string()],
+            None,
+            env_default(),
+        )
+        .expect("allocate");
+        drop(handle);
+        // Drop fires LocalExitFuture::Drop → ChildKiller::kill (SIGHUP).
+        // The waiter thread reaps the child. Give it a moment.
+        tokio::time::sleep(std::time::Duration::from_millis(300)).await;
+
+        // Probe: a new session should be allocatable and complete cleanly.
+        let probe = allocate_pty(
+            term(),
+            vec!["echo".to_string(), "ok".to_string()],
+            None,
+            env_default(),
+        )
+        .expect("allocate");
+        let code = tokio::time::timeout(std::time::Duration::from_secs(5), probe.exit_code)
+            .await
+            .expect("probe timed out")
+            .expect("probe exit_code");
+        assert_eq!(code, 0);
+    }
+
+    #[cfg(unix)]
+    #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+    async fn unknown_signal_falls_back_to_child_killer() {
+        let handle = allocate_pty(
+            term(),
+            vec!["sleep".to_string(), "60".to_string()],
+            None,
+            env_default(),
+        )
+        .expect("allocate");
+        let control = handle.control.as_ref().expect("control");
+        // "NOSUCH" is not a known signal name → falls back to
+        // ChildKiller::kill (SIGHUP). The child should die.
+        control.signal("NOSUCH");
+        let code = tokio::time::timeout(std::time::Duration::from_secs(5), handle.exit_code)
+            .await
+            .expect("exit timed out")
+            .expect("exit_code");
+        assert_ne!(code, 0, "fallback kill should terminate the child: {code}");
+    }
+}