feat(select): fd arms in select + timed fd waits (RFC 008 phase 1)

FdArm composes fd readiness with channel arms on one wait epoch.
Selectable grows fallible sel_register and an eager-cleanup hook;
losing/stop-unwound/timed-out fd arms are unregistered (waiters entry
+ kernel ONESHOT) so the fd is never poisoned. try_select /
try_select_timeout surface registration errors (EBADF, EMFILE,
AlreadyExists) instead of RFC 008's permanently-ready lean, which
would busy-loop a healthy-but-unregistrable fd; select/select_timeout
stay infallible for channel-only arms. Adds wait_readable_timeout /
wait_writable_timeout as one-arm selects.

Known benign race (pre-existing, slightly widened): a queued FdReady
racing the cleanup DEL can spuriously wake a fresh waiter on that fd;
absorbed by select's defensive re-loop. Fixable by epoch-stamping
completions.

src/channel.rs

@@ -361,7 +361,7 @@ impl<T: Send + 'static> crate::timer::TimerTarget for RawMutex<Inner<T>> {
 // select — ready-index wait over multiple receivers
 // ---------------------------------------------------------------------------
 
-mod sealed {
+pub(crate) mod sealed {
     pub trait Sealed {}
 }
 impl<T> sealed::Sealed for Receiver<T> {}
@@ -371,28 +371,46 @@ impl<T> sealed::Sealed for Receiver<T> {}
 ///
 /// Contract (all under the arm's own lock): `sel_register` checks-or-
 /// registers atomically — if the arm is ready it does NOT register and
-/// returns `false`; otherwise it publishes `(pid, epoch)` where its wakers
+/// returns `Ok(false)`; otherwise it publishes `(pid, epoch)` where its
-/// will find it. "Ready" means a receive would not park: a message is
+/// wakers will find it and returns `Ok(true)`. "Ready" means a receive
-/// queued, or the arm is closed.
+/// would not park: a message is queued, or the arm is closed. `Err` means
+/// the arm could not register at all (only fd arms can fail; channel
+/// registration is infallible) — the wait must be retired and earlier
+/// eager-cleanup arms unregistered.
 pub trait Selectable: sealed::Sealed {
     #[doc(hidden)]
-    fn sel_register(&self, pid: Pid, epoch: u32) -> bool;
+    fn sel_register(&self, pid: Pid, epoch: u32) -> std::io::Result<bool>;
     #[doc(hidden)]
     fn sel_ready(&self) -> bool;
+    /// Remove this arm's `(pid, epoch)` registration if — and only if — it
+    /// is still in place. Default no-op: a losing channel arm's stale
+    /// registration is inert (its wakers die at the epoch CAS; the next
+    /// wait overwrites the slot). Fd arms override this: their staleness
+    /// poisons the fd (waiters entry + kernel-side ONESHOT registration)
+    /// and needs an eager cleanup pass.
+    #[doc(hidden)]
+    fn sel_unregister(&self, _pid: Pid, _epoch: u32) {}
+    /// Whether this arm requires the eager cleanup pass at all. Gates the
+    /// post-wake `sel_unregister` sweep so channel-only selects keep
+    /// today's zero-cancellation hot path.
+    #[doc(hidden)]
+    fn sel_eager_cleanup(&self) -> bool {
+        false
+    }
 }
 
 impl<T> Selectable for Receiver<T> {
-    fn sel_register(&self, pid: Pid, epoch: u32) -> bool {
+    fn sel_register(&self, pid: Pid, epoch: u32) -> std::io::Result<bool> {
         let mut g = self.inner.lock();
         if !g.queue.is_empty() || g.senders == 0 {
-            return false;
+            return Ok(false);
         }
         debug_assert!(
             g.parked_receiver.is_none_or(|(p, _)| p == pid),
             "channel has more than one receiver"
         );
         g.parked_receiver = Some((pid, epoch));
-        true
+        Ok(true)
     }
 
     fn sel_ready(&self) -> bool {
@@ -429,30 +447,95 @@ impl<T> Selectable for Receiver<T> {
 /// self-clean at their wakers' failed CAS, or get overwritten by this
 /// receiver's next wait on that channel.
 ///
-/// Panics if `arms` is empty, or when called outside an actor.
+/// Panics if `arms` is empty, when called outside an actor, or if an fd
+/// arm fails to register (EBADF, EMFILE, a second waiter on one fd —
+/// see [`try_select`] for the fallible form; channel-only selects cannot
+/// fail).
 pub fn select(arms: &[&dyn Selectable]) -> usize {
+    try_select(arms).expect("select(): fd arm failed to register (use try_select)")
+}
+
+/// [`select`], fallible: `Err` when an arm fails to register (only fd
+/// arms can — EBADF, EMFILE on the epoll set, or a second waiter on an
+/// fd that already has one). On `Err` the wait is fully retired and no
+/// registration is left behind: every arm registered before the failing
+/// one has been unregistered.
+pub fn try_select(arms: &[&dyn Selectable]) -> std::io::Result<usize> {
     assert!(!arms.is_empty(), "select() on an empty arm list");
     let me = crate::actor::current_pid().expect("select() called outside an actor");
     loop {
         let epoch = crate::scheduler::begin_wait();
-        if let Some(i) = register_arms(me, epoch, arms) {
+        if let Some(i) = register_arms(me, epoch, arms)? {
-            return i;
+            return Ok(i);
         }
 
+        // Stale fd registrations are not harmless (a losing fd arm's
+        // waiters entry poisons the fd with AlreadyExists and its
+        // kernel-side ONESHOT registration can fire arbitrarily late), so
+        // selects containing fd arms run an eager cleanup pass after the
+        // park — including when a terminal stop unwinds out of it, via
+        // the guard. Channel-only selects skip all of it: `eager` is
+        // false, the guard is disarmed, and the loser-arm self-cleaning
+        // story is unchanged.
+        let eager = arms.iter().any(|a| a.sel_eager_cleanup());
+        let mut guard = UnregisterGuard { arms, me, epoch, armed: eager };
+
         crate::scheduler::park_current();
+
+        if eager {
+            unregister_arms(arms, me, epoch);
+        }
+        guard.armed = false;
+        drop(guard);
+
         // Woken precisely: an arm's send (message) or last-sender drop
         // (closure) consumed our epoch, and both leave their arm ready —
         // return the first one, in priority order (which may be a
         // different, higher-priority arm than the one that woke us; its
         // message stays queued and re-reports ready on the next call).
+        // Fd arms classify by a fresh zero-timeout poll, so they too are
+        // a pure function of state — independent of the registration the
+        // cleanup pass just removed.
         for (i, arm) in arms.iter().enumerate() {
             if arm.sel_ready() {
-                return i;
+                return Ok(i);
             }
         }
         // Unreachable by protocol (a stop wake unwinds out of
         // park_current). Defensive: re-open the wait and re-register —
-        // stale own-registrations are overwritten.
+        // stale own-registrations are overwritten (channels) or were
+        // removed by the cleanup pass above (fds).
+    }
+}
+
+/// Eager-cleanup sweep: remove every fd arm's registration that is still
+/// ours. No-op per channel arm (one virtual call); one io-lock visit per
+/// fd arm.
+fn unregister_arms(arms: &[&dyn Selectable], me: Pid, epoch: u32) {
+    for arm in arms {
+        if arm.sel_eager_cleanup() {
+            arm.sel_unregister(me, epoch);
+        }
+    }
+}
+
+/// Stop-unwind twin of the explicit cleanup pass: a terminal stop unwinds
+/// out of `park_current`, and a registered fd arm must not outlive its
+/// actor (the generalization of `wait_fd`'s `Dereg`). Disarmed on the
+/// normal path after the explicit pass runs; never armed when no fd arm
+/// registered, keeping the channel-only path guard-free in effect.
+struct UnregisterGuard<'a> {
+    arms: &'a [&'a dyn Selectable],
+    me: Pid,
+    epoch: u32,
+    armed: bool,
+}
+
+impl Drop for UnregisterGuard<'_> {
+    fn drop(&mut self) {
+        if self.armed {
+            unregister_arms(self.arms, self.me, self.epoch);
+        }
     }
 }
 
@@ -462,19 +545,35 @@ pub fn select(arms: &[&dyn Selectable]) -> usize {
 /// right after its registration wakes the caller through the protocol (the
 /// prep-to-park window is closed by RunningNotified).
 ///
-/// `Some(i)` = arm `i` was ready, the pass stopped, and the wait has been
+/// `Ok(Some(i))` = arm `i` was ready, the pass stopped, and the wait has
-/// RETIRED (no park may follow): earlier arms hold live-epoch
+/// been RETIRED (no park may follow): earlier arms hold live-epoch
-/// registrations, so the epoch is bumped, a landed notification eaten, and
+/// registrations, so earlier *fd* arms are unregistered eagerly, then the
-/// a pending stop re-observed — without which a stale arm wake could fault
+/// epoch is bumped, a landed notification eaten, and a pending stop
-/// a later one-shot park. `None` = every arm registered; the caller parks.
+/// re-observed — without which a stale arm wake could fault a later
-fn register_arms(me: Pid, epoch: u32, arms: &[&dyn Selectable]) -> Option<usize> {
+/// one-shot park. `Err` = an arm failed to register; identical unwind
+/// (earlier fd arms unregistered, wait retired). `Ok(None)` = every arm
+/// registered; the caller parks.
+fn register_arms(
+    me: Pid,
+    epoch: u32,
+    arms: &[&dyn Selectable],
+) -> std::io::Result<Option<usize>> {
     for (i, arm) in arms.iter().enumerate() {
-        if !arm.sel_register(me, epoch) {
+        let registered = match arm.sel_register(me, epoch) {
+            Ok(r) => r,
+            Err(e) => {
+                unregister_arms(&arms[..i], me, epoch);
+                crate::scheduler::retire_wait();
+                return Err(e);
+            }
+        };
+        if !registered {
+            unregister_arms(&arms[..i], me, epoch);
             crate::scheduler::retire_wait();
-            return Some(i);
+            return Ok(Some(i));
         }
     }
-    None
+    Ok(None)
 }
 
 /// The [`select_timeout`] timer target: stateless, because precise wakes
@@ -506,16 +605,29 @@ impl crate::timer::TimerTarget for SelectTimeout {
 /// `Duration::ZERO` is a valid timeout: it parks until the immediately-due
 /// timer is drained, then reports `None` unless an arm was already ready.
 ///
-/// Panics if `arms` is empty, or when called outside an actor.
+/// Panics if `arms` is empty, when called outside an actor, or if an fd
+/// arm fails to register (see [`try_select_timeout`] for the fallible
+/// form; channel-only selects cannot fail).
 pub fn select_timeout(
     arms: &[&dyn Selectable],
     timeout: std::time::Duration,
 ) -> Option<usize> {
+    try_select_timeout(arms, timeout)
+        .expect("select_timeout(): fd arm failed to register (use try_select_timeout)")
+}
+
+/// [`select_timeout`], fallible: `Err` when an arm fails to register
+/// (only fd arms can). On `Err` the wait is fully retired and no
+/// registration — arm-side or kernel-side — is left behind.
+pub fn try_select_timeout(
+    arms: &[&dyn Selectable],
+    timeout: std::time::Duration,
+) -> std::io::Result<Option<usize>> {
     assert!(!arms.is_empty(), "select_timeout() on an empty arm list");
     let me = crate::actor::current_pid().expect("select_timeout() called outside an actor");
     let epoch = crate::scheduler::begin_wait();
-    if let Some(i) = register_arms(me, epoch, arms) {
+    if let Some(i) = register_arms(me, epoch, arms)? {
-        return Some(i); // ready now: the timer was never armed
+        return Ok(Some(i)); // ready now: the timer was never armed
     }
 
     // Arm the timer after the registration pass, outside every Channel
@@ -524,7 +636,22 @@ pub fn select_timeout(
     let target: std::sync::Arc<dyn crate::timer::TimerTarget> = std::sync::Arc::new(SelectTimeout);
     crate::scheduler::insert_wait_timer(deadline, me, target, epoch);
 
+    // Same eager-cleanup story as `try_select`: the timer arm needs none
+    // (stateless, stale entries die at the epoch CAS), channel arms need
+    // none, fd arms do — and a timer win in particular leaves every fd
+    // arm's registration behind, which without this pass would poison
+    // those fds until a kernel event happened to fire.
+    let eager = arms.iter().any(|a| a.sel_eager_cleanup());
+    let mut guard = UnregisterGuard { arms, me, epoch, armed: eager };
+
     crate::scheduler::park_current();
+
+    if eager {
+        unregister_arms(arms, me, epoch);
+    }
+    guard.armed = false;
+    drop(guard);
+
     // Woken precisely: an arm (ready below) or the timer (nothing ready).
-    arms.iter().position(|arm| arm.sel_ready())
+    Ok(arms.iter().position(|arm| arm.sel_ready()))
 }

src/lib.rs

@@ -45,7 +45,10 @@ static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
 // Public API re-exports
 // ---------------------------------------------------------------------------
 
-pub use channel::{channel, select, select_timeout, Receiver, RecvError, RecvTimeoutError, Selectable, Sender};
+pub use channel::{
+    channel, select, select_timeout, try_select, try_select_timeout, Receiver, RecvError,
+    RecvTimeoutError, Selectable, Sender,
+};
 pub use gen_server::{CallError, CallTimeoutError, CastError, GenServer, ServerBuilder, ServerCtx, ServerRef, Watcher};
 pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
@@ -55,7 +58,8 @@ pub use registry::{name_of, register, unregister, whereis, RegisterError};
 pub use runtime::{init, Config, Runtime};
 pub use scheduler::{
     block_on_io, request_stop, run, self_pid, sleep, spawn, spawn_under, wait_readable,
-    wait_writable, yield_now, JoinError, JoinHandle,
+    wait_readable_timeout, wait_writable, wait_writable_timeout, yield_now, FdArm, JoinError,
+    JoinHandle,
 };
 pub use supervisor::{ChildSpec, OneForOne, Restart, Signal, Strategy};
 

src/scheduler.rs

@@ -432,6 +432,138 @@ fn wait_fd(fd: std::os::fd::RawFd, readable: bool, writable: bool) -> std::io::R
     Ok(())
 }
 
+// ---------------------------------------------------------------------------
+// FdArm — fd readiness as a select arm (RFC 008)
+// ---------------------------------------------------------------------------
+
+/// An fd-readiness arm for [`crate::select`] / [`crate::select_timeout`]:
+/// ready when the fd is readable (resp. writable), composable with channel
+/// receivers on one wait epoch. Phase-1 rules apply: one waiter per fd at a
+/// time, one direction per arm (duplex on a single fd needs `dup`; epoll
+/// registrations key on the open file description, so dup'd fds register
+/// independently).
+pub struct FdArm {
+    fd: std::os::fd::RawFd,
+    readable: bool,
+    writable: bool,
+}
+
+impl FdArm {
+    pub fn readable(fd: std::os::fd::RawFd) -> Self {
+        FdArm { fd, readable: true, writable: false }
+    }
+
+    pub fn writable(fd: std::os::fd::RawFd) -> Self {
+        FdArm { fd, readable: false, writable: true }
+    }
+}
+
+impl crate::channel::sealed::Sealed for FdArm {}
+
+impl crate::channel::Selectable for FdArm {
+    /// Ready-now check is a zero-timeout `poll(2)`; if the requested events
+    /// are pending the wait is retired without registering (`Ok(false)`,
+    /// the channel-arm contract). Otherwise register with the io thread —
+    /// every failure surfaces as `Err` (EBADF including a closed-fd
+    /// POLLNVAL, EMFILE on the epoll set, AlreadyExists for a second
+    /// waiter on the fd): the fallible-out, nothing-left-behind rule, in
+    /// deviation from RFC 008's permanently-ready lean, which would spin a
+    /// consumer whose fd is healthy but unregistrable (EMFILE).
+    fn sel_register(&self, pid: Pid, epoch: u32) -> std::io::Result<bool> {
+        if poll_events(self.fd, self.readable, self.writable)? {
+            return Ok(false);
+        }
+        with_runtime(|inner| {
+            let mut io = inner.io.lock().unwrap();
+            io.as_mut()
+                .expect("io thread not started")
+                .epoll_register(self.fd, pid, epoch, self.readable, self.writable)
+        })?;
+        Ok(true)
+    }
+
+    /// Classification is the same zero-timeout poll: a pure function of fd
+    /// state, independent of the registration the cleanup pass removed. An
+    /// error here (EBADF: fd closed mid-wait) reports READY — the
+    /// consumer's read/write surfaces the errno; a dead arm is an event,
+    /// not a hang.
+    fn sel_ready(&self) -> bool {
+        poll_events(self.fd, self.readable, self.writable).unwrap_or(true)
+    }
+
+    /// `wait_fd`'s `Dereg` compare, verbatim: remove the waiters entry and
+    /// kernel-side registration iff the entry is still `(pid, epoch)`-ours.
+    /// A `FdReady` racing the wake may have consumed it (it removes + DELs
+    /// under the io lock), after which the fd may even carry ANOTHER
+    /// actor's fresh registration; in that case touch nothing.
+    fn sel_unregister(&self, pid: Pid, epoch: u32) {
+        with_runtime(|inner| {
+            let mut io = inner.io.lock().unwrap();
+            if let Some(io) = io.as_mut() {
+                if io.waiters.get(&self.fd) == Some(&(pid, epoch)) {
+                    io.waiters.remove(&self.fd);
+                    io.epoll_deregister(self.fd);
+                }
+            }
+        });
+    }
+
+    fn sel_eager_cleanup(&self) -> bool {
+        true
+    }
+}
+
+/// Zero-timeout `poll(2)`: are any of the requested events (or ERR/HUP,
+/// which make the consumer's read/write fail loudly rather than park
+/// forever) pending on `fd`? POLLNVAL maps to `Err(EBADF)`.
+fn poll_events(fd: std::os::fd::RawFd, readable: bool, writable: bool) -> std::io::Result<bool> {
+    let mut events: libc::c_short = 0;
+    if readable {
+        events |= libc::POLLIN;
+    }
+    if writable {
+        events |= libc::POLLOUT;
+    }
+    let mut pfd = libc::pollfd { fd, events, revents: 0 };
+    loop {
+        let r = unsafe { libc::poll(&mut pfd, 1, 0) };
+        if r < 0 {
+            let e = std::io::Error::last_os_error();
+            if e.kind() == std::io::ErrorKind::Interrupted {
+                continue;
+            }
+            return Err(e);
+        }
+        if r == 0 {
+            return Ok(false);
+        }
+        if pfd.revents & libc::POLLNVAL != 0 {
+            return Err(std::io::Error::from_raw_os_error(libc::EBADF));
+        }
+        return Ok(pfd.revents & (events | libc::POLLERR | libc::POLLHUP) != 0);
+    }
+}
+
+/// Wait until `fd` is readable or `timeout` elapses: `Ok(true)` = ready,
+/// `Ok(false)` = timed out. A one-arm [`crate::try_select_timeout`].
+pub fn wait_readable_timeout(
+    fd: std::os::fd::RawFd,
+    timeout: std::time::Duration,
+) -> std::io::Result<bool> {
+    let arm = FdArm::readable(fd);
+    Ok(crate::channel::try_select_timeout(&[&arm], timeout)?.is_some())
+}
+
+/// Wait until `fd` is writable or `timeout` elapses: `Ok(true)` = ready,
+/// `Ok(false)` = timed out.
+pub fn wait_writable_timeout(
+    fd: std::os::fd::RawFd,
+    timeout: std::time::Duration,
+) -> std::io::Result<bool> {
+    let arm = FdArm::writable(fd);
+    Ok(crate::channel::try_select_timeout(&[&arm], timeout)?.is_some())
+}
+
 pub fn read(fd: std::os::fd::RawFd, buf: &mut [u8]) -> std::io::Result<usize> {
     wait_readable(fd)?;
     let n = unsafe { libc::read(fd, buf.as_mut_ptr() as *mut _, buf.len()) };

tests/fd_select.rs

@@ -0,0 +1,403 @@
+//! RFC 008 — fd arms in select. Beyond the functional cases, the
+//! *_stays_usable tests are the soundness probes for the one asymmetry the
+//! RFC must close: a losing CHANNEL arm's stale registration is inert, but
+//! a losing FD arm's registration (waiters entry + kernel ONESHOT) poisons
+//! the fd with AlreadyExists until the eager cleanup pass removes it. Every
+//! "loser" scenario therefore re-waits on the same fd afterwards and must
+//! succeed — pre-cleanup, each of those re-waits errors or hangs.
+//!
+//! House pattern: actor panics are trampoline-caught and `run` returns
+//! normally, so every test funnels its result into an outcome flag asserted
+//! OUTSIDE `run` — an in-actor assertion alone passes vacuously.
+
+use smarm::{
+    channel, run, select, select_timeout, spawn, try_select, wait_readable,
+    wait_readable_timeout, wait_writable_timeout, yield_now, FdArm,
+};
+use std::os::fd::RawFd;
+use std::sync::atomic::{AtomicBool, AtomicU32, Ordering};
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+// ---------------------------------------------------------------------------
+// Pipe helper (as in io_epoll.rs)
+// ---------------------------------------------------------------------------
+
+struct Pipe {
+    read: RawFd,
+    write: RawFd,
+}
+
+impl Pipe {
+    fn new() -> Self {
+        let mut fds: [libc::c_int; 2] = [0; 2];
+        let r = unsafe { libc::pipe2(fds.as_mut_ptr(), libc::O_CLOEXEC | libc::O_NONBLOCK) };
+        assert_eq!(r, 0, "pipe2 failed");
+        Pipe { read: fds[0], write: fds[1] }
+    }
+}
+
+impl Drop for Pipe {
+    fn drop(&mut self) {
+        unsafe {
+            libc::close(self.read);
+            libc::close(self.write);
+        }
+    }
+}
+
+fn raw_write(fd: RawFd, buf: &[u8]) -> isize {
+    unsafe { libc::write(fd, buf.as_ptr() as *const _, buf.len()) }
+}
+
+fn raw_read(fd: RawFd, buf: &mut [u8]) -> isize {
+    unsafe { libc::read(fd, buf.as_mut_ptr() as *mut _, buf.len()) }
+}
+
+fn flag() -> (Arc<AtomicBool>, Arc<AtomicBool>) {
+    let f = Arc::new(AtomicBool::new(false));
+    (f.clone(), f)
+}
+
+// ---------------------------------------------------------------------------
+// Ready-now: data already pending retires the wait without parking.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn fd_arm_ready_now_returns_without_parking() {
+    let (ok, ok2) = flag();
+    run(move || {
+        let p = Pipe::new();
+        assert_eq!(raw_write(p.write, b"x"), 1);
+        let (_tx, rx) = channel::<i64>();
+        let fd_arm = FdArm::readable(p.read);
+        // fd arm at index 1: the ready-now path must also work for a
+        // non-first arm (and clean nothing — channel arms are inert).
+        let i = select(&[&rx, &fd_arm]);
+        assert_eq!(i, 1);
+        let mut buf = [0u8; 1];
+        assert_eq!(raw_read(p.read, &mut buf), 1);
+        ok2.store(true, Ordering::SeqCst);
+    });
+    assert!(ok.load(Ordering::SeqCst));
+}
+
+// ---------------------------------------------------------------------------
+// Park-then-wake: fd arm wins against an idle channel arm.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn fd_arm_parks_until_data_then_wins() {
+    let got = Arc::new(AtomicU32::new(0));
+    let got2 = got.clone();
+    run(move || {
+        let p = Pipe::new();
+        let (rfd, wfd) = (p.read, p.write);
+        let (_tx_keepalive, rx) = channel::<i64>();
+
+        let h = spawn(move || {
+            let fd_arm = FdArm::readable(rfd);
+            let i = select(&[&fd_arm, &rx]);
+            assert_eq!(i, 0);
+            let mut buf = [0u8; 1];
+            assert_eq!(raw_read(rfd, &mut buf), 1);
+            got2.store(buf[0] as u32, Ordering::SeqCst);
+        });
+        yield_now(); // let it park
+        assert_eq!(raw_write(wfd, b"y"), 1);
+        let _ = h.join();
+    });
+    assert_eq!(got.load(Ordering::SeqCst), b'y' as u32);
+}
+
+// ---------------------------------------------------------------------------
+// THE asymmetry probe: channel arm wins, losing fd arm must be cleaned —
+// the same actor (and the io thread) must be able to wait that fd again.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn losing_fd_arm_is_cleaned_up_and_fd_stays_usable() {
+    let got = Arc::new(AtomicU32::new(0));
+    let got2 = got.clone();
+    run(move || {
+        let p = Pipe::new();
+        let (rfd, wfd) = (p.read, p.write);
+        let (tx, rx) = channel::<i64>();
+
+        let h = spawn(move || {
+            let fd_arm = FdArm::readable(rfd);
+            // Channel wins; the fd arm registered and lost.
+            let i = select(&[&fd_arm, &rx]);
+            assert_eq!(i, 1);
+            assert_eq!(rx.try_recv().unwrap(), Some(7));
+
+            // Pre-cleanup this wait_readable fails AlreadyExists (the
+            // waiters entry is stale-ours) — the cleanup pass must have
+            // removed it.
+            wait_readable(rfd).unwrap();
+            let mut buf = [0u8; 1];
+            assert_eq!(raw_read(rfd, &mut buf), 1);
+            got2.store(buf[0] as u32, Ordering::SeqCst);
+        });
+        yield_now(); // let it park in the select
+        tx.send(7).unwrap();
+        yield_now(); // let it reach the second wait
+        assert_eq!(raw_write(wfd, b"z"), 1);
+        let _ = h.join();
+    });
+    assert_eq!(got.load(Ordering::SeqCst), b'z' as u32);
+}
+
+// ---------------------------------------------------------------------------
+// Ready-now on a LATER arm must unregister an earlier fd arm (the
+// register_arms prefix-cleanup path: no park ever happens).
+// ---------------------------------------------------------------------------
+
+#[test]
+fn ready_now_later_arm_cleans_earlier_fd_arm() {
+    let (ok, ok2) = flag();
+    run(move || {
+        let p = Pipe::new();
+        let (rfd, wfd) = (p.read, p.write);
+        let (tx, rx) = channel::<i64>();
+        tx.send(1).unwrap(); // arm 1 ready before the select
+
+        let fd_arm = FdArm::readable(rfd);
+        let i = select(&[&fd_arm, &rx]);
+        assert_eq!(i, 1);
+        assert_eq!(rx.try_recv().unwrap(), Some(1));
+
+        // The fd arm registered (idle pipe), then arm 1 retired the wait.
+        // Its registration must have been removed in the same pass.
+        assert_eq!(raw_write(wfd, b"a"), 1);
+        wait_readable(rfd).unwrap();
+        let mut buf = [0u8; 1];
+        assert_eq!(raw_read(rfd, &mut buf), 1);
+        ok2.store(true, Ordering::SeqCst);
+    });
+    assert!(ok.load(Ordering::SeqCst));
+}
+
+// ---------------------------------------------------------------------------
+// Two fd arms in one select (phase-1: distinct fds, nothing special).
+// ---------------------------------------------------------------------------
+
+#[test]
+fn two_fd_arms_second_fires_first_stays_usable() {
+    let (ok, ok2) = flag();
+    run(move || {
+        let pa = Pipe::new();
+        let pb = Pipe::new();
+        let (rfd_a, wfd_a) = (pa.read, pa.write);
+        let (rfd_b, wfd_b) = (pb.read, pb.write);
+
+        let h = spawn(move || {
+            let a = FdArm::readable(rfd_a);
+            let b = FdArm::readable(rfd_b);
+            let i = select(&[&a, &b]);
+            assert_eq!(i, 1);
+            let mut buf = [0u8; 1];
+            assert_eq!(raw_read(rfd_b, &mut buf), 1);
+
+            // Arm a lost; its fd must be immediately re-waitable.
+            assert_eq!(raw_write(wfd_a, b"q"), 1);
+            wait_readable(rfd_a).unwrap();
+            assert_eq!(raw_read(rfd_a, &mut buf), 1);
+            assert_eq!(buf[0], b'q');
+            ok2.store(true, Ordering::SeqCst);
+        });
+        yield_now();
+        assert_eq!(raw_write(wfd_b, b"b"), 1);
+        let _ = h.join();
+    });
+    assert!(ok.load(Ordering::SeqCst));
+}
+
+// ---------------------------------------------------------------------------
+// select_timeout: timer wins over an idle fd arm; the fd is left clean.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn select_timeout_timer_beats_idle_fd_arm_and_fd_stays_usable() {
+    let (ok, ok2) = flag();
+    run(move || {
+        let p = Pipe::new();
+        let (rfd, wfd) = (p.read, p.write);
+
+        let fd_arm = FdArm::readable(rfd);
+        let start = Instant::now();
+        let r = select_timeout(&[&fd_arm], Duration::from_millis(30));
+        assert!(r.is_none(), "idle fd must time out");
+        assert!(start.elapsed() >= Duration::from_millis(30));
+
+        // Timer win is exactly the case where the fd arm's registration is
+        // left behind without an eager pass.
+        assert_eq!(raw_write(wfd, b"c"), 1);
+        wait_readable(rfd).unwrap();
+        let mut buf = [0u8; 1];
+        assert_eq!(raw_read(rfd, &mut buf), 1);
+        ok2.store(true, Ordering::SeqCst);
+    });
+    assert!(ok.load(Ordering::SeqCst));
+}
+
+// ---------------------------------------------------------------------------
+// Derived wrappers.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn wait_readable_timeout_times_out_then_succeeds_with_data() {
+    let (ok, ok2) = flag();
+    run(move || {
+        let p = Pipe::new();
+        let (rfd, wfd) = (p.read, p.write);
+
+        let start = Instant::now();
+        assert_eq!(wait_readable_timeout(rfd, Duration::from_millis(30)).unwrap(), false);
+        assert!(start.elapsed() >= Duration::from_millis(30));
+
+        // Timed-out wait must leave the fd clean; ready path returns true.
+        assert_eq!(raw_write(wfd, b"d"), 1);
+        assert_eq!(wait_readable_timeout(rfd, Duration::from_secs(5)).unwrap(), true);
+        let mut buf = [0u8; 1];
+        assert_eq!(raw_read(rfd, &mut buf), 1);
+        ok2.store(true, Ordering::SeqCst);
+    });
+    assert!(ok.load(Ordering::SeqCst));
+}
+
+#[test]
+fn wait_readable_timeout_wakes_on_late_data() {
+    let got = Arc::new(AtomicU32::new(0));
+    let got2 = got.clone();
+    run(move || {
+        let p = Pipe::new();
+        let (rfd, wfd) = (p.read, p.write);
+        let h = spawn(move || {
+            assert_eq!(wait_readable_timeout(rfd, Duration::from_secs(5)).unwrap(), true);
+            let mut buf = [0u8; 1];
+            assert_eq!(raw_read(rfd, &mut buf), 1);
+            got2.store(buf[0] as u32, Ordering::SeqCst);
+        });
+        yield_now();
+        assert_eq!(raw_write(wfd, b"e"), 1);
+        let _ = h.join();
+    });
+    assert_eq!(got.load(Ordering::SeqCst), b'e' as u32);
+}
+
+#[test]
+fn wait_writable_timeout_ready_now_on_empty_pipe() {
+    let (ok, ok2) = flag();
+    run(move || {
+        let p = Pipe::new();
+        // An empty pipe's write end is writable: ready-now path, no park.
+        assert_eq!(wait_writable_timeout(p.write, Duration::from_secs(5)).unwrap(), true);
+        ok2.store(true, Ordering::SeqCst);
+    });
+    assert!(ok.load(Ordering::SeqCst));
+}
+
+// ---------------------------------------------------------------------------
+// Error surface: registration failure is an Err from try_select, with the
+// wait retired (the actor can immediately wait on something else).
+// ---------------------------------------------------------------------------
+
+#[test]
+fn try_select_surfaces_registration_error_and_retires_the_wait() {
+    let (ok, ok2) = flag();
+    run(move || {
+        let bad: RawFd = {
+            let p = Pipe::new();
+            p.read
+        }; // both ends closed by Drop: EBADF on registration
+
+        let fd_arm = FdArm::readable(bad);
+        let err = try_select(&[&fd_arm]).unwrap_err();
+        // EBADF, whether the pre-poll or epoll_ctl ADD reports it.
+        assert_eq!(err.raw_os_error(), Some(libc::EBADF));
+
+        // The wait was retired: a normal select right after works.
+        let (tx, rx) = channel::<i64>();
+        tx.send(9).unwrap();
+        let i = select(&[&rx]);
+        assert_eq!(i, 0);
+        assert_eq!(rx.try_recv().unwrap(), Some(9));
+        ok2.store(true, Ordering::SeqCst);
+    });
+    assert!(ok.load(Ordering::SeqCst));
+}
+
+// ---------------------------------------------------------------------------
+// Stop-unwind: an actor stopped while parked in an fd-arm select must not
+// poison the fd (the UnregisterGuard generalization of wait_fd's Dereg).
+// Mirrors io_epoll.rs::stopped_waiter_does_not_poison_the_fd.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn stopped_selector_does_not_poison_the_fd() {
+    let seen = Arc::new(AtomicU32::new(0));
+    let seen_outer = seen.clone();
+    run(move || {
+        let p = Pipe::new();
+        let (rfd, wfd) = (p.read, p.write);
+        let (_tx_keepalive, rx) = channel::<i64>();
+
+        let h = spawn(move || {
+            let fd_arm = FdArm::readable(rfd);
+            select(&[&fd_arm, &rx]);
+            unreachable!("neither arm ever fires while this actor lives");
+        });
+        yield_now(); // let it reach the park
+        smarm::request_stop(h.pid());
+        let _ = h.join(); // Ok(()): stopped, not panicked
+
+        // Second waiter on the SAME fd must register and be woken.
+        let seen2 = seen.clone();
+        let h2 = spawn(move || {
+            wait_readable(rfd).unwrap();
+            let mut buf = [0u8; 1];
+            assert_eq!(raw_read(rfd, &mut buf), 1);
+            seen2.store(buf[0] as u32, Ordering::SeqCst);
+        });
+        yield_now();
+        assert_eq!(raw_write(wfd, b"x"), 1);
+        let _ = h2.join();
+    });
+    assert_eq!(seen_outer.load(Ordering::SeqCst), b'x' as u32);
+}
+
+// ---------------------------------------------------------------------------
+// Phase-1 misuse: a second waiter on an fd that already has one is an Err
+// (AlreadyExists), not a hang — and does not disturb the first waiter.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn second_waiter_on_same_fd_errs_without_disturbing_the_first() {
+    let got = Arc::new(AtomicU32::new(0));
+    let got2 = got.clone();
+    let (ok, ok2) = flag();
+    run(move || {
+        let p = Pipe::new();
+        let (rfd, wfd) = (p.read, p.write);
+
+        let h = spawn(move || {
+            wait_readable(rfd).unwrap();
+            let mut buf = [0u8; 1];
+            assert_eq!(raw_read(rfd, &mut buf), 1);
+            got2.store(buf[0] as u32, Ordering::SeqCst);
+        });
+        yield_now(); // first waiter parked
+
+        let fd_arm = FdArm::readable(rfd);
+        let err = try_select(&[&fd_arm]).unwrap_err();
+        assert_eq!(err.kind(), std::io::ErrorKind::AlreadyExists);
+
+        // First waiter still wakes normally.
+        assert_eq!(raw_write(wfd, b"w"), 1);
+        let _ = h.join();
+        ok2.store(true, Ordering::SeqCst);
+    });
+    assert_eq!(got.load(Ordering::SeqCst), b'w' as u32);
+    assert!(ok.load(Ordering::SeqCst));
+}