smarm/src/channel.rs

//! Unbounded MPSC channels.
//!
//! Single-threaded scheduler: the inner state is `Rc<RefCell<Inner<T>>>`,
//! not `Arc<Mutex>`. We hand-implement `Send` for `Sender<T>` and
//! `Receiver<T>` when `T: Send`, on the basis that the only way two actor
//! contexts touch the same channel is by being scheduled on the *same* OS
//! thread (v0.1 has exactly one). When we add a second scheduler thread,
//! this lie must be retired: replace `Rc<RefCell>` with `Arc<Mutex>` (or a
//! lock-free queue) and remove the unsafe Send impls.
//!
//! Semantics:
//!   - Senders are clonable; the last sender drop closes the channel.
//!   - `Receiver::recv` on an empty open channel parks the receiver.
//!   - `Receiver::recv` on an empty closed channel returns `Err(RecvError)`.
//!   - `Sender::send` on an open channel always succeeds.
//!   - `Sender::send` on a closed channel (receiver dropped) returns
//!     `Err(SendError(value))`.
//!   - When a send pushes to a previously empty queue and a receiver is
//!     parked, the receiver is unparked.

use crate::pid::Pid;
use std::cell::RefCell;
use std::collections::VecDeque;
use std::rc::Rc;

pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
    let inner = Rc::new(RefCell::new(Inner {
        queue: VecDeque::new(),
        parked_receiver: None,
        senders: 1,
        receiver_alive: true,
    }));
    (Sender { inner: inner.clone() }, Receiver { inner })
}

struct Inner<T> {
    queue: VecDeque<T>,
    parked_receiver: Option<Pid>,
    senders: usize,
    receiver_alive: bool,
}

pub struct Sender<T> {
    inner: Rc<RefCell<Inner<T>>>,
}

pub struct Receiver<T> {
    inner: Rc<RefCell<Inner<T>>>,
}

// SAFETY (v0.1 only): the scheduler is single-threaded. Sender/Receiver can
// be captured into actor closures (which require Send), but they will only
// ever be touched from one OS thread. When multi-threading lands, swap the
// `Rc<RefCell>` for `Arc<Mutex>` and remove these.
unsafe impl<T: Send> Send for Sender<T> {}
unsafe impl<T: Send> Send for Receiver<T> {}

#[derive(Debug, PartialEq, Eq)]
pub struct SendError<T>(pub T);

#[derive(Debug, PartialEq, Eq, Clone, Copy)]
pub struct RecvError;

impl std::fmt::Display for RecvError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "channel closed")
    }
}

impl std::error::Error for RecvError {}

impl<T> Clone for Sender<T> {
    fn clone(&self) -> Self {
        self.inner.borrow_mut().senders += 1;
        Sender { inner: self.inner.clone() }
    }
}

impl<T> Drop for Sender<T> {
    fn drop(&mut self) {
        let unpark = {
            let mut g = self.inner.borrow_mut();
            g.senders -= 1;
            if g.senders == 0 && g.queue.is_empty() {
                // Channel closed and drained. Wake the receiver so it can
                // see RecvError.
                g.parked_receiver.take()
            } else {
                None
            }
        };
        if let Some(pid) = unpark {
            crate::scheduler::unpark(pid);
        }
    }
}

impl<T> Drop for Receiver<T> {
    fn drop(&mut self) {
        self.inner.borrow_mut().receiver_alive = false;
    }
}

impl<T> Sender<T> {
    pub fn send(&self, value: T) -> Result<(), SendError<T>> {
        let unpark = {
            let mut g = self.inner.borrow_mut();
            if !g.receiver_alive {
                return Err(SendError(value));
            }
            g.queue.push_back(value);
            // If the receiver is parked, unpark it.
            g.parked_receiver.take()
        };
        if let Some(pid) = unpark {
            crate::scheduler::unpark(pid);
        }
        Ok(())
    }
}

impl<T> Receiver<T> {
    pub fn recv(&self) -> Result<T, RecvError> {
        loop {
            // Try to take a message.
            {
                let mut g = self.inner.borrow_mut();
                if let Some(v) = g.queue.pop_front() {
                    return Ok(v);
                }
                if g.senders == 0 {
                    return Err(RecvError);
                }
                // Empty + open: register and park.
                let me = crate::actor::current_pid()
                    .expect("recv() called outside an actor");
                debug_assert!(
                    g.parked_receiver.is_none(),
                    "channel has more than one receiver"
                );
                g.parked_receiver = Some(me);
            }
            // Release the borrow before parking — the unparker will need it.
            crate::scheduler::park_current();
            // Loop: the message that woke us might already have been taken
            // (it can't, with one receiver, but the senders=0 path can fire
            // here too).
        }
    }

    /// Non-blocking. `Ok(Some(v))` if a message was available, `Ok(None)` if
    /// the channel is empty but open, `Err(RecvError)` if closed and drained.
    pub fn try_recv(&self) -> Result<Option<T>, RecvError> {
        let mut g = self.inner.borrow_mut();
        if let Some(v) = g.queue.pop_front() {
            return Ok(Some(v));
        }
        if g.senders == 0 {
            return Err(RecvError);
        }
        Ok(None)
    }
}