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oneshot.rs
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oneshot.rs
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//! A one-time send - receive channel.
use std::cell::UnsafeCell;
use std::future::Future;
use std::marker::PhantomData;
use std::rc::Rc;
use std::task::{Poll, Waker};
use thiserror::Error;
/// Error returned by awaiting the [`Receiver`].
#[derive(Debug, Error)]
#[error("channel has been closed.")]
pub struct RecvError {
_marker: PhantomData<()>,
}
#[derive(Debug)]
struct Inner<T> {
rx_waker: Option<Waker>,
closed: bool,
item: Option<T>,
}
/// The receiver of a oneshot channel.
#[derive(Debug)]
pub struct Receiver<T> {
inner: Rc<UnsafeCell<Inner<T>>>,
}
impl<T> Future for Receiver<T> {
type Output = Result<T, RecvError>;
fn poll(self: std::pin::Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
// SAFETY:
//
// We can acquire a mutable reference without checking as:
//
// - This type is !Send.
// - This function is not used by any other functions and hence uniquely owns the
// mutable reference.
// - The mutable reference is dropped at the end of this function.
let inner = unsafe { &mut *self.inner.get() };
// Implementation Note:
//
// It might be neater to use a match pattern here.
// However, this will slow down the polling process by 10%.
if let Some(m) = inner.item.take() {
return Poll::Ready(Ok(m));
}
if inner.closed {
return Poll::Ready(Err(RecvError {
_marker: PhantomData,
}));
}
inner.rx_waker = Some(cx.waker().clone());
Poll::Pending
}
}
impl<T> Drop for Receiver<T> {
fn drop(&mut self) {
// SAFETY:
//
// We can acquire a mutable reference without checking as:
//
// - This type is !Send.
// - This function is not used by any other functions and hence uniquely owns the
// mutable reference.
// - The mutable reference is dropped at the end of this function.
let inner = unsafe { &mut *self.inner.get() };
inner.closed = true;
}
}
/// The sender of a oneshot channel.
#[derive(Debug)]
pub struct Sender<T> {
inner: Rc<UnsafeCell<Inner<T>>>,
}
impl<T> Sender<T> {
/// Send an item to the other side of the channel, consumes the sender.
pub fn send(self, item: T) -> Result<(), T> {
// SAFETY:
//
// We can acquire a mutable reference without checking as:
//
// - This type is !Send.
// - This function is not used by any other functions and hence uniquely owns the
// mutable reference.
// - The mutable reference is dropped at the end of this function.
let inner = unsafe { &mut *self.inner.get() };
if inner.closed {
return Err(item);
}
inner.item = Some(item);
if let Some(ref m) = inner.rx_waker {
m.wake_by_ref();
}
Ok(())
}
}
impl<T> Drop for Sender<T> {
fn drop(&mut self) {
// SAFETY:
//
// We can acquire a mutable reference without checking as:
//
// - This type is !Send.
// - This function is not used by any other functions and hence uniquely owns the
// mutable reference.
// - The mutable reference is dropped at the end of this function.
let inner = unsafe { &mut *self.inner.get() };
inner.closed = true;
if inner.item.is_none() {
if let Some(ref m) = inner.rx_waker {
m.wake_by_ref();
}
}
}
}
/// Creates a oneshot channel.
pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
let inner = Rc::new(UnsafeCell::new(Inner {
rx_waker: None,
closed: false,
item: None,
}));
(
Sender {
inner: inner.clone(),
},
Receiver { inner },
)
}
#[cfg(not(target_arch = "wasm32"))]
#[cfg(feature = "tokio")]
#[cfg(test)]
mod tests {
use std::sync::Arc;
use std::time::Duration;
use tokio::sync::Barrier;
use tokio::task::LocalSet;
use tokio::test;
use super::*;
use crate::platform::spawn_local;
use crate::platform::time::sleep;
#[test]
async fn oneshot_works() {
let (tx, rx) = channel();
tx.send(0).expect("failed to send.");
assert_eq!(rx.await.expect("failed to receive."), 0);
}
#[test]
async fn oneshot_drops_sender() {
let local_set = LocalSet::new();
local_set
.run_until(async {
let (tx, rx) = channel::<usize>();
spawn_local(async move {
sleep(Duration::from_millis(1)).await;
drop(tx);
});
rx.await.expect_err("successful to receive.");
})
.await;
}
#[test]
async fn oneshot_drops_receiver() {
let local_set = LocalSet::new();
local_set
.run_until(async {
let (tx, rx) = channel::<usize>();
let bar = Arc::new(Barrier::new(2));
{
let bar = bar.clone();
spawn_local(async move {
sleep(Duration::from_millis(1)).await;
drop(rx);
bar.wait().await;
});
}
bar.wait().await;
tx.send(0).expect_err("successful to send.");
})
.await;
}
}