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use std::cell::Cell;
use std::future::Future;
use std::marker::PhantomData;
use std::pin::Pin;
use std::ptr;
use std::task::{Context, Poll};
#[derive(Debug)]
pub struct Sender<T> {
_p: PhantomData<T>,
}
#[derive(Debug)]
pub struct Receiver<T> {
_p: PhantomData<T>,
}
pub(crate) struct Enter<'a, T> {
_rx: &'a mut Receiver<T>,
prev: *mut (),
}
pub fn pair<T>() -> (Sender<T>, Receiver<T>) {
let tx = Sender { _p: PhantomData };
let rx = Receiver { _p: PhantomData };
(tx, rx)
}
// Tracks the pointer to `Option<T>`.
//
// TODO: Ensure wakers match?
thread_local!(static STORE: Cell<*mut ()> = Cell::new(ptr::null_mut()));
// ===== impl Sender =====
impl<T> Sender<T> {
pub fn send(&mut self, value: T) -> impl Future<Output = ()> {
Send { value: Some(value) }
}
}
struct Send<T> {
value: Option<T>,
}
impl<T> Unpin for Send<T> {}
impl<T> Future for Send<T> {
type Output = ();
fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<()> {
if self.value.is_none() {
return Poll::Ready(());
}
STORE.with(|cell| unsafe {
let ptr = cell.get() as *mut Option<T>;
let option_ref = ptr.as_mut().expect("invalid usage");
if option_ref.is_none() {
*option_ref = self.value.take();
}
Poll::Pending
})
}
}
// ===== impl Receiver =====
impl<T> Receiver<T> {
pub(crate) fn enter<'a>(&'a mut self, dst: &'a mut Option<T>) -> Enter<'a, T> {
let prev = STORE.with(|cell| {
let prev = cell.get();
cell.set(dst as *mut _ as *mut ());
prev
});
Enter { _rx: self, prev }
}
}
// ===== impl Enter =====
impl<'a, T> Drop for Enter<'a, T> {
fn drop(&mut self) {
STORE.with(|cell| cell.set(self.prev));
}
}
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