Add simple mailbox

src/mailbox.rs

@@ -0,0 +1,114 @@
+use std::{
+	future::Future,
+	sync::Mutex,
+	task::{Context, Poll, Waker},
+};
+
+pub struct Mailbox<T>(Mutex<MailboxInner<T>>);
+
+impl<T> Mailbox<T> {
+	pub fn new() -> Self {
+		Mailbox(Mutex::new(MailboxInner {
+			value: None,
+			waker: None,
+		}))
+	}
+}
+
+struct MailboxInner<T> {
+	value: Option<T>,
+	waker: Option<Waker>,
+}
+
+struct MailboxFuture<'a, T>(&'a Mailbox<T>);
+impl<'a, T> Future for MailboxFuture<'a, T> {
+	type Output = T;
+
+	fn poll(self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+		let mut mb = self.0 .0.lock().unwrap();
+		if let Some(value) = mb.value.take() {
+			return Poll::Ready(value);
+		}
+		mb.waker = Some(cx.waker().clone());
+		Poll::Pending
+	}
+}
+
+impl<T> Mailbox<T> {
+	pub fn put(&self, value: T) {
+		let mut mb = self.0.lock().unwrap();
+		mb.value = Some(value);
+		let Some(waker) = mb.waker.take() else {
+			return;
+		};
+		drop(mb);
+		waker.wake();
+	}
+
+	pub fn get(&self) -> impl Future<Output = T> + '_ {
+		MailboxFuture(&self)
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use std::{pin::pin, sync::Arc, task::Wake};
+
+	use super::*;
+
+	struct NoopWaker;
+	impl Wake for NoopWaker {
+		fn wake(self: std::sync::Arc<Self>) {}
+	}
+
+	fn ready(f: impl Future) -> bool {
+		let f = pin!(f);
+		matches!(
+			f.poll(&mut Context::from_waker(&Arc::new(NoopWaker).into())),
+			Poll::Ready(_)
+		)
+	}
+
+	#[test]
+	fn test_mailbox_empty() {
+		let mb = Mailbox::<i32>::new();
+		assert!(!ready(mb.get()));
+	}
+
+	#[test]
+	fn test_mailbox_once() {
+		let mb = Mailbox::<i32>::new();
+		mb.put(42);
+		assert_eq!(pollster::block_on(mb.get()), 42);
+		assert!(!ready(mb.get()));
+	}
+
+	#[test]
+	fn test_mailbox_once_oor() {
+		let mb = Mailbox::<i32>::new();
+		let f = mb.get();
+		mb.put(42);
+		assert_eq!(pollster::block_on(f), 42);
+	}
+
+	#[test]
+	fn test_mailbox_twice_no_wait() {
+		let mb = Mailbox::<i32>::new();
+		mb.put(42);
+		mb.put(13);
+		assert_eq!(pollster::block_on(mb.get()), 13);
+		assert!(!ready(mb.get()));
+	}
+
+	#[test]
+	fn test_mailbox_twice_in_order() {
+		let mb = Mailbox::<i32>::new();
+		let f = mb.get();
+		mb.put(42);
+		assert_eq!(pollster::block_on(f), 42);
+		let f = mb.get();
+		mb.put(13);
+		assert_eq!(pollster::block_on(f), 13);
+		assert!(!ready(mb.get()));
+	}
+}

src/main.rs

@@ -6,6 +6,8 @@ use winit::{
 	window::{Window, WindowAttributes},
 };
 
+mod mailbox;
+
 fn main() {
 	let event_loop = EventLoop::new().unwrap();