use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use pin_project::pin_project;
pub(crate) fn lazy<F, R>(f: F) -> Lazy<F, R>
where
F: FnOnce() -> R,
R: Future,
{
Lazy {
inner: InnerLazy::Init(f),
}
}
#[pin_project]
pub(crate) struct Lazy<F, R> {
#[pin]
inner: InnerLazy<F, R>,
}
#[pin_project(project = InnerProj, project_replace = InnerProjReplace)]
enum InnerLazy<F, R> {
Init(F),
Future(#[pin] R),
Empty,
}
impl<F, R> Future for Lazy<F, R>
where
F: FnOnce() -> R,
R: Future,
{
type Output = R::Output;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut this = self.project();
loop {
if let InnerProj::Future(future) = this.inner.as_mut().project() {
return future.poll(cx);
}
if let InnerProjReplace::Init(f) = this.inner.as_mut().project_replace(InnerLazy::Empty)
{
this.inner.set(InnerLazy::Future(f()));
}
if let InnerProj::Empty = this.inner.as_mut().project() {
panic!("Lazy future polled after completion");
}
}
}
}
#[cfg(test)]
mod tests {
use std::sync::atomic::AtomicUsize;
use futures_util::poll;
use super::*;
#[tokio::test]
async fn lazy_future() {
let (tx, rx) = tokio::sync::oneshot::channel();
let count = AtomicUsize::new(0);
let mut future = std::pin::pin!(lazy(|| async {
rx.await.unwrap();
count.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
}));
assert_eq!(count.load(std::sync::atomic::Ordering::SeqCst), 0);
assert_eq!(poll!(future.as_mut()), Poll::Pending);
tx.send(()).unwrap();
assert_eq!(poll!(future.as_mut()), Poll::Ready(()));
assert_eq!(count.load(std::sync::atomic::Ordering::SeqCst), 1);
}
#[cfg(not(tarpaulin))]
#[tokio::test]
#[should_panic]
async fn lazy_future_panic() {
let count = AtomicUsize::new(0);
let mut future = std::pin::pin!(lazy(|| async {
count.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
}));
assert_eq!(count.load(std::sync::atomic::Ordering::SeqCst), 0);
assert!(poll!(future.as_mut()).is_ready());
assert_eq!(count.load(std::sync::atomic::Ordering::SeqCst), 1);
let _ = poll!(future.as_mut());
}
}