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#![no_std]
#![forbid(unsafe_code)]
#![deny(missing_docs)]
/*!
Foreback provides some future adapters for driving background work concurrently
with foreground work. See [`FutureExt`] for details.
*/
use core::{
convert::Infallible,
future::Future,
pin::Pin,
task::ready,
task::{Context, Poll},
};
use futures_core::future::{FusedFuture, TryFuture};
use pin_project::pin_project;
/// Future for [`with_try_background`][FutureExt::with_try_background]
#[pin_project]
#[derive(Debug, Clone, Default)]
pub struct TryForegroundBackground<F, B> {
#[pin]
foreground: F,
#[pin]
background: B,
}
#[must_use]
#[inline(always)]
fn assert_future<F: Future>(fut: F) -> F {
fut
}
#[must_use]
#[inline(always)]
fn unwrap_infallible<T>(res: Result<T, Infallible>) -> T {
res.expect("can never be Infallible")
}
impl<F, B> Future for TryForegroundBackground<F, B>
where
F: Future,
B: TryFuture<Ok = ()> + FusedFuture,
{
type Output = Result<F::Output, B::Error>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let this = self.project();
match this.foreground.poll(cx) {
Poll::Ready(value) => Poll::Ready(Ok(value)),
Poll::Pending if this.background.is_terminated() => Poll::Pending,
Poll::Pending => match ready!(this.background.try_poll(cx)) {
Ok(()) => Poll::Pending,
Err(err) => Poll::Ready(Err(err)),
},
}
}
}
#[pin_project]
#[derive(Debug, Clone, Default)]
struct NeverError<F> {
#[pin]
fut: F,
}
impl<F: Future> Future for NeverError<F> {
type Output = Result<F::Output, Infallible>;
#[inline]
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
self.project().fut.poll(cx).map(Ok)
}
}
impl<F: FusedFuture> FusedFuture for NeverError<F> {
fn is_terminated(&self) -> bool {
self.fut.is_terminated()
}
}
/// Future for [`with_background`][FutureExt::with_background]
#[pin_project]
#[derive(Debug, Clone, Default)]
pub struct ForegroundBackground<F, B> {
#[pin]
inner: TryForegroundBackground<F, NeverError<B>>,
}
impl<F, B> Future for ForegroundBackground<F, B>
where
F: Future,
B: Future<Output = ()> + FusedFuture,
{
type Output = F::Output;
#[inline(always)]
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
self.project().inner.poll(cx).map(unwrap_infallible)
}
}
impl<F, B> FusedFuture for ForegroundBackground<F, B>
where
F: FusedFuture,
B: Future<Output = ()> + FusedFuture,
{
fn is_terminated(&self) -> bool {
self.inner.foreground.is_terminated()
}
}
/**
Extension trait for [`Future`] providing adapters to run other futures
concurrently in the background.
*/
pub trait FutureExt: Sized + Future {
/**
Arrange for a future to run concurrently with a background future.
This method returns a new future that waits for `self` to complete
while also concurrently running `background`. If `background` finishes
before `self`, it will still wait for `self` to complete. If `self`
finishes first, the future will resolve immediately and no longer poll
`background`.
# Example
```rust
use std::time::Duration;
use futures::future::FutureExt as _;
use futures::executor::block_on;
use async_channel::{bounded, Receiver, Sender};
use foreback::FutureExt as _;
/// Send pings forever to a channel
async fn ticker(sender: Sender<()>) {
while let Ok(()) = sender.send(()).await {}
}
block_on(async {
let (send, recv) = bounded(1);
let result = async move {
let mut counter = 0;
for _ in 0..5 {
let () = recv.recv().await.unwrap();
counter += 1;
}
counter
}
.with_background(ticker(send).fuse())
.await;
assert_eq!(result, 5);
});
```
*/
#[must_use = "futures do nothing unless you `.await` or poll them"]
#[inline]
fn with_background<B>(self, background: B) -> ForegroundBackground<Self, B>
where
B: Future<Output = ()> + FusedFuture,
{
assert_future(ForegroundBackground {
inner: self.with_try_background(NeverError { fut: background }),
})
}
/**
Arrange for a future to run concurrently with a fallible background
future.
This method returns a new future that waits for `self` to complete
while also concurrently running `background`. If `background` finishes
before `self`, it will still wait for `self` to complete. If `self`
finishes first, the future will resolve immediately and no longer poll
`background`. If `background` finishes with an error, that error will
be returned immediately, and `self` will no longer be polled.
# Example
```rust
use std::time::Duration;
use futures::future::{FutureExt as _, pending};
use futures::executor::block_on;
use async_channel::{bounded, Receiver, Sender, SendError};
use foreback::FutureExt as _;
/// Send pings forever to a channel. Return an error if the channel closes.
async fn ticker(sender: Sender<()>) -> Result<(), SendError<()>> {
loop {
match sender.send(()).await {
Ok(()) => {},
Err(err) => return Err(err),
}
}
}
block_on(async {
let (send, recv) = bounded(1);
let result = async move {
let mut counter = 0;
for _ in 0..5 {
let () = recv.recv().await.unwrap();
counter += 1;
}
counter
}
.with_try_background(ticker(send).fuse())
.await
.unwrap();
assert_eq!(result, 5);
});
block_on(async {
let (send, recv) = bounded(1);
let result = async move {
let mut counter = 0;
for _ in 0..5 {
let () = recv.recv().await.unwrap();
counter += 1;
}
drop(recv);
let () = pending().await;
counter
}
.with_try_background(ticker(send).fuse())
.await
.unwrap_err();
});
```
*/
#[must_use = "futures do nothing unless you `.await` or poll them"]
#[inline]
fn with_try_background<B>(self, background: B) -> TryForegroundBackground<Self, B>
where
B: TryFuture<Ok = ()> + FusedFuture,
{
assert_future(TryForegroundBackground {
foreground: self,
background,
})
}
}
impl<F: Future> FutureExt for F {}