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//! Concurrency extensions for `Future`.
//!
//! # Examples
//!
//! ```
//! use futures_lite::future::block_on;
//! use std::future::ready;
//! use futures_concurrency::prelude::*;
//!
//! fn main() {
//! block_on(async {
//! // Await multiple similarly-typed futures.
//! let fut = [ready(1u8), ready(2u8), ready(3u8)].join();
//! let [a, b, c] = fut.await;
//! println!("{} {} {}", a, b, c);
//!
//! // Await multiple differently-typed futures.
//! let fut = (ready(1u8), ready("hello"), ready(3u8)).join();
//! let (a, b, c) = fut.await;
//! println!("{} {} {}", a, b, c);
//!
//! // It even works with vectors of futures, providing an alternative
//! // to futures-rs' `join_all`.
//! let numbers = std::iter::repeat(12u8).take(6);
//! let fut: Vec<_> = numbers.map(ready).collect();
//! println!("{:?}", fut.join().await);
//! })
//! }
//! ```
//!
//! # Progress
//!
//! The following traits have been implemented.
//!
//! - [x] `Join`
//! - [ ] `TryJoin`
//! - [ ] `Race`
//! - [ ] `TryRace`
//!
//! # Base Futures Concurrency
//!
//! Often it's desireable to await multiple futures as if it was a single
//! future. The `join` family of operations converts multiple futures into a
//! single future that returns all of their outputs. The `race` family of
//! operations converts multiple future into a single future that returns the
//! first output.
//!
//! For operating on futures the following functions can be used:
//!
//! | Name | Return signature | When does it return? |
//! | --- | --- | --- |
//! | `Join` | `(T1, T2)` | Wait for all to complete
//! | `Race` | `T` | Return on first value
//!
//! ## Fallible Futures Concurrency
//!
//! For operating on futures that return `Result` additional `try_` variants of
//! the functions mentioned before can be used. These functions are aware of `Result`,
//! and will behave slightly differently from their base variants.
//!
//! In the case of `try_join`, if any of the futures returns `Err` all
//! futures are dropped and an error is returned. This is referred to as
//! "short-circuiting".
//!
//! In the case of `try_race`, instead of returning the first future that
//! completes it returns the first future that _successfully_ completes. This
//! means `try_race` will keep going until any one of the futures returns
//! `Ok`, or _all_ futures have returned `Err`.
//!
//! However sometimes it can be useful to use the base variants of the functions
//! even on futures that return `Result`. Here is an overview of operations that
//! work on `Result`, and their respective semantics:
//!
//! | Name | Return signature | When does it return? |
//! | --- | --- | --- |
//! | `Join` | `(Result<T, E>, Result<T, E>)` | Wait for all to complete
//! | `TryJoin` | `Result<(T1, T2), E>` | Return on first `Err`, wait for all to complete
//! | `Race` | `Result<T, E>` | Return on first value
//! | `Try_race` | `Result<T, E>` | Return on first `Ok`, reject on last Err
#![deny(missing_debug_implementations, nonstandard_style)]
#![warn(missing_docs, unreachable_pub)]
#![allow(non_snake_case)]
#![feature(maybe_uninit_uninit_array)]
mod maybe_done;
use core::future::Future;
use std::pin::Pin;
pub(crate) use maybe_done::MaybeDone;
/// The futures concurrency prelude.
pub mod prelude {
pub use super::Join;
}
/// Wait for multiple futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the futures
/// once both complete.
pub trait Join {
/// The resulting joined future.
type Output: Future;
/// Waits for multiple futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the futures once both complete.
///
/// This function returns a new future which polls both futures concurrently.
fn join(self) -> Self::Output;
}
/// Implementations for the Array type.
pub mod array {
use super::{Join as JoinTrait, MaybeDone};
use core::fmt;
use core::future::Future;
use core::pin::Pin;
use core::task::{Context, Poll};
use pin_project::pin_project;
impl<T, const N: usize> JoinTrait for [T; N]
where
T: Future,
{
type Output = Join<T, N>;
fn join(self) -> Self::Output {
Join {
elems: self.map(MaybeDone::new),
}
}
}
/// Waits for two similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
#[must_use = "futures do nothing unless you `.await` or poll them"]
#[pin_project]
pub struct Join<F, const N: usize>
where
F: Future,
{
elems: [MaybeDone<F>; N],
}
impl<F, const N: usize> fmt::Debug for Join<F, N>
where
F: Future + fmt::Debug,
F::Output: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Join").field("elems", &self.elems).finish()
}
}
impl<F, const N: usize> Future for Join<F, N>
where
F: Future,
{
type Output = [F::Output; N];
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut all_done = true;
let this = self.project();
for elem in this.elems.iter_mut() {
let elem = unsafe { Pin::new_unchecked(elem) };
if elem.poll(cx).is_pending() {
all_done = false;
}
}
if all_done {
use core::mem::MaybeUninit;
// Create the result array based on the indices
let mut out: [MaybeUninit<F::Output>; N] = MaybeUninit::uninit_array();
// NOTE: this clippy attribute can be removed once we can `collect` into `[usize; K]`.
#[allow(clippy::clippy::needless_range_loop)]
for (i, el) in this.elems.iter_mut().enumerate() {
let el = unsafe { Pin::new_unchecked(el) }.take().unwrap();
out[i] = MaybeUninit::new(el);
}
let result = unsafe { out.as_ptr().cast::<[F::Output; N]>().read() };
Poll::Ready(result)
} else {
Poll::Pending
}
}
}
}
/// Implementations for the Vec type.
pub mod vec {
use crate::iter_pin_mut;
use super::{Join as JoinTrait, MaybeDone};
use core::fmt;
use core::future::Future;
use core::mem;
use core::pin::Pin;
use core::task::{Context, Poll};
use std::boxed::Box;
use std::vec::Vec;
impl<T> JoinTrait for Vec<T>
where
T: Future,
{
type Output = Join<T>;
fn join(self) -> Self::Output {
let elems: Box<[_]> = self.into_iter().map(MaybeDone::new).collect();
Join {
elems: elems.into(),
}
}
}
/// Waits for two similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct Join<F>
where
F: Future,
{
elems: Pin<Box<[MaybeDone<F>]>>,
}
impl<F> fmt::Debug for Join<F>
where
F: Future + fmt::Debug,
F::Output: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Join").field("elems", &self.elems).finish()
}
}
impl<F> Future for Join<F>
where
F: Future,
{
type Output = Vec<F::Output>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut all_done = true;
for elem in iter_pin_mut(self.elems.as_mut()) {
if elem.poll(cx).is_pending() {
all_done = false;
}
}
if all_done {
let mut elems = mem::replace(&mut self.elems, Box::pin([]));
let result = iter_pin_mut(elems.as_mut())
.map(|e| e.take().unwrap())
.collect();
Poll::Ready(result)
} else {
Poll::Pending
}
}
}
}
/// Implementations for the tuple type.
pub mod tuple {
use super::{Join as JoinTrait, MaybeDone};
use core::fmt;
use core::future::Future;
use core::pin::Pin;
use core::task::{Context, Poll};
use pin_project::pin_project;
macro_rules! generate {
($(
$(#[$doc:meta])*
($Join:ident, <$($Fut:ident),*>),
)*) => ($(
$(#[$doc])*
#[pin_project]
#[must_use = "futures do nothing unless you `.await` or poll them"]
#[allow(non_snake_case)]
pub struct $Join<$($Fut: Future),*> {
$(#[pin] $Fut: MaybeDone<$Fut>,)*
}
impl<$($Fut),*> fmt::Debug for $Join<$($Fut),*>
where
$(
$Fut: Future + fmt::Debug,
$Fut::Output: fmt::Debug,
)*
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct(stringify!($Join))
$(.field(stringify!($Fut), &self.$Fut))*
.finish()
}
}
impl<$($Fut: Future),*> $Join<$($Fut),*> {
fn new(($($Fut),*): ($($Fut),*)) -> Self {
Self {
$($Fut: MaybeDone::new($Fut)),*
}
}
}
impl<$($Fut),*> JoinTrait for ($($Fut),*)
where
$(
$Fut: Future,
)*
{
type Output = $Join<$($Fut),*>;
fn join(self) -> Self::Output {
$Join::new(self)
}
}
impl<$($Fut: Future),*> Future for $Join<$($Fut),*> {
type Output = ($($Fut::Output),*);
fn poll(
self: Pin<&mut Self>, cx: &mut Context<'_>
) -> Poll<Self::Output> {
let mut all_done = true;
let mut futures = self.project();
$(
all_done &= futures.$Fut.as_mut().poll(cx).is_ready();
)*
if all_done {
Poll::Ready(($(futures.$Fut.take().unwrap()), *))
} else {
Poll::Pending
}
}
}
)*)
}
generate! {
/// Waits for two similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join2, <A, B>),
/// Waits for three similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join3, <A, B, C>),
/// Waits for four similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join4, <A, B, C, D>),
/// Waits for five similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join5, <A, B, C, D, E>),
/// Waits for six similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join6, <A, B, C, D, E, F>),
/// Waits for seven similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join7, <A, B, C, D, E, F, G>),
/// Waits for eight similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join8, <A, B, C, D, E, F, G, H>),
/// Waits for nine similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join9, <A, B, C, D, E, F, G, H, I>),
/// Waits for ten similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join10, <A, B, C, D, E, F, G, H, I, J>),
/// Waits for eleven similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join11, <A, B, C, D, E, F, G, H, I, J, K>),
/// Waits for twelve similarly-typed futures to complete.
///
/// Awaits multiple futures simultaneously, returning the output of the
/// futures once both complete.
(Join12, <A, B, C, D, E, F, G, H, I, J, K, L>),
}
}
pub(crate) fn iter_pin_mut<T>(slice: Pin<&mut [T]>) -> impl Iterator<Item = Pin<&mut T>> {
// Safety: `std` _could_ make this unsound if it were to decide Pin's
// invariants aren't required to transmit through slices. Otherwise this has
// the same safety as a normal field pin projection.
unsafe { slice.get_unchecked_mut() }
.iter_mut()
.map(|t| unsafe { Pin::new_unchecked(t) })
}