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//! Definition of the `JoinAll` combinator, waiting for all of a list of futures
//! to finish.
use alloc::boxed::Box;
use alloc::vec::Vec;
use core::fmt;
use core::future::Future;
use core::iter::FromIterator;
use core::mem;
use core::pin::Pin;
use core::task::{Context, Poll};
use super::{assert_future, MaybeDone};
#[cfg(not(futures_no_atomic_cas))]
use crate::stream::{Collect, FuturesOrdered, StreamExt};
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) })
}
#[must_use = "futures do nothing unless you `.await` or poll them"]
/// Future for the [`join_all`] function.
pub struct JoinAll<F>
where
F: Future,
{
kind: JoinAllKind<F>,
}
#[cfg(not(futures_no_atomic_cas))]
pub(crate) const SMALL: usize = 30;
enum JoinAllKind<F>
where
F: Future,
{
Small {
elems: Pin<Box<[MaybeDone<F>]>>,
},
#[cfg(not(futures_no_atomic_cas))]
Big {
fut: Collect<FuturesOrdered<F>, Vec<F::Output>>,
},
}
impl<F> fmt::Debug for JoinAll<F>
where
F: Future + fmt::Debug,
F::Output: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.kind {
JoinAllKind::Small { ref elems } => {
f.debug_struct("JoinAll").field("elems", elems).finish()
}
#[cfg(not(futures_no_atomic_cas))]
JoinAllKind::Big { ref fut, .. } => fmt::Debug::fmt(fut, f),
}
}
}
/// Creates a future which represents a collection of the outputs of the futures
/// given.
///
/// The returned future will drive execution for all of its underlying futures,
/// collecting the results into a destination `Vec<T>` in the same order as they
/// were provided.
///
/// This function is only available when the `std` or `alloc` feature of this
/// library is activated, and it is activated by default.
///
/// # See Also
///
/// `join_all` will switch to the more powerful [`FuturesOrdered`] for performance
/// reasons if the number of futures is large. You may want to look into using it or
/// it's counterpart [`FuturesUnordered`][crate::stream::FuturesUnordered] directly.
///
/// Some examples for additional functionality provided by these are:
///
/// * Adding new futures to the set even after it has been started.
///
/// * Only polling the specific futures that have been woken. In cases where
/// you have a lot of futures this will result in much more efficient polling.
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::join_all;
///
/// async fn foo(i: u32) -> u32 { i }
///
/// let futures = vec![foo(1), foo(2), foo(3)];
///
/// assert_eq!(join_all(futures).await, [1, 2, 3]);
/// # });
/// ```
pub fn join_all<I>(iter: I) -> JoinAll<I::Item>
where
I: IntoIterator,
I::Item: Future,
{
let iter = iter.into_iter();
#[cfg(futures_no_atomic_cas)]
{
let kind =
JoinAllKind::Small { elems: iter.map(MaybeDone::Future).collect::<Box<[_]>>().into() };
assert_future::<Vec<<I::Item as Future>::Output>, _>(JoinAll { kind })
}
#[cfg(not(futures_no_atomic_cas))]
{
let kind = match iter.size_hint().1 {
Some(max) if max <= SMALL => JoinAllKind::Small {
elems: iter.map(MaybeDone::Future).collect::<Box<[_]>>().into(),
},
_ => JoinAllKind::Big { fut: iter.collect::<FuturesOrdered<_>>().collect() },
};
assert_future::<Vec<<I::Item as Future>::Output>, _>(JoinAll { kind })
}
}
impl<F> Future for JoinAll<F>
where
F: Future,
{
type Output = Vec<F::Output>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
match &mut self.kind {
JoinAllKind::Small { elems } => {
let mut all_done = true;
for elem in iter_pin_mut(elems.as_mut()) {
if elem.poll(cx).is_pending() {
all_done = false;
}
}
if all_done {
let mut elems = mem::replace(elems, Box::pin([]));
let result =
iter_pin_mut(elems.as_mut()).map(|e| e.take_output().unwrap()).collect();
Poll::Ready(result)
} else {
Poll::Pending
}
}
#[cfg(not(futures_no_atomic_cas))]
JoinAllKind::Big { fut } => Pin::new(fut).poll(cx),
}
}
}
impl<F: Future> FromIterator<F> for JoinAll<F> {
fn from_iter<T: IntoIterator<Item = F>>(iter: T) -> Self {
join_all(iter)
}
}