1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157

//! Definition of the `JoinAll` combinator, waiting for all of a list of futures
//! to finish.

use core::fmt;
use core::future::Future;
use core::iter::FromIterator;
use core::mem;
use core::pin::Pin;
use core::task::{Context, Poll};
use alloc::boxed::Box;
use alloc::vec::Vec;

#[derive(Debug)]
enum ElemState<F>
where
    F: Future,
{
    Pending(F),
    Done(Option<F::Output>),
}

impl<F> ElemState<F>
where
    F: Future,
{
    fn pending_pin_mut<'a>(self: Pin<&'a mut Self>) -> Option<Pin<&'a mut F>> {
        // Safety: Basic enum pin projection, no drop + optionally Unpin based
        // on the type of this variant
        match unsafe { self.get_unchecked_mut() } {
            ElemState::Pending(f) => Some(unsafe { Pin::new_unchecked(f) }),
            ElemState::Done(_) => None,
        }
    }

    fn take_done(self: Pin<&mut Self>) -> Option<F::Output> {
        // Safety: Going from pin to a variant we never pin-project
        match unsafe { self.get_unchecked_mut() } {
            ElemState::Pending(_) => None,
            ElemState::Done(output) => output.take(),
        }
    }
}

impl<F> Unpin for ElemState<F>
where
    F: Future + Unpin,
{
}

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) })
}

/// Future for the [`join_all`] function.
#[must_use = "futures do nothing unless polled"]
pub struct JoinAll<F>
where
    F: Future,
{
    elems: Pin<Box<[ElemState<F>]>>,
}

impl<F> fmt::Debug for JoinAll<F>
where
    F: Future + fmt::Debug,
    F::Output: fmt::Debug,
{
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("JoinAll")
            .field("elems", &self.elems)
            .finish()
    }
}

/// 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.
///
/// # See Also
///
/// This is purposefully a very simple API for basic use-cases. In a lot of
/// cases you will want to use the more powerful
/// [`FuturesUnordered`][crate::stream::FuturesUnordered] APIs, some
/// examples of additional functionality that provides:
///
///  * 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
///
/// ```
/// #![feature(async_await, await_macro, futures_api)]
/// # 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!(await!(join_all(futures)), [1, 2, 3]);
/// # });
/// ```
pub fn join_all<I>(i: I) -> JoinAll<I::Item>
where
    I: IntoIterator,
    I::Item: Future,
{
    let elems: Box<[_]> = i.into_iter().map(ElemState::Pending).collect();
    JoinAll { elems: Box::into_pin(elems) }
}

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> {
        let mut all_done = true;

        for mut elem in iter_pin_mut(self.elems.as_mut()) {
            if let Some(pending) = elem.as_mut().pending_pin_mut() {
                if let Poll::Ready(output) = pending.poll(cx) {
                    elem.set(ElemState::Done(Some(output)));
                } else {
                    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_done().unwrap())
                .collect();
            Poll::Ready(result)
        } else {
            Poll::Pending
        }
    }
}

impl<F: Future> FromIterator<F> for JoinAll<F> {
    fn from_iter<T: IntoIterator<Item = F>>(iter: T) -> Self {
        join_all(iter)
    }
}