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
158
159
160
161
162
163
164
165
166
167

//! 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};

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))]
const SMALL: usize = 30;

pub(crate) 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,
{
    #[cfg(futures_no_atomic_cas)]
    {
        let elems = iter.into_iter().map(MaybeDone::Future).collect::<Box<[_]>>().into();
        let kind = JoinAllKind::Small { elems };
        assert_future::<Vec<<I::Item as Future>::Output>, _>(JoinAll { kind })
    }
    #[cfg(not(futures_no_atomic_cas))]
    {
        let iter = iter.into_iter();
        let kind = match iter.size_hint().1 {
            None => JoinAllKind::Big { fut: iter.collect::<FuturesOrdered<_>>().collect() },
            Some(max) => {
                if max <= SMALL {
                    let elems = iter.map(MaybeDone::Future).collect::<Box<[_]>>().into();
                    JoinAllKind::Small { elems }
                } else {
                    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)
    }
}