async_core/
defer.rs

1#![cfg(not(feature = "no_std"))]
2extern crate std;
3
4use core::{
5    future::Future,
6    pin::Pin,
7    task::{Context, Poll},
8};
9use std::{
10    sync::mpsc::{channel, Receiver},
11    thread,
12};
13
14/// A future that runs a function in a new thread without blocking.
15pub struct DeferredFuture<Args, T, F>
16where
17    Args: Send + 'static,
18    T: Send + 'static,
19    F: (FnOnce(Args) -> T) + Send + 'static,
20{
21    has_started: bool,
22    args: Option<Args>,
23    f: Option<F>,
24    receiver: Option<Receiver<T>>,
25}
26
27impl<Args, T, F> DeferredFuture<Args, T, F>
28where
29    Args: Send + 'static,
30    T: Send + 'static,
31    F: (FnOnce(Args) -> T) + Send + 'static,
32{
33    fn new(f: F, args: Args) -> Self {
34        Self {
35            has_started: false,
36            args: Some(args),
37            f: Some(f),
38            receiver: None,
39        }
40    }
41}
42
43impl<Args, T, F> Future for DeferredFuture<Args, T, F>
44where
45    Args: Send + 'static,
46    T: Send + 'static,
47    F: (FnOnce(Args) -> T) + Send + 'static,
48{
49    type Output = T;
50
51    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
52        // SAFETY: The pin necessarily lives as long as poll() and is owned, so won't be modified
53        let me = unsafe { self.get_unchecked_mut() };
54        // SAFETY: All .take().unwrap() calls can never panic because has_started will only be
55        // false if the Options are still present.
56        if !me.has_started {
57            let (arg_sender, arg_receiver) = channel();
58            let (t_sender, t_receiver) = channel();
59            let f = me.f.take().unwrap();
60            me.receiver = Some(t_receiver);
61            arg_sender
62                .send(me.args.take().unwrap())
63                .expect("broken channel");
64            thread::spawn(move || {
65                let (sender, receiver) = (t_sender, arg_receiver);
66                let args = receiver.recv().expect("unable to recv args");
67                sender
68                    .send(f(args))
69                    .expect("DeferredFuture completed, but thread was not ready yet.");
70            });
71            me.has_started = true;
72            return Poll::Pending;
73        }
74        if let Ok(x) = me.receiver.as_ref().unwrap().try_recv() {
75            Poll::Ready(x)
76        } else {
77            Poll::Pending
78        }
79    }
80}
81
82/// Defers execution of a blocking function to another thread returning a DeferredFuture, which
83/// does nothing until it is awaited.
84pub fn defer<Args, T, F>(f: F, args: Args) -> DeferredFuture<Args, T, F>
85where
86    Args: Send + 'static,
87    T: Send + 'static,
88    F: (FnOnce(Args) -> T) + Send + 'static,
89{
90    DeferredFuture::new(f, args)
91}
async_core/defer.rs

async_core/
defer.rs
