fuchsia_async/runtime/fuchsia/executor/atomic_future/
spawnable_future.rs

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1// Copyright 2025 The Fuchsia Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5use super::super::common::TaskHandle;
6use super::{AtomicFutureHandle, Bomb, Meta, DONE, RESULT_TAKEN};
7use crate::scope::Spawnable;
8use crate::ScopeHandle;
9use std::future::Future;
10use std::marker::PhantomData;
11use std::pin::Pin;
12use std::task::{ready, Context, Poll};
13
14/// `SpawnableFuture` is a boxed future that can be spawned without incurring any more allocations
15/// i.e. it doesn't end up with the double boxing that you end up with if you try and spawn `Box<dyn
16/// Future>`.  It can be used in place of `BoxFuture` although it carries more overhead than
17/// `BoxFuture`, so it shouldn't be used if it isn't going to be spawned on a scope.
18/// `SpawnableFuture` implements `Future` but the future will not be running as a separate task if
19/// used this way.  If polled and then later spawned, the spawned task will be polled again and any
20/// waker recorded when polled prior to spawning will be impotent.
21pub struct SpawnableFuture<'a, O>(AtomicFutureHandle<'a>, PhantomData<O>);
22
23impl<O> Unpin for SpawnableFuture<'_, O> {}
24
25impl<'a, O> SpawnableFuture<'a, O> {
26    /// Creates a new spawnable future. To spawn the future on a scope, use either `spawn_on` or
27    /// `compute_on`.
28    pub fn new<F: Future<Output = O> + Send + 'a>(future: F) -> Self
29    where
30        O: Send + 'a,
31    {
32        Self(AtomicFutureHandle::new(None, 0, future), PhantomData)
33    }
34
35    fn meta(&mut self) -> &mut Meta {
36        // SAFETY: This is safe because we know there is only one reference to the handle.
37        unsafe { &mut *self.0 .0.as_mut() }
38    }
39}
40
41impl<O> Spawnable for SpawnableFuture<'static, O> {
42    type Output = O;
43
44    fn into_task(mut self, scope: ScopeHandle) -> TaskHandle {
45        let meta = self.meta();
46        meta.id = scope.executor().next_task_id();
47        meta.scope = Some(scope);
48        self.0
49    }
50}
51
52// This is intentionally &mut as otherwise we can't have a specialized implementation of
53// `Spawnable`. This should work just the same because of Rust's autoref behaviour.
54impl<O> Future for &mut SpawnableFuture<'_, O> {
55    type Output = O;
56
57    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
58        // We cannot recover from panics.
59        let bomb = Bomb;
60
61        let meta = self.meta();
62        let result = unsafe { (meta.vtable.poll)(meta.into(), cx) };
63
64        std::mem::forget(bomb);
65
66        ready!(result);
67
68        let result = unsafe { ((meta.vtable.get_result)(meta.into()) as *const O).read() };
69        *meta.state.get_mut() = DONE | RESULT_TAKEN;
70
71        Poll::Ready(result)
72    }
73}
74
75#[cfg(test)]
76mod tests {
77    use super::SpawnableFuture;
78    use crate::{Scope, SendExecutor};
79    use std::future::poll_fn;
80    use std::sync::atomic::AtomicU64;
81    use std::sync::atomic::Ordering::Relaxed;
82    use std::sync::Arc;
83    use std::task::Poll;
84
85    #[test]
86    fn test_spawnable_future() {
87        let mut executor = SendExecutor::new(2);
88        executor.run(async move {
89            let counter = Arc::new(AtomicU64::new(0));
90            let counter2 = Arc::clone(&counter);
91            let mut task1 = SpawnableFuture::new(async move {
92                let () = poll_fn(|_cx| {
93                    if counter2.fetch_add(1, Relaxed) == 1 {
94                        Poll::Ready(())
95                    } else {
96                        Poll::Pending
97                    }
98                })
99                .await;
100            });
101            let old = counter.load(Relaxed);
102            assert!(futures::poll!(&mut task1).is_pending());
103            assert_eq!(counter.load(Relaxed), old + 1);
104
105            Scope::current().spawn(task1).await;
106
107            assert_eq!(counter.load(Relaxed), old + 2);
108        });
109    }
110}
fuchsia_async/runtime/fuchsia/executor/atomic_future/spawnable_future.rs

fuchsia_async/runtime/fuchsia/executor/atomic_future/
spawnable_future.rs
