futures_task/
waker.rs

1use super::arc_wake::ArcWake;
2use alloc::sync::Arc;
3use core::mem;
4use core::task::{RawWaker, RawWakerVTable, Waker};
5
6pub(super) fn waker_vtable<W: ArcWake + 'static>() -> &'static RawWakerVTable {
7    &RawWakerVTable::new(
8        clone_arc_raw::<W>,
9        wake_arc_raw::<W>,
10        wake_by_ref_arc_raw::<W>,
11        drop_arc_raw::<W>,
12    )
13}
14
15/// Creates a [`Waker`] from an `Arc<impl ArcWake>`.
16///
17/// The returned [`Waker`] will call
18/// [`ArcWake.wake()`](ArcWake::wake) if awoken.
19pub fn waker<W>(wake: Arc<W>) -> Waker
20where
21    W: ArcWake + 'static,
22{
23    let ptr = Arc::into_raw(wake).cast::<()>();
24
25    unsafe { Waker::from_raw(RawWaker::new(ptr, waker_vtable::<W>())) }
26}
27
28// FIXME: panics on Arc::clone / refcount changes could wreak havoc on the
29// code here. We should guard against this by aborting.
30
31#[allow(clippy::redundant_clone)] // The clone here isn't actually redundant.
32unsafe fn increase_refcount<T: ArcWake + 'static>(data: *const ()) {
33    // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
34    let arc = mem::ManuallyDrop::new(unsafe { Arc::<T>::from_raw(data.cast::<T>()) });
35    // Now increase refcount, but don't drop new refcount either
36    let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
37}
38
39// used by `waker_ref`
40#[inline(always)]
41unsafe fn clone_arc_raw<T: ArcWake + 'static>(data: *const ()) -> RawWaker {
42    unsafe { increase_refcount::<T>(data) }
43    RawWaker::new(data, waker_vtable::<T>())
44}
45
46unsafe fn wake_arc_raw<T: ArcWake + 'static>(data: *const ()) {
47    let arc: Arc<T> = unsafe { Arc::from_raw(data.cast::<T>()) };
48    ArcWake::wake(arc);
49}
50
51// used by `waker_ref`
52unsafe fn wake_by_ref_arc_raw<T: ArcWake + 'static>(data: *const ()) {
53    // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
54    let arc = mem::ManuallyDrop::new(unsafe { Arc::<T>::from_raw(data.cast::<T>()) });
55    ArcWake::wake_by_ref(&arc);
56}
57
58unsafe fn drop_arc_raw<T: ArcWake + 'static>(data: *const ()) {
59    drop(unsafe { Arc::<T>::from_raw(data.cast::<T>()) })
60}