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use core::mem::MaybeUninit;
use core::ptr;
use std::task::Waker;
const NUM_WAKERS: usize = 32;
/// A list of wakers to be woken.
///
/// # Invariants
///
/// The first `curr` elements of `inner` are initialized.
pub(crate) struct WakeList {
inner: [MaybeUninit<Waker>; NUM_WAKERS],
curr: usize,
}
impl WakeList {
pub(crate) fn new() -> Self {
const UNINIT_WAKER: MaybeUninit<Waker> = MaybeUninit::uninit();
Self {
inner: [UNINIT_WAKER; NUM_WAKERS],
curr: 0,
}
}
#[inline]
pub(crate) fn can_push(&self) -> bool {
self.curr < NUM_WAKERS
}
pub(crate) fn push(&mut self, val: Waker) {
debug_assert!(self.can_push());
self.inner[self.curr] = MaybeUninit::new(val);
self.curr += 1;
}
pub(crate) fn wake_all(&mut self) {
struct DropGuard {
start: *mut Waker,
end: *mut Waker,
}
impl Drop for DropGuard {
fn drop(&mut self) {
// SAFETY: Both pointers are part of the same object, with `start <= end`.
let len = unsafe { self.end.offset_from(self.start) } as usize;
let slice = ptr::slice_from_raw_parts_mut(self.start, len);
// SAFETY: All elements in `start..len` are initialized, so we can drop them.
unsafe { ptr::drop_in_place(slice) };
}
}
debug_assert!(self.curr <= NUM_WAKERS);
let mut guard = {
let start = self.inner.as_mut_ptr().cast::<Waker>();
// SAFETY: The resulting pointer is in bounds or one after the length of the same object.
let end = unsafe { start.add(self.curr) };
// Transfer ownership of the wakers in `inner` to `DropGuard`.
self.curr = 0;
DropGuard { start, end }
};
while !ptr::eq(guard.start, guard.end) {
// SAFETY: `start` is always initialized if `start != end`.
let waker = unsafe { ptr::read(guard.start) };
// SAFETY: The resulting pointer is in bounds or one after the length of the same object.
guard.start = unsafe { guard.start.add(1) };
// If this panics, then `guard` will clean up the remaining wakers.
waker.wake();
}
}
}
impl Drop for WakeList {
fn drop(&mut self) {
let slice =
ptr::slice_from_raw_parts_mut(self.inner.as_mut_ptr().cast::<Waker>(), self.curr);
// SAFETY: The first `curr` elements are initialized, so we can drop them.
unsafe { ptr::drop_in_place(slice) };
}
}