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use core::panic::{RefUnwindSafe, UnwindSafe};
use portable_atomic::{AtomicBool, Ordering};
use critical_section::{CriticalSection, Mutex};
use crate::unsync;
pub(crate) struct OnceCell<T> {
initialized: AtomicBool,
// Use `unsync::OnceCell` internally since `Mutex` does not provide
// interior mutability and to be able to re-use `get_or_try_init`.
value: Mutex<unsync::OnceCell<T>>,
}
// Why do we need `T: Send`?
// Thread A creates a `OnceCell` and shares it with
// scoped thread B, which fills the cell, which is
// then destroyed by A. That is, destructor observes
// a sent value.
unsafe impl<T: Sync + Send> Sync for OnceCell<T> {}
unsafe impl<T: Send> Send for OnceCell<T> {}
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T> {}
impl<T: UnwindSafe> UnwindSafe for OnceCell<T> {}
impl<T> OnceCell<T> {
pub(crate) const fn new() -> OnceCell<T> {
OnceCell { initialized: AtomicBool::new(false), value: Mutex::new(unsync::OnceCell::new()) }
}
pub(crate) const fn with_value(value: T) -> OnceCell<T> {
OnceCell {
initialized: AtomicBool::new(true),
value: Mutex::new(unsync::OnceCell::with_value(value)),
}
}
#[inline]
pub(crate) fn is_initialized(&self) -> bool {
self.initialized.load(Ordering::Acquire)
}
#[cold]
pub(crate) fn initialize<F, E>(&self, f: F) -> Result<(), E>
where
F: FnOnce() -> Result<T, E>,
{
critical_section::with(|cs| {
let cell = self.value.borrow(cs);
cell.get_or_try_init(f).map(|_| {
self.initialized.store(true, Ordering::Release);
})
})
}
/// Get the reference to the underlying value, without checking if the cell
/// is initialized.
///
/// # Safety
///
/// Caller must ensure that the cell is in initialized state, and that
/// the contents are acquired by (synchronized to) this thread.
pub(crate) unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.is_initialized());
// SAFETY: The caller ensures that the value is initialized and access synchronized.
self.value.borrow(CriticalSection::new()).get().unwrap_unchecked()
}
#[inline]
pub(crate) fn get_mut(&mut self) -> Option<&mut T> {
self.value.get_mut().get_mut()
}
#[inline]
pub(crate) fn into_inner(self) -> Option<T> {
self.value.into_inner().into_inner()
}
}
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