use core::panic::{RefUnwindSafe, UnwindSafe}; use atomic_polyfill::{AtomicBool, Ordering}; use critical_section::{CriticalSection, Mutex}; use crate::unsync; pub(crate) struct OnceCell { 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>, } // 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 Sync for OnceCell {} unsafe impl Send for OnceCell {} impl RefUnwindSafe for OnceCell {} impl UnwindSafe for OnceCell {} impl OnceCell { pub(crate) const fn new() -> OnceCell { OnceCell { initialized: AtomicBool::new(false), value: Mutex::new(unsync::OnceCell::new()) } } pub(crate) const fn with_value(value: T) -> OnceCell { 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(&self, f: F) -> Result<(), E> where F: FnOnce() -> Result, { 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 { self.value.into_inner().into_inner() } }