| use std::{ |
| cell::UnsafeCell, |
| hint, |
| panic::{RefUnwindSafe, UnwindSafe}, |
| sync::atomic::{AtomicBool, Ordering}, |
| }; |
| |
| use parking_lot::Mutex; |
| |
| pub(crate) struct OnceCell<T> { |
| mutex: Mutex<()>, |
| is_initialized: AtomicBool, |
| value: UnsafeCell<Option<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 { |
| mutex: parking_lot::const_mutex(()), |
| is_initialized: AtomicBool::new(false), |
| value: UnsafeCell::new(None), |
| } |
| } |
| |
| /// Safety: synchronizes with store to value via Release/Acquire. |
| #[inline] |
| pub(crate) fn is_initialized(&self) -> bool { |
| self.is_initialized.load(Ordering::Acquire) |
| } |
| |
| /// Safety: synchronizes with store to value via `is_initialized` or mutex |
| /// lock/unlock, writes value only once because of the mutex. |
| #[cold] |
| pub(crate) fn initialize<F, E>(&self, f: F) -> Result<(), E> |
| where |
| F: FnOnce() -> Result<T, E>, |
| { |
| let _guard = self.mutex.lock(); |
| if !self.is_initialized() { |
| // We are calling user-supplied function and need to be careful. |
| // - if it returns Err, we unlock mutex and return without touching anything |
| // - if it panics, we unlock mutex and propagate panic without touching anything |
| // - if it calls `set` or `get_or_try_init` re-entrantly, we get a deadlock on |
| // mutex, which is important for safety. We *could* detect this and panic, |
| // but that is more complicated |
| // - finally, if it returns Ok, we store the value and store the flag with |
| // `Release`, which synchronizes with `Acquire`s. |
| let value = f()?; |
| // Safe b/c we have a unique access and no panic may happen |
| // until the cell is marked as initialized. |
| let slot: &mut Option<T> = unsafe { &mut *self.value.get() }; |
| debug_assert!(slot.is_none()); |
| *slot = Some(value); |
| self.is_initialized.store(true, Ordering::Release); |
| } |
| Ok(()) |
| } |
| |
| /// 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()); |
| let slot: &Option<T> = &*self.value.get(); |
| match slot { |
| Some(value) => value, |
| // This unsafe does improve performance, see `examples/bench`. |
| None => { |
| debug_assert!(false); |
| hint::unreachable_unchecked() |
| } |
| } |
| } |
| |
| /// Gets the mutable reference to the underlying value. |
| /// Returns `None` if the cell is empty. |
| pub(crate) fn get_mut(&mut self) -> Option<&mut T> { |
| // Safe b/c we have an exclusive access |
| let slot: &mut Option<T> = unsafe { &mut *self.value.get() }; |
| slot.as_mut() |
| } |
| |
| /// Consumes this `OnceCell`, returning the wrapped value. |
| /// Returns `None` if the cell was empty. |
| pub(crate) fn into_inner(self) -> Option<T> { |
| self.value.into_inner() |
| } |
| } |
| |
| #[test] |
| fn test_size() { |
| use std::mem::size_of; |
| |
| assert_eq!(size_of::<OnceCell<bool>>(), 2 * size_of::<bool>() + size_of::<u8>()); |
| } |