| use std::{ |
| cell::UnsafeCell, |
| panic::{RefUnwindSafe, UnwindSafe}, |
| sync::atomic::{AtomicU8, Ordering}, |
| }; |
| |
| pub(crate) struct OnceCell<T> { |
| state: AtomicU8, |
| value: UnsafeCell<Option<T>>, |
| } |
| |
| const INCOMPLETE: u8 = 0x0; |
| const RUNNING: u8 = 0x1; |
| const COMPLETE: u8 = 0x2; |
| |
| // 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 { state: AtomicU8::new(INCOMPLETE), value: UnsafeCell::new(None) } |
| } |
| |
| pub(crate) const fn with_value(value: T) -> OnceCell<T> { |
| OnceCell { state: AtomicU8::new(COMPLETE), value: UnsafeCell::new(Some(value)) } |
| } |
| |
| /// Safety: synchronizes with store to value via Release/Acquire. |
| #[inline] |
| pub(crate) fn is_initialized(&self) -> bool { |
| self.state.load(Ordering::Acquire) == COMPLETE |
| } |
| |
| /// 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 mut f = Some(f); |
| let mut res: Result<(), E> = Ok(()); |
| let slot: *mut Option<T> = self.value.get(); |
| initialize_inner(&self.state, &mut || { |
| // 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 f = unsafe { crate::unwrap_unchecked(f.take()) }; |
| match f() { |
| Ok(value) => unsafe { |
| // Safe b/c we have a unique access and no panic may happen |
| // until the cell is marked as initialized. |
| debug_assert!((*slot).is_none()); |
| *slot = Some(value); |
| true |
| }, |
| Err(err) => { |
| res = Err(err); |
| false |
| } |
| } |
| }); |
| res |
| } |
| |
| #[cold] |
| pub(crate) fn wait(&self) { |
| let key = &self.state as *const _ as usize; |
| unsafe { |
| parking_lot_core::park( |
| key, |
| || self.state.load(Ordering::Acquire) != COMPLETE, |
| || (), |
| |_, _| (), |
| parking_lot_core::DEFAULT_PARK_TOKEN, |
| None, |
| ); |
| } |
| } |
| |
| /// 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 = &*self.value.get(); |
| crate::unwrap_unchecked(slot.as_ref()) |
| } |
| |
| /// 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() |
| } |
| } |
| |
| struct Guard<'a> { |
| state: &'a AtomicU8, |
| new_state: u8, |
| } |
| |
| impl<'a> Drop for Guard<'a> { |
| fn drop(&mut self) { |
| self.state.store(self.new_state, Ordering::Release); |
| unsafe { |
| let key = self.state as *const AtomicU8 as usize; |
| parking_lot_core::unpark_all(key, parking_lot_core::DEFAULT_UNPARK_TOKEN); |
| } |
| } |
| } |
| |
| // Note: this is intentionally monomorphic |
| #[inline(never)] |
| fn initialize_inner(state: &AtomicU8, init: &mut dyn FnMut() -> bool) { |
| loop { |
| let exchange = |
| state.compare_exchange_weak(INCOMPLETE, RUNNING, Ordering::Acquire, Ordering::Acquire); |
| match exchange { |
| Ok(_) => { |
| let mut guard = Guard { state, new_state: INCOMPLETE }; |
| if init() { |
| guard.new_state = COMPLETE; |
| } |
| return; |
| } |
| Err(COMPLETE) => return, |
| Err(RUNNING) => unsafe { |
| let key = state as *const AtomicU8 as usize; |
| parking_lot_core::park( |
| key, |
| || state.load(Ordering::Relaxed) == RUNNING, |
| || (), |
| |_, _| (), |
| parking_lot_core::DEFAULT_PARK_TOKEN, |
| None, |
| ); |
| }, |
| Err(INCOMPLETE) => (), |
| Err(_) => debug_assert!(false), |
| } |
| } |
| } |
| |
| #[test] |
| fn test_size() { |
| use std::mem::size_of; |
| |
| assert_eq!(size_of::<OnceCell<bool>>(), 1 * size_of::<bool>() + size_of::<u8>()); |
| } |