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Fixes #3212. With the release of `v1.15`, the once_cell crate is breaking our current MSRV. Vendoring it will allow us to keep using certain APIs from that crate without compromising our MSRV.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,173 @@ | ||
| use std::{ | ||
| cell::UnsafeCell, | ||
| hint, | ||
| 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 { super::take_unchecked(&mut f) }; | ||
| 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 | ||
| } | ||
|
|
||
| /// 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() | ||
| } | ||
| } | ||
|
|
||
| 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>() | ||
| ); | ||
| } |
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