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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 { f.take().unwrap_unchecked() };
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();
slot.as_ref().unwrap_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()
}
}
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>());
}