use std::{
cell::UnsafeCell,
sync::atomic::{AtomicBool, Ordering},
hint::unreachable_unchecked,
panic::{UnwindSafe, RefUnwindSafe},
fmt,
};
use parking_lot::{
RawMutex,
lock_api::RawMutex as _RawMutex,
};
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: fmt::Debug> fmt::Debug for OnceCell<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("OnceCell").field("value", &self.get()).finish()
}
}
impl<T> OnceCell<T> {
pub(crate) const fn new() -> OnceCell<T> {
OnceCell {
mutex: Mutex::new(),
is_initialized: AtomicBool::new(false),
value: UnsafeCell::new(None),
}
}
pub(crate) fn get(&self) -> Option<&T> {
if self.is_initialized.load(Ordering::Acquire) {
// This is safe: if we've read `true` with `Acquire`, that means
// we've are paired with `Release` store, which sets the value.
// Additionally, no one invalidates value after `is_initialized` is
// set to `true`
let value: &Option<T> = unsafe { &*self.value.get() };
value.as_ref()
} else {
None
}
}
pub(crate) fn set(&self, value: T) -> Result<(), T> {
let mut value = Some(value);
{
// We *could* optimistically check here if cell is initialized, but
// we don't do that, assuming that `set` actually sets the value
// most of the time.
let _guard = self.mutex.lock();
// Relaxed loads are OK under the mutex, because it's mutex
// unlock/lock that establishes "happens before".
if !self.is_initialized.load(Ordering::Relaxed) {
// Uniqueness of reference is guaranteed my mutex and flag
let slot: &mut Option<T> = unsafe { &mut *self.value.get() };
debug_assert!(slot.is_none());
*slot = value.take();
// This `Release` guarantees that `get` sees only fully stored
// value
self.is_initialized.store(true, Ordering::Release);
}
}
match value {
None => Ok(()),
Some(value) => Err(value),
}
}
pub(crate) fn get_or_init<F: FnOnce() -> T>(&self, f: F) -> &T {
enum Void {}
match self.get_or_try_init(|| Ok::<T, Void>(f())) {
Ok(val) => val,
Err(void) => match void {},
}
}
pub(crate) fn get_or_try_init<F: FnOnce() -> Result<T, E>, E>(&self, f: F) -> Result<&T, E> {
// Standard double-checked locking pattern.
// Optimistically check if value is initialized, without locking a
// mutex.
if !self.is_initialized.load(Ordering::Acquire) {
let _guard = self.mutex.lock();
// Relaxed is OK, because mutex unlock/lock establishes "happens
// before".
if !self.is_initialized.load(Ordering::Relaxed) {
// 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()?;
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);
}
}
// Value is initialized here, because we've read `true` from
// `is_initialized`, and have a "happens before" due to either
// Acquire/Release pair (fast path) or mutex unlock (slow path).
// While we could have just called `get`, that would be twice
// as slow!
let value: &Option<T> = unsafe { &*self.value.get() };
return match value.as_ref() {
Some(it) => Ok(it),
None => {
debug_assert!(false);
unsafe { unreachable_unchecked() }
}
};
}
pub(crate) fn into_inner(self) -> Option<T> {
// Because `into_inner` takes `self` by value, the compiler statically verifies
// that it is not currently borrowed. So it is safe to move out `Option<T>`.
self.value.into_inner()
}
}
/// Wrapper around parking_lot's `RawMutex` which has `const fn` new.
struct Mutex {
inner: RawMutex,
}
impl Mutex {
const fn new() -> Mutex {
Mutex { inner: RawMutex::INIT }
}
fn lock(&self) -> MutexGuard<'_> {
self.inner.lock();
MutexGuard { inner: &self.inner }
}
}
struct MutexGuard<'a> {
inner: &'a RawMutex,
}
impl Drop for MutexGuard<'_> {
fn drop(&mut self) {
self.inner.unlock();
}
}
#[test]
#[cfg(pointer_width = "64")]
fn test_size() {
use std::mem::size_of;
assert_eq!(size_of::<OnceCell<u32>>, 2 * size_of::<u32>);
}