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use crate::sync::{AtomicBool, ReadGuard, RwLock, WriteGuard};
#[cfg(parallel_compiler)]
use crate::sync::{DynSend, DynSync};
use std::{
cell::UnsafeCell,
ops::{Deref, DerefMut},
sync::atomic::Ordering,
};
/// A type which allows mutation using a lock until
/// the value is frozen and can be accessed lock-free.
///
/// Unlike `RwLock`, it can be used to prevent mutation past a point.
#[derive(Default)]
pub struct FreezeLock<T> {
data: UnsafeCell<T>,
frozen: AtomicBool,
/// This lock protects writes to the `data` and `frozen` fields.
lock: RwLock<()>,
}
#[cfg(parallel_compiler)]
unsafe impl<T: DynSync + DynSend> DynSync for FreezeLock<T> {}
impl<T> FreezeLock<T> {
#[inline]
pub fn new(value: T) -> Self {
Self { data: UnsafeCell::new(value), frozen: AtomicBool::new(false), lock: RwLock::new(()) }
}
#[inline]
pub fn read(&self) -> FreezeReadGuard<'_, T> {
FreezeReadGuard {
_lock_guard: if self.frozen.load(Ordering::Acquire) {
None
} else {
Some(self.lock.read())
},
// SAFETY: If this is not frozen, `_lock_guard` holds the lock to the `UnsafeCell` so
// this has shared access until the `FreezeReadGuard` is dropped. If this is frozen,
// the data cannot be modified and shared access is sound.
data: unsafe { &*self.data.get() },
}
}
#[inline]
#[track_caller]
pub fn write(&self) -> FreezeWriteGuard<'_, T> {
let _lock_guard = self.lock.write();
// Use relaxed ordering since we're in the write lock.
assert!(!self.frozen.load(Ordering::Relaxed), "still mutable");
FreezeWriteGuard {
_lock_guard,
// SAFETY: `_lock_guard` holds the lock to the `UnsafeCell` so this has mutable access
// until the `FreezeWriteGuard` is dropped.
data: unsafe { &mut *self.data.get() },
}
}
#[inline]
pub fn freeze(&self) -> &T {
if !self.frozen.load(Ordering::Acquire) {
// Get the lock to ensure no concurrent writes and that we release the latest write.
let _lock = self.lock.write();
self.frozen.store(true, Ordering::Release);
}
// SAFETY: This is frozen so the data cannot be modified and shared access is sound.
unsafe { &*self.data.get() }
}
}
/// A guard holding shared access to a `FreezeLock` which is in a locked state or frozen.
#[must_use = "if unused the FreezeLock may immediately unlock"]
pub struct FreezeReadGuard<'a, T> {
_lock_guard: Option<ReadGuard<'a, ()>>,
data: &'a T,
}
impl<'a, T: 'a> Deref for FreezeReadGuard<'a, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
self.data
}
}
/// A guard holding mutable access to a `FreezeLock` which is in a locked state or frozen.
#[must_use = "if unused the FreezeLock may immediately unlock"]
pub struct FreezeWriteGuard<'a, T> {
_lock_guard: WriteGuard<'a, ()>,
data: &'a mut T,
}
impl<'a, T: 'a> Deref for FreezeWriteGuard<'a, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
self.data
}
}
impl<'a, T: 'a> DerefMut for FreezeWriteGuard<'a, T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
self.data
}
}
|
