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//! A racily-initialized alternative to `OnceLock<Box<T>>`.
//!
//! This is used to implement synchronization primitives that need allocation,
//! like the pthread versions.
#![allow(dead_code)] // Only used on some platforms.
use crate::mem::replace;
use crate::ptr::null_mut;
use crate::sync::atomic::AtomicPtr;
use crate::sync::atomic::Ordering::{Acquire, Relaxed, Release};
pub(crate) struct OnceBox<T> {
ptr: AtomicPtr<T>,
}
impl<T> OnceBox<T> {
#[inline]
pub const fn new() -> Self {
Self { ptr: AtomicPtr::new(null_mut()) }
}
/// Gets access to the value, assuming it is already initialized and this
/// initialization has been observed by the current thread.
///
/// Since all modifications to the pointer have already been observed, the
/// pointer load in this function can be performed with relaxed ordering,
/// potentially allowing the optimizer to turn code like this:
/// ```rust, ignore
/// once_box.get_or_init(|| Box::new(42));
/// unsafe { once_box.get_unchecked() }
/// ```
/// into
/// ```rust, ignore
/// once_box.get_or_init(|| Box::new(42))
/// ```
///
/// # Safety
/// This causes undefined behavior if the assumption above is violated.
#[inline]
pub unsafe fn get_unchecked(&self) -> &T {
unsafe { &*self.ptr.load(Relaxed) }
}
#[inline]
pub fn get_or_init(&self, f: impl FnOnce() -> Box<T>) -> &T {
let ptr = self.ptr.load(Acquire);
match unsafe { ptr.as_ref() } {
Some(val) => val,
None => self.initialize(f),
}
}
#[inline]
pub fn take(&mut self) -> Option<Box<T>> {
let ptr = replace(self.ptr.get_mut(), null_mut());
if !ptr.is_null() { Some(unsafe { Box::from_raw(ptr) }) } else { None }
}
#[cold]
fn initialize(&self, f: impl FnOnce() -> Box<T>) -> &T {
let new_ptr = Box::into_raw(f());
match self.ptr.compare_exchange(null_mut(), new_ptr, Release, Acquire) {
Ok(_) => unsafe { &*new_ptr },
Err(ptr) => {
// Lost the race to another thread.
// Drop the value we created, and use the one from the other thread instead.
drop(unsafe { Box::from_raw(new_ptr) });
unsafe { &*ptr }
}
}
}
}
unsafe impl<T: Send> Send for OnceBox<T> {}
unsafe impl<T: Send + Sync> Sync for OnceBox<T> {}
impl<T> Drop for OnceBox<T> {
fn drop(&mut self) {
self.take();
}
}
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