std/sys/sync/
once_box.rs

Help
1//! A racily-initialized alternative to `OnceLock<Box<T>>`.
2//!
3//! This is used to implement synchronization primitives that need allocation,
4//! like the pthread versions.
5
6#![allow(dead_code)] // Only used on some platforms.
7
8use crate::mem::replace;
9use crate::pin::Pin;
10use crate::ptr::null_mut;
11use crate::sync::atomic::AtomicPtr;
12use crate::sync::atomic::Ordering::{Acquire, Relaxed, Release};
13
14pub(crate) struct OnceBox<T> {
15    ptr: AtomicPtr<T>,
16}
17
18impl<T> OnceBox<T> {
19    #[inline]
20    pub const fn new() -> Self {
21        Self { ptr: AtomicPtr::new(null_mut()) }
22    }
23
24    /// Gets access to the value, assuming it is already initialized and this
25    /// initialization has been observed by the current thread.
26    ///
27    /// Since all modifications to the pointer have already been observed, the
28    /// pointer load in this function can be performed with relaxed ordering,
29    /// potentially allowing the optimizer to turn code like this:
30    /// ```rust, ignore
31    /// once_box.get_or_init(|| Box::pin(42));
32    /// unsafe { once_box.get_unchecked() }
33    /// ```
34    /// into
35    /// ```rust, ignore
36    /// once_box.get_or_init(|| Box::pin(42))
37    /// ```
38    ///
39    /// # Safety
40    /// This causes undefined behavior if the assumption above is violated.
41    #[inline]
42    pub unsafe fn get_unchecked(&self) -> Pin<&T> {
43        unsafe { Pin::new_unchecked(&*self.ptr.load(Relaxed)) }
44    }
45
46    #[inline]
47    pub fn get_or_init(&self, f: impl FnOnce() -> Pin<Box<T>>) -> Pin<&T> {
48        let ptr = self.ptr.load(Acquire);
49        match unsafe { ptr.as_ref() } {
50            Some(val) => unsafe { Pin::new_unchecked(val) },
51            None => self.initialize(f),
52        }
53    }
54
55    #[inline]
56    pub fn take(&mut self) -> Option<Pin<Box<T>>> {
57        let ptr = replace(self.ptr.get_mut(), null_mut());
58        if !ptr.is_null() { Some(unsafe { Pin::new_unchecked(Box::from_raw(ptr)) }) } else { None }
59    }
60
61    #[cold]
62    fn initialize(&self, f: impl FnOnce() -> Pin<Box<T>>) -> Pin<&T> {
63        let new_ptr = Box::into_raw(unsafe { Pin::into_inner_unchecked(f()) });
64        match self.ptr.compare_exchange(null_mut(), new_ptr, Release, Acquire) {
65            Ok(_) => unsafe { Pin::new_unchecked(&*new_ptr) },
66            Err(ptr) => {
67                // Lost the race to another thread.
68                // Drop the value we created, and use the one from the other thread instead.
69                drop(unsafe { Box::from_raw(new_ptr) });
70                unsafe { Pin::new_unchecked(&*ptr) }
71            }
72        }
73    }
74}
75
76unsafe impl<T: Send> Send for OnceBox<T> {}
77unsafe impl<T: Send + Sync> Sync for OnceBox<T> {}
78
79impl<T> Drop for OnceBox<T> {
80    fn drop(&mut self) {
81        self.take();
82    }
83}
std/sys/sync/once_box.rs

std/sys/sync/
once_box.rs
