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::Ordering::{Acquire, Relaxed, Release};
12use crate::sync::atomic::{Atomic, AtomicPtr};
13
14pub(crate) struct OnceBox<T> {
15 ptr: Atomic<*mut 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}
84