once_box.rs - source
Description: Source of the Rust file `library/std/src/sys/sync/once_box.rs`.

#
std/sys/sync/
once_box.rs

    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}