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#![allow(dead_code)] // Only used on some platforms.
// This is used to wrap pthread {Mutex, Condvar, RwLock} in.
use crate::marker::PhantomData;
use crate::ops::{Deref, DerefMut};
use crate::ptr::null_mut;
use crate::sync::atomic::{
AtomicPtr,
Ordering::{AcqRel, Acquire},
};
pub(crate) struct LazyBox<T: LazyInit> {
ptr: AtomicPtr<T>,
_phantom: PhantomData<T>,
}
pub(crate) trait LazyInit {
/// This is called before the box is allocated, to provide the value to
/// move into the new box.
///
/// It might be called more than once per LazyBox, as multiple threads
/// might race to initialize it concurrently, each constructing and initializing
/// their own box. All but one of them will be passed to `cancel_init` right after.
fn init() -> Box<Self>;
/// Any surplus boxes from `init()` that lost the initialization race
/// are passed to this function for disposal.
///
/// The default implementation calls destroy().
fn cancel_init(x: Box<Self>) {
Self::destroy(x);
}
/// This is called to destroy a used box.
///
/// The default implementation just drops it.
fn destroy(_: Box<Self>) {}
}
impl<T: LazyInit> LazyBox<T> {
#[inline]
pub const fn new() -> Self {
Self { ptr: AtomicPtr::new(null_mut()), _phantom: PhantomData }
}
#[inline]
fn get_pointer(&self) -> *mut T {
let ptr = self.ptr.load(Acquire);
if ptr.is_null() { self.initialize() } else { ptr }
}
#[cold]
fn initialize(&self) -> *mut T {
let new_ptr = Box::into_raw(T::init());
match self.ptr.compare_exchange(null_mut(), new_ptr, AcqRel, Acquire) {
Ok(_) => new_ptr,
Err(ptr) => {
// Lost the race to another thread.
// Drop the box we created, and use the one from the other thread instead.
T::cancel_init(unsafe { Box::from_raw(new_ptr) });
ptr
}
}
}
}
impl<T: LazyInit> Deref for LazyBox<T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
unsafe { &*self.get_pointer() }
}
}
impl<T: LazyInit> DerefMut for LazyBox<T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.get_pointer() }
}
}
impl<T: LazyInit> Drop for LazyBox<T> {
fn drop(&mut self) {
let ptr = *self.ptr.get_mut();
if !ptr.is_null() {
T::destroy(unsafe { Box::from_raw(ptr) });
}
}
}
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