|
| 1 | +#![cfg_attr(loom, allow(dead_code))] |
| 2 | +use std::cell::UnsafeCell; |
| 3 | +use std::mem::MaybeUninit; |
| 4 | +use std::sync::Once; |
| 5 | + |
| 6 | +pub(crate) struct OnceCell<T> { |
| 7 | + once: Once, |
| 8 | + value: UnsafeCell<MaybeUninit<T>>, |
| 9 | +} |
| 10 | + |
| 11 | +unsafe impl<T: Send + Sync> Send for OnceCell<T> {} |
| 12 | +unsafe impl<T: Send + Sync> Sync for OnceCell<T> {} |
| 13 | + |
| 14 | +impl<T> OnceCell<T> { |
| 15 | + pub(crate) const fn new() -> Self { |
| 16 | + Self { |
| 17 | + once: Once::new(), |
| 18 | + value: UnsafeCell::new(MaybeUninit::uninit()), |
| 19 | + } |
| 20 | + } |
| 21 | + |
| 22 | + /// Get the value inside this cell, intiailizing it using the provided |
| 23 | + /// function if necessary. |
| 24 | + /// |
| 25 | + /// If the `init` closure panics, then the `OnceCell` is poisoned and all |
| 26 | + /// future calls to `get` will panic. |
| 27 | + #[inline] |
| 28 | + pub(crate) fn get(&self, init: fn() -> T) -> &T { |
| 29 | + if !self.once.is_completed() { |
| 30 | + self.do_init(init); |
| 31 | + } |
| 32 | + |
| 33 | + // Safety: The `std::sync::Once` guarantees that we can only reach this |
| 34 | + // line if a `call_once` closure has been run exactly once and without |
| 35 | + // panicking. Thus, the value is not uninitialized. |
| 36 | + // |
| 37 | + // There is also no race because the only `&self` method that modifies |
| 38 | + // `value` is `do_init`, but if the `call_once` closure is still |
| 39 | + // running, then no thread has gotten past the `call_once`. |
| 40 | + unsafe { &*(self.value.get() as *const T) } |
| 41 | + } |
| 42 | + |
| 43 | + #[cold] |
| 44 | + fn do_init(&self, init: fn() -> T) { |
| 45 | + let value_ptr = self.value.get() as *mut T; |
| 46 | + |
| 47 | + self.once.call_once(|| { |
| 48 | + let set_to = init(); |
| 49 | + |
| 50 | + // Safety: The `std::sync::Once` guarantees that this initialization |
| 51 | + // will run at most once, and that no thread can get past the |
| 52 | + // `call_once` until it has run exactly once. Thus, we have |
| 53 | + // exclusive access to `value`. |
| 54 | + unsafe { |
| 55 | + std::ptr::write(value_ptr, set_to); |
| 56 | + } |
| 57 | + }); |
| 58 | + } |
| 59 | +} |
| 60 | + |
| 61 | +impl<T> Drop for OnceCell<T> { |
| 62 | + fn drop(&mut self) { |
| 63 | + if self.once.is_completed() { |
| 64 | + let value_ptr = self.value.get() as *mut T; |
| 65 | + unsafe { |
| 66 | + std::ptr::drop_in_place(value_ptr); |
| 67 | + } |
| 68 | + } |
| 69 | + } |
| 70 | +} |
0 commit comments