1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174
use std::{
cell::UnsafeCell,
panic::{RefUnwindSafe, UnwindSafe},
sync::atomic::{AtomicU8, Ordering},
};
pub(crate) struct OnceCell<T> {
state: AtomicU8,
value: UnsafeCell<Option<T>>,
}
const INCOMPLETE: u8 = 0x0;
const RUNNING: u8 = 0x1;
const COMPLETE: u8 = 0x2;
// Why do we need `T: Send`?
// Thread A creates a `OnceCell` and shares it with
// scoped thread B, which fills the cell, which is
// then destroyed by A. That is, destructor observes
// a sent value.
unsafe impl<T: Sync + Send> Sync for OnceCell<T> {}
unsafe impl<T: Send> Send for OnceCell<T> {}
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T> {}
impl<T: UnwindSafe> UnwindSafe for OnceCell<T> {}
impl<T> OnceCell<T> {
pub(crate) const fn new() -> OnceCell<T> {
OnceCell { state: AtomicU8::new(INCOMPLETE), value: UnsafeCell::new(None) }
}
pub(crate) const fn with_value(value: T) -> OnceCell<T> {
OnceCell { state: AtomicU8::new(COMPLETE), value: UnsafeCell::new(Some(value)) }
}
/// Safety: synchronizes with store to value via Release/Acquire.
#[inline]
pub(crate) fn is_initialized(&self) -> bool {
self.state.load(Ordering::Acquire) == COMPLETE
}
/// Safety: synchronizes with store to value via `is_initialized` or mutex
/// lock/unlock, writes value only once because of the mutex.
#[cold]
pub(crate) fn initialize<F, E>(&self, f: F) -> Result<(), E>
where
F: FnOnce() -> Result<T, E>,
{
let mut f = Some(f);
let mut res: Result<(), E> = Ok(());
let slot: *mut Option<T> = self.value.get();
initialize_inner(&self.state, &mut || {
// We are calling user-supplied function and need to be careful.
// - if it returns Err, we unlock mutex and return without touching anything
// - if it panics, we unlock mutex and propagate panic without touching anything
// - if it calls `set` or `get_or_try_init` re-entrantly, we get a deadlock on
// mutex, which is important for safety. We *could* detect this and panic,
// but that is more complicated
// - finally, if it returns Ok, we store the value and store the flag with
// `Release`, which synchronizes with `Acquire`s.
let f = unsafe { f.take().unwrap_unchecked() };
match f() {
Ok(value) => unsafe {
// Safe b/c we have a unique access and no panic may happen
// until the cell is marked as initialized.
debug_assert!((*slot).is_none());
*slot = Some(value);
true
},
Err(err) => {
res = Err(err);
false
}
}
});
res
}
#[cold]
pub(crate) fn wait(&self) {
let key = &self.state as *const _ as usize;
unsafe {
parking_lot_core::park(
key,
|| self.state.load(Ordering::Acquire) != COMPLETE,
|| (),
|_, _| (),
parking_lot_core::DEFAULT_PARK_TOKEN,
None,
);
}
}
/// Get the reference to the underlying value, without checking if the cell
/// is initialized.
///
/// # Safety
///
/// Caller must ensure that the cell is in initialized state, and that
/// the contents are acquired by (synchronized to) this thread.
pub(crate) unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.is_initialized());
let slot = &*self.value.get();
slot.as_ref().unwrap_unchecked()
}
/// Gets the mutable reference to the underlying value.
/// Returns `None` if the cell is empty.
pub(crate) fn get_mut(&mut self) -> Option<&mut T> {
// Safe b/c we have an exclusive access
let slot: &mut Option<T> = unsafe { &mut *self.value.get() };
slot.as_mut()
}
/// Consumes this `OnceCell`, returning the wrapped value.
/// Returns `None` if the cell was empty.
pub(crate) fn into_inner(self) -> Option<T> {
self.value.into_inner()
}
}
struct Guard<'a> {
state: &'a AtomicU8,
new_state: u8,
}
impl<'a> Drop for Guard<'a> {
fn drop(&mut self) {
self.state.store(self.new_state, Ordering::Release);
unsafe {
let key = self.state as *const AtomicU8 as usize;
parking_lot_core::unpark_all(key, parking_lot_core::DEFAULT_UNPARK_TOKEN);
}
}
}
// Note: this is intentionally monomorphic
#[inline(never)]
fn initialize_inner(state: &AtomicU8, init: &mut dyn FnMut() -> bool) {
loop {
let exchange =
state.compare_exchange_weak(INCOMPLETE, RUNNING, Ordering::Acquire, Ordering::Acquire);
match exchange {
Ok(_) => {
let mut guard = Guard { state, new_state: INCOMPLETE };
if init() {
guard.new_state = COMPLETE;
}
return;
}
Err(COMPLETE) => return,
Err(RUNNING) => unsafe {
let key = state as *const AtomicU8 as usize;
parking_lot_core::park(
key,
|| state.load(Ordering::Relaxed) == RUNNING,
|| (),
|_, _| (),
parking_lot_core::DEFAULT_PARK_TOKEN,
None,
);
},
Err(INCOMPLETE) => (),
Err(_) => debug_assert!(false),
}
}
}
#[test]
fn test_size() {
use std::mem::size_of;
assert_eq!(size_of::<OnceCell<bool>>(), 1 * size_of::<bool>() + size_of::<u8>());
}