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// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the License for the
// specific language governing permissions and limitations
// under the License..
use crate::once::Once;
use core::cell::UnsafeCell;
use core::fmt;
use core::marker::PhantomData;
use core::mem::MaybeUninit;
use core::ops::Drop;
use core::panic::{RefUnwindSafe, UnwindSafe};
/// A synchronization primitive which can be written to only once.
///
/// This type is a thread-safe `OnceCell`.
///
pub struct OnceLock<T> {
once: Once,
// Whether or not the value is initialized is tracked by `state_and_queue`.
value: UnsafeCell<MaybeUninit<T>>,
/// `PhantomData` to make sure dropck understands we're dropping T in our Drop impl.
_marker: PhantomData<T>,
}
impl<T> OnceLock<T> {
/// Creates a new empty cell.
#[must_use]
pub const fn new() -> OnceLock<T> {
OnceLock {
once: Once::new(),
value: UnsafeCell::new(MaybeUninit::uninit()),
_marker: PhantomData,
}
}
/// Gets the reference to the underlying value.
///
/// Returns `None` if the cell is empty, or being initialized. This
/// method never blocks.
///
pub fn get(&self) -> Option<&T> {
if self.is_initialized() {
// Safe b/c checked is_initialized
Some(unsafe { self.get_unchecked() })
} else {
None
}
}
/// Gets the mutable reference to the underlying value.
///
/// Returns `None` if the cell is empty. This method never blocks.
pub fn get_mut(&mut self) -> Option<&mut T> {
if self.is_initialized() {
// Safe b/c checked is_initialized and we have a unique access
Some(unsafe { self.get_unchecked_mut() })
} else {
None
}
}
/// Sets the contents of this cell to `value`.
///
/// May block if another thread is currently attempting to initialize the cell. The cell is
/// guaranteed to contain a value when set returns, though not necessarily the one provided.
///
/// Returns `Ok(())` if the cell's value was set by this call.
///
pub fn set(&self, value: T) -> Result<(), T> {
let mut value = Some(value);
self.get_or_init(|| value.take().unwrap());
match value {
None => Ok(()),
Some(value) => Err(value),
}
}
/// Gets the contents of the cell, initializing it with `f` if the cell
/// was empty.
///
/// Many threads may call `get_or_init` concurrently with different
/// initializing functions, but it is guaranteed that only one function
/// will be executed.
///
/// # Panics
///
/// If `f` panics, the panic is propagated to the caller, and the cell
/// remains uninitialized.
///
/// It is an error to reentrantly initialize the cell from `f`. The
/// exact outcome is unspecified. Current implementation deadlocks, but
/// this may be changed to a panic in the future.
///
pub fn get_or_init<F>(&self, f: F) -> &T
where
F: FnOnce() -> T,
{
match self.get_or_try_init(|| Ok::<T, !>(f())) {
Ok(val) => val,
Err(_) => unreachable!(),
}
}
/// Gets the contents of the cell, initializing it with `f` if
/// the cell was empty. If the cell was empty and `f` failed, an
/// error is returned.
///
/// # Panics
///
/// If `f` panics, the panic is propagated to the caller, and
/// the cell remains uninitialized.
///
/// It is an error to reentrantly initialize the cell from `f`.
/// The exact outcome is unspecified. Current implementation
/// deadlocks, but this may be changed to a panic in the future.
///
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
where
F: FnOnce() -> Result<T, E>,
{
// Fast path check
// NOTE: We need to perform an acquire on the state in this method
// in order to correctly synchronize `LazyLock::force`. This is
// currently done by calling `self.get()`, which in turn calls
// `self.is_initialized()`, which in turn performs the acquire.
if let Some(value) = self.get() {
return Ok(value);
}
self.initialize(f)?;
debug_assert!(self.is_initialized());
// SAFETY: The inner value has been initialized
Ok(unsafe { self.get_unchecked() })
}
/// Consumes the `OnceLock`, returning the wrapped value. Returns
/// `None` if the cell was empty.
///
pub fn into_inner(mut self) -> Option<T> {
self.take()
}
/// Takes the value out of this `OnceLock`, moving it back to an uninitialized state.
///
/// Has no effect and returns `None` if the `OnceLock` hasn't been initialized.
///
/// Safety is guaranteed by requiring a mutable reference.
///
pub fn take(&mut self) -> Option<T> {
if self.is_initialized() {
self.once = Once::new();
// SAFETY: `self.value` is initialized and contains a valid `T`.
// `self.once` is reset, so `is_initialized()` will be false again
// which prevents the value from being read twice.
unsafe { Some((*self.value.get()).assume_init_read()) }
} else {
None
}
}
#[inline]
fn is_initialized(&self) -> bool {
self.once.is_completed()
}
#[cold]
fn initialize<F, E>(&self, f: F) -> Result<(), E>
where
F: FnOnce() -> Result<T, E>,
{
let mut res: Result<(), E> = Ok(());
let slot = &self.value;
// Ignore poisoning from other threads
// If another thread panics, then we'll be able to run our closure
self.once.call_once_force(|p| {
match f() {
Ok(value) => {
unsafe { (*slot.get()).write(value) };
}
Err(e) => {
res = Err(e);
// Treat the underlying `Once` as poisoned since we
// failed to initialize our value. Calls
p.poison();
}
}
});
res
}
/// # Safety
///
/// The value must be initialized
unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.is_initialized());
(*self.value.get()).assume_init_ref()
}
/// # Safety
///
/// The value must be initialized
unsafe fn get_unchecked_mut(&mut self) -> &mut T {
debug_assert!(self.is_initialized());
(*self.value.get()).assume_init_mut()
}
}
unsafe impl<T: Sync + Send> Sync for OnceLock<T> {}
unsafe impl<T: Send> Send for OnceLock<T> {}
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceLock<T> {}
impl<T: UnwindSafe> UnwindSafe for OnceLock<T> {}
impl<T> const Default for OnceLock<T> {
/// Creates a new empty cell.
///
fn default() -> OnceLock<T> {
OnceLock::new()
}
}
impl<T: fmt::Debug> fmt::Debug for OnceLock<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.get() {
Some(v) => f.debug_tuple("Once").field(v).finish(),
None => f.write_str("Once(Uninit)"),
}
}
}
impl<T: Clone> Clone for OnceLock<T> {
fn clone(&self) -> OnceLock<T> {
let cell = Self::new();
if let Some(value) = self.get() {
match cell.set(value.clone()) {
Ok(()) => (),
Err(_) => unreachable!(),
}
}
cell
}
}
impl<T> From<T> for OnceLock<T> {
/// Create a new cell with its contents set to `value`.
///
fn from(value: T) -> Self {
let cell = Self::new();
match cell.set(value) {
Ok(()) => cell,
Err(_) => unreachable!(),
}
}
}
impl<T: PartialEq> PartialEq for OnceLock<T> {
fn eq(&self, other: &OnceLock<T>) -> bool {
self.get() == other.get()
}
}
impl<T: Eq> Eq for OnceLock<T> {}
unsafe impl<#[may_dangle] T> Drop for OnceLock<T> {
fn drop(&mut self) {
if self.is_initialized() {
// SAFETY: The cell is initialized and being dropped, so it can't
// be accessed again. We also don't touch the `T` other than
// dropping it, which validates our usage of #[may_dangle].
unsafe { (*self.value.get()).assume_init_drop() };
}
}
}