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use super::{Semaphore, SemaphorePermit, TryAcquireError};
use crate::loom::cell::UnsafeCell;
use std::error::Error;
use std::fmt;
use std::future::Future;
use std::mem::MaybeUninit;
use std::ops::Drop;
use std::ptr;
use std::sync::atomic::{AtomicBool, Ordering};
// This file contains an implementation of an OnceCell. The principle
// behind the safety the of the cell is that any thread with an `&OnceCell` may
// access the `value` field according the following rules:
//
// 1. When `value_set` is false, the `value` field may be modified by the
// thread holding the permit on the semaphore.
// 2. When `value_set` is true, the `value` field may be accessed immutably by
// any thread.
//
// It is an invariant that if the semaphore is closed, then `value_set` is true.
// The reverse does not necessarily hold — but if not, the semaphore may not
// have any available permits.
//
// A thread with a `&mut OnceCell` may modify the value in any way it wants as
// long as the invariants are upheld.
/// A thread-safe cell that can be written to only once.
///
/// A `OnceCell` is typically used for global variables that need to be
/// initialized once on first use, but need no further changes. The `OnceCell`
/// in Tokio allows the initialization procedure to be asynchronous.
///
/// # Examples
///
/// ```
/// use tokio::sync::OnceCell;
///
/// async fn some_computation() -> u32 {
/// 1 + 1
/// }
///
/// static ONCE: OnceCell<u32> = OnceCell::const_new();
///
/// #[tokio::main]
/// async fn main() {
/// let result = ONCE.get_or_init(some_computation).await;
/// assert_eq!(*result, 2);
/// }
/// ```
///
/// It is often useful to write a wrapper method for accessing the value.
///
/// ```
/// use tokio::sync::OnceCell;
///
/// static ONCE: OnceCell<u32> = OnceCell::const_new();
///
/// async fn get_global_integer() -> &'static u32 {
/// ONCE.get_or_init(|| async {
/// 1 + 1
/// }).await
/// }
///
/// #[tokio::main]
/// async fn main() {
/// let result = get_global_integer().await;
/// assert_eq!(*result, 2);
/// }
/// ```
pub struct OnceCell<T> {
value_set: AtomicBool,
value: UnsafeCell<MaybeUninit<T>>,
semaphore: Semaphore,
}
impl<T> Default for OnceCell<T> {
fn default() -> OnceCell<T> {
OnceCell::new()
}
}
impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("OnceCell")
.field("value", &self.get())
.finish()
}
}
impl<T: Clone> Clone for OnceCell<T> {
fn clone(&self) -> OnceCell<T> {
OnceCell::new_with(self.get().cloned())
}
}
impl<T: PartialEq> PartialEq for OnceCell<T> {
fn eq(&self, other: &OnceCell<T>) -> bool {
self.get() == other.get()
}
}
impl<T: Eq> Eq for OnceCell<T> {}
impl<T> Drop for OnceCell<T> {
fn drop(&mut self) {
if self.initialized_mut() {
unsafe {
self.value
.with_mut(|ptr| ptr::drop_in_place((*ptr).as_mut_ptr()));
};
}
}
}
impl<T> From<T> for OnceCell<T> {
fn from(value: T) -> Self {
let semaphore = Semaphore::new(0);
semaphore.close();
OnceCell {
value_set: AtomicBool::new(true),
value: UnsafeCell::new(MaybeUninit::new(value)),
semaphore,
}
}
}
impl<T> OnceCell<T> {
/// Creates a new empty `OnceCell` instance.
pub fn new() -> Self {
OnceCell {
value_set: AtomicBool::new(false),
value: UnsafeCell::new(MaybeUninit::uninit()),
semaphore: Semaphore::new(1),
}
}
/// Creates a new `OnceCell` that contains the provided value, if any.
///
/// If the `Option` is `None`, this is equivalent to `OnceCell::new`.
///
/// [`OnceCell::new`]: crate::sync::OnceCell::new
pub fn new_with(value: Option<T>) -> Self {
if let Some(v) = value {
OnceCell::from(v)
} else {
OnceCell::new()
}
}
/// Creates a new empty `OnceCell` instance.
///
/// Equivalent to `OnceCell::new`, except that it can be used in static
/// variables.
///
/// # Example
///
/// ```
/// use tokio::sync::OnceCell;
///
/// static ONCE: OnceCell<u32> = OnceCell::const_new();
///
/// async fn get_global_integer() -> &'static u32 {
/// ONCE.get_or_init(|| async {
/// 1 + 1
/// }).await
/// }
///
/// #[tokio::main]
/// async fn main() {
/// let result = get_global_integer().await;
/// assert_eq!(*result, 2);
/// }
/// ```
#[cfg(all(feature = "parking_lot", not(all(loom, test))))]
#[cfg_attr(docsrs, doc(cfg(feature = "parking_lot")))]
pub const fn const_new() -> Self {
OnceCell {
value_set: AtomicBool::new(false),
value: UnsafeCell::new(MaybeUninit::uninit()),
semaphore: Semaphore::const_new(1),
}
}
/// Returns `true` if the `OnceCell` currently contains a value, and `false`
/// otherwise.
pub fn initialized(&self) -> bool {
// Using acquire ordering so any threads that read a true from this
// atomic is able to read the value.
self.value_set.load(Ordering::Acquire)
}
/// Returns `true` if the `OnceCell` currently contains a value, and `false`
/// otherwise.
fn initialized_mut(&mut self) -> bool {
*self.value_set.get_mut()
}
// SAFETY: The OnceCell must not be empty.
unsafe fn get_unchecked(&self) -> &T {
&*self.value.with(|ptr| (*ptr).as_ptr())
}
// SAFETY: The OnceCell must not be empty.
unsafe fn get_unchecked_mut(&mut self) -> &mut T {
&mut *self.value.with_mut(|ptr| (*ptr).as_mut_ptr())
}
fn set_value(&self, value: T, permit: SemaphorePermit<'_>) -> &T {
// SAFETY: We are holding the only permit on the semaphore.
unsafe {
self.value.with_mut(|ptr| (*ptr).as_mut_ptr().write(value));
}
// Using release ordering so any threads that read a true from this
// atomic is able to read the value we just stored.
self.value_set.store(true, Ordering::Release);
self.semaphore.close();
permit.forget();
// SAFETY: We just initialized the cell.
unsafe { self.get_unchecked() }
}
/// Returns a reference to the value currently stored in the `OnceCell`, or
/// `None` if the `OnceCell` is empty.
pub fn get(&self) -> Option<&T> {
if self.initialized() {
Some(unsafe { self.get_unchecked() })
} else {
None
}
}
/// Returns a mutable reference to the value currently stored in the
/// `OnceCell`, or `None` if the `OnceCell` is empty.
///
/// Since this call borrows the `OnceCell` mutably, it is safe to mutate the
/// value inside the `OnceCell` — the mutable borrow statically guarantees
/// no other references exist.
pub fn get_mut(&mut self) -> Option<&mut T> {
if self.initialized_mut() {
Some(unsafe { self.get_unchecked_mut() })
} else {
None
}
}
/// Sets the value of the `OnceCell` to the given value if the `OnceCell` is
/// empty.
///
/// If the `OnceCell` already has a value, this call will fail with an
/// [`SetError::AlreadyInitializedError`].
///
/// If the `OnceCell` is empty, but some other task is currently trying to
/// set the value, this call will fail with [`SetError::InitializingError`].
///
/// [`SetError::AlreadyInitializedError`]: crate::sync::SetError::AlreadyInitializedError
/// [`SetError::InitializingError`]: crate::sync::SetError::InitializingError
pub fn set(&self, value: T) -> Result<(), SetError<T>> {
if self.initialized() {
return Err(SetError::AlreadyInitializedError(value));
}
// Another task might be initializing the cell, in which case
// `try_acquire` will return an error. If we succeed to acquire the
// permit, then we can set the value.
match self.semaphore.try_acquire() {
Ok(permit) => {
debug_assert!(!self.initialized());
self.set_value(value, permit);
Ok(())
}
Err(TryAcquireError::NoPermits) => {
// Some other task is holding the permit. That task is
// currently trying to initialize the value.
Err(SetError::InitializingError(value))
}
Err(TryAcquireError::Closed) => {
// The semaphore was closed. Some other task has initialized
// the value.
Err(SetError::AlreadyInitializedError(value))
}
}
}
/// Gets the value currently in the `OnceCell`, or initialize it with the
/// given asynchronous operation.
///
/// If some other task is currently working on initializing the `OnceCell`,
/// this call will wait for that other task to finish, then return the value
/// that the other task produced.
///
/// If the provided operation is cancelled or panics, the initialization
/// attempt is cancelled. If there are other tasks waiting for the value to
/// be initialized, one of them will start another attempt at initializing
/// the value.
///
/// This will deadlock if `f` tries to initialize the cell recursively.
pub async fn get_or_init<F, Fut>(&self, f: F) -> &T
where
F: FnOnce() -> Fut,
Fut: Future<Output = T>,
{
crate::trace::async_trace_leaf().await;
if self.initialized() {
// SAFETY: The OnceCell has been fully initialized.
unsafe { self.get_unchecked() }
} else {
// Here we try to acquire the semaphore permit. Holding the permit
// will allow us to set the value of the OnceCell, and prevents
// other tasks from initializing the OnceCell while we are holding
// it.
match self.semaphore.acquire().await {
Ok(permit) => {
debug_assert!(!self.initialized());
// If `f()` panics or `select!` is called, this
// `get_or_init` call is aborted and the semaphore permit is
// dropped.
let value = f().await;
self.set_value(value, permit)
}
Err(_) => {
debug_assert!(self.initialized());
// SAFETY: The semaphore has been closed. This only happens
// when the OnceCell is fully initialized.
unsafe { self.get_unchecked() }
}
}
}
}
/// Gets the value currently in the `OnceCell`, or initialize it with the
/// given asynchronous operation.
///
/// If some other task is currently working on initializing the `OnceCell`,
/// this call will wait for that other task to finish, then return the value
/// that the other task produced.
///
/// If the provided operation returns an error, is cancelled or panics, the
/// initialization attempt is cancelled. If there are other tasks waiting
/// for the value to be initialized, one of them will start another attempt
/// at initializing the value.
///
/// This will deadlock if `f` tries to initialize the cell recursively.
pub async fn get_or_try_init<E, F, Fut>(&self, f: F) -> Result<&T, E>
where
F: FnOnce() -> Fut,
Fut: Future<Output = Result<T, E>>,
{
crate::trace::async_trace_leaf().await;
if self.initialized() {
// SAFETY: The OnceCell has been fully initialized.
unsafe { Ok(self.get_unchecked()) }
} else {
// Here we try to acquire the semaphore permit. Holding the permit
// will allow us to set the value of the OnceCell, and prevents
// other tasks from initializing the OnceCell while we are holding
// it.
match self.semaphore.acquire().await {
Ok(permit) => {
debug_assert!(!self.initialized());
// If `f()` panics or `select!` is called, this
// `get_or_try_init` call is aborted and the semaphore
// permit is dropped.
let value = f().await;
match value {
Ok(value) => Ok(self.set_value(value, permit)),
Err(e) => Err(e),
}
}
Err(_) => {
debug_assert!(self.initialized());
// SAFETY: The semaphore has been closed. This only happens
// when the OnceCell is fully initialized.
unsafe { Ok(self.get_unchecked()) }
}
}
}
}
/// Takes the value from the cell, destroying the cell in the process.
/// Returns `None` if the cell is empty.
pub fn into_inner(mut self) -> Option<T> {
if self.initialized_mut() {
// Set to uninitialized for the destructor of `OnceCell` to work properly
*self.value_set.get_mut() = false;
Some(unsafe { self.value.with(|ptr| ptr::read(ptr).assume_init()) })
} else {
None
}
}
/// Takes ownership of the current value, leaving the cell empty. Returns
/// `None` if the cell is empty.
pub fn take(&mut self) -> Option<T> {
std::mem::take(self).into_inner()
}
}
// Since `get` gives us access to immutable references of the OnceCell, OnceCell
// can only be Sync if T is Sync, otherwise OnceCell would allow sharing
// references of !Sync values across threads. We need T to be Send in order for
// OnceCell to by Sync because we can use `set` on `&OnceCell<T>` to send values
// (of type T) across threads.
unsafe impl<T: Sync + Send> Sync for OnceCell<T> {}
// Access to OnceCell's value is guarded by the semaphore permit
// and atomic operations on `value_set`, so as long as T itself is Send
// it's safe to send it to another thread
unsafe impl<T: Send> Send for OnceCell<T> {}
/// Errors that can be returned from [`OnceCell::set`].
///
/// [`OnceCell::set`]: crate::sync::OnceCell::set
#[derive(Debug, PartialEq, Eq)]
pub enum SetError<T> {
/// The cell was already initialized when [`OnceCell::set`] was called.
///
/// [`OnceCell::set`]: crate::sync::OnceCell::set
AlreadyInitializedError(T),
/// The cell is currently being initialized.
InitializingError(T),
}
impl<T> fmt::Display for SetError<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
SetError::AlreadyInitializedError(_) => write!(f, "AlreadyInitializedError"),
SetError::InitializingError(_) => write!(f, "InitializingError"),
}
}
}
impl<T: fmt::Debug> Error for SetError<T> {}
impl<T> SetError<T> {
/// Whether `SetError` is `SetError::AlreadyInitializedError`.
pub fn is_already_init_err(&self) -> bool {
match self {
SetError::AlreadyInitializedError(_) => true,
SetError::InitializingError(_) => false,
}
}
/// Whether `SetError` is `SetError::InitializingError`
pub fn is_initializing_err(&self) -> bool {
match self {
SetError::AlreadyInitializedError(_) => false,
SetError::InitializingError(_) => true,
}
}
}