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Diffstat (limited to 'third_party/rust/tokio/src/sync/once_cell.rs')
| -rw-r--r-- | third_party/rust/tokio/src/sync/once_cell.rs | 457 |
1 files changed, 457 insertions, 0 deletions
diff --git a/third_party/rust/tokio/src/sync/once_cell.rs b/third_party/rust/tokio/src/sync/once_cell.rs new file mode 100644 index 0000000000..d31a40e2c8 --- /dev/null +++ b/third_party/rust/tokio/src/sync/once_cell.rs @@ -0,0 +1,457 @@ +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((&mut *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>, + { + 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>>, + { + 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)] +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, + } + } +} |