src/sync/once_cell.rs

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
        }
    }

    /// Set 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))
            }
        }
    }

    /// Get 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() }
                }
            }
        }
    }

    /// Get 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()) }
                }
            }
        }
    }

    /// Take 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,
        }
    }
}