once_cell 1.4.0

use std::{
    cell::UnsafeCell,
    mem::{self, MaybeUninit},
    panic::{RefUnwindSafe, UnwindSafe},
    ptr,
    sync::atomic::{AtomicBool, Ordering},
};

use parking_lot::{lock_api::RawMutex as _RawMutex, RawMutex};

pub(crate) struct OnceCell<T> {
    mutex: Mutex,
    is_initialized: AtomicBool,
    value: UnsafeCell<MaybeUninit<T>>,
}

// 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 {
            mutex: Mutex::new(),
            is_initialized: AtomicBool::new(false),
            value: UnsafeCell::new(MaybeUninit::uninit()),
        }
    }

    /// Safety: synchronizes with store to value via Release/Acquire.
    #[inline]
    pub(crate) fn is_initialized(&self) -> bool {
        self.is_initialized.load(Ordering::Acquire)
    }

    /// 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 _guard = self.mutex.lock();
        if !self.is_initialized() {
            // 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 value = f()?;
            // Safe b/c we have a unique access and no panic may happen
            // until the cell is marked as initialized.
            unsafe { self.as_mut_ptr().write(value) };
            self.is_initialized.store(true, Ordering::Release);
        }
        Ok(())
    }

    /// 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());
        &*self.as_ptr()
    }

    /// 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> {
        if self.is_initialized() {
            // Safe b/c we have a unique access and value is initialized.
            Some(unsafe { &mut *self.as_mut_ptr() })
        } else {
            None
        }
    }

    /// Consumes this `OnceCell`, returning the wrapped value.
    /// Returns `None` if the cell was empty.
    pub(crate) fn into_inner(self) -> Option<T> {
        if !self.is_initialized() {
            return None;
        }

        // Safe b/c we have a unique access and value is initialized.
        let value: T = unsafe { ptr::read(self.as_ptr()) };

        // It's OK to `mem::forget` without dropping, because both `self.mutex`
        // and `self.is_initialized` are not heap-allocated.
        mem::forget(self);

        Some(value)
    }

    fn as_ptr(&self) -> *const T {
        unsafe {
            let slot: &MaybeUninit<T> = &*self.value.get();
            slot.as_ptr()
        }
    }

    fn as_mut_ptr(&self) -> *mut T {
        unsafe {
            let slot: &mut MaybeUninit<T> = &mut *self.value.get();
            slot.as_mut_ptr()
        }
    }
}

impl<T> Drop for OnceCell<T> {
    fn drop(&mut self) {
        if self.is_initialized() {
            // Safe b/c we have a unique access and value is initialized.
            unsafe { ptr::drop_in_place(self.as_mut_ptr()) };
        }
    }
}

/// Wrapper around parking_lot's `RawMutex` which has `const fn` new.
struct Mutex {
    inner: RawMutex,
}

impl Mutex {
    const fn new() -> Mutex {
        Mutex { inner: RawMutex::INIT }
    }

    fn lock(&self) -> MutexGuard<'_> {
        self.inner.lock();
        MutexGuard { inner: &self.inner }
    }
}

struct MutexGuard<'a> {
    inner: &'a RawMutex,
}

impl Drop for MutexGuard<'_> {
    fn drop(&mut self) {
        self.inner.unlock();
    }
}

#[test]
fn test_size() {
    use std::mem::size_of;

    assert_eq!(size_of::<OnceCell<bool>>(), 2 * size_of::<bool>() + size_of::<u8>());
}