# once.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/spin-0.7.1/src/once.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Synchronization primitives for one-time evaluation.

    use core::{
        cell::UnsafeCell,
        mem::MaybeUninit,
        sync::atomic::{AtomicUsize, Ordering},
        fmt,
    };

    /// A primitive that provides lazy one-time initialization.
    ///
    /// Unlike its `std::sync` equivalent, this is generalized such that the closure returns a
    /// value to be stored by the [`Once`] (`std::sync::Once` can be trivially emulated with
    /// `Once<()>`).
    ///
    /// Because [`Once::new`] is `const`, this primitive may be used to safely initialize statics.
    ///
    /// # Examples
    ///
    /// ```
    /// use spin;
    ///
    /// static START: spin::Once<()> = spin::Once::new();
    ///
    /// START.call_once(|| {
    ///     // run initialization here
    /// });
    /// ```
    pub struct Once<T> {
        state: AtomicUsize,
        data: UnsafeCell<MaybeUninit<T>>,
    }

    impl<T: fmt::Debug> fmt::Debug for Once<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match self.get() {
                Some(s) => write!(f, "Once {{ data: ")
    				.and_then(|()| s.fmt(f))
    				.and_then(|()| write!(f, "}}")),
                None => write!(f, "Once {{ <uninitialized> }}")
            }
        }
    }

    // Same unsafe impls as `std::sync::RwLock`, because this also allows for
    // concurrent reads.
    unsafe impl<T: Send + Sync> Sync for Once<T> {}
    unsafe impl<T: Send> Send for Once<T> {}

    // Four states that a Once can be in, encoded into the lower bits of `state` in
    // the Once structure.
    const INCOMPLETE: usize = 0x0;
    const RUNNING: usize = 0x1;
    const COMPLETE: usize = 0x2;
    const PANICKED: usize = 0x3;

    use core::hint::unreachable_unchecked as unreachable;

    impl<T> Once<T> {
        /// Initialization constant of [`Once`].
        #[allow(clippy::declare_interior_mutable_const)]
        pub const INIT: Self = Self {
            state: AtomicUsize::new(INCOMPLETE),
            data: UnsafeCell::new(MaybeUninit::uninit()),
        };

        /// Creates a new [`Once`].
        pub const fn new() -> Once<T> {
            Self::INIT
        }

        /// Creates a new initialized [`Once`].
        pub const fn initialized(data: T) -> Once<T> {
            Self {
                state: AtomicUsize::new(COMPLETE),
                data: UnsafeCell::new(MaybeUninit::new(data)),
            }
        }

        /// Get a reference to the initialized instance. Must only be called once COMPLETE.
        unsafe fn force_get(&self) -> &T {
            // SAFETY:
            // * `UnsafeCell`/inner deref: data never changes again
            // * `MaybeUninit`/outer deref: data was initialized
            &*(*self.data.get()).as_ptr()
        }

        /// Get a reference to the initialized instance. Must only be called once COMPLETE.
        unsafe fn force_get_mut(&mut self) -> &mut T {
            // SAFETY:
            // * `UnsafeCell`/inner deref: data never changes again
            // * `MaybeUninit`/outer deref: data was initialized
            &mut *(*self.data.get()).as_mut_ptr()
        }

        /// Get a reference to the initialized instance. Must only be called once COMPLETE.
        unsafe fn force_into_inner(self) -> T {
            // SAFETY:
            // * `UnsafeCell`/inner deref: data never changes again
            // * `MaybeUninit`/outer deref: data was initialized
            (*self.data.get()).as_ptr().read()
        }

        /// Performs an initialization routine once and only once. The given closure
        /// will be executed if this is the first time `call_once` has been called,
        /// and otherwise the routine will *not* be invoked.
        ///
        /// This method will block the calling thread if another initialization
        /// routine is currently running.
        ///
        /// When this function returns, it is guaranteed that some initialization
        /// has run and completed (it may not be the closure specified). The
        /// returned pointer will point to the result from the closure that was
        /// run.
        ///
        /// # Panics
        ///
        /// This function will panic if the [`Once`] previously panicked while attempting
        /// to initialize. This is similar to the poisoning behaviour of `std::sync`'s
        /// primitives.
        ///
        /// # Examples
        ///
        /// ```
        /// use spin;
        ///
        /// static INIT: spin::Once<usize> = spin::Once::new();
        ///
        /// fn get_cached_val() -> usize {
        ///     *INIT.call_once(expensive_computation)
        /// }
        ///
        /// fn expensive_computation() -> usize {
        ///     // ...
        /// # 2
        /// }
        /// ```
        pub fn call_once<F: FnOnce() -> T>(&self, f: F) -> &T {
            let mut status = self.state.load(Ordering::SeqCst);

            if status == INCOMPLETE {
                status = self.state.compare_and_swap(
                    INCOMPLETE,
                    RUNNING,
                    Ordering::SeqCst,
                );

                if status == INCOMPLETE { // We init
                    // We use a guard (Finish) to catch panics caused by builder
                    let mut finish = Finish { state: &self.state, panicked: true };
                    unsafe {
                        // SAFETY:
                        // `UnsafeCell`/deref: currently the only accessor, mutably
                        // and immutably by cas exclusion.
                        // `write`: pointer comes from `MaybeUninit`.
                        (*self.data.get()).as_mut_ptr().write(f())
                    };
                    finish.panicked = false;

                    status = COMPLETE;
                    self.state.store(status, Ordering::SeqCst);

                    // This next line is strictly an optimization
                    return unsafe { self.force_get() };
                }
            }

            self
                .poll()
                .unwrap_or_else(|| unreachable!("Encountered INCOMPLETE when polling Once"))
        }

        /// Returns a reference to the inner value if the [`Once`] has been initialized.
        pub fn get(&self) -> Option<&T> {
            match self.state.load(Ordering::SeqCst) {
                COMPLETE => Some(unsafe { self.force_get() }),
                _ => None,
            }
        }

        /// Returns a mutable reference to the inner value if the [`Once`] has been initialized.
        ///
        /// Because this method requires a mutable reference to the [`Once`], no synchronization
        /// overhead is required to access the inner value. In effect, it is zero-cost.
        pub fn get_mut(&mut self) -> Option<&mut T> {
            match *self.state.get_mut() {
                COMPLETE => Some(unsafe { self.force_get_mut() }),
                _ => None,
            }
        }

        /// Returns a the inner value if the [`Once`] has been initialized.
        ///
        /// Because this method requires ownershup of the [`Once`], no synchronization overhead
        /// is required to access the inner value. In effect, it is zero-cost.
        pub fn try_into_inner(mut self) -> Option<T> {
            match *self.state.get_mut() {
                COMPLETE => Some(unsafe { self.force_into_inner() }),
                _ => None,
            }
        }

        /// Returns a reference to the inner value if the [`Once`] has been initialized.
        pub fn is_completed(&self) -> bool {
            self.state.load(Ordering::SeqCst) == COMPLETE
        }

        /// Spins until the [`Once`] contains a value.
        ///
        /// Note that in releases prior to `0.7`, this function had the behaviour of [`Once::poll`].
        ///
        /// # Panics
        ///
        /// This function will panic if the [`Once`] previously panicked while attempting
        /// to initialize. This is similar to the poisoning behaviour of `std::sync`'s
        /// primitives.
        pub fn wait(&self) -> &T {
            loop {
                match self.poll() {
                    Some(x) => break x,
                    None => crate::relax(),
                }
            }
        }

        /// Like [`Once::get`], but will spin if the [`Once`] is in the process of being
        /// initialized. If initialization has not even begun, `None` will be returned.
        ///
        /// Note that in releases prior to `0.7`, this function was named `wait`.
        ///
        /// # Panics
        ///
        /// This function will panic if the [`Once`] previously panicked while attempting
        /// to initialize. This is similar to the poisoning behaviour of `std::sync`'s
        /// primitives.
        pub fn poll(&self) -> Option<&T> {
            loop {
                match self.state.load(Ordering::SeqCst) {
                    INCOMPLETE => return None,
                    RUNNING => crate::relax(), // We spin
                    COMPLETE => return Some(unsafe { self.force_get() }),
                    PANICKED => panic!("Once previously poisoned by a panicked"),
                    _ => unsafe { unreachable() },
                }
            }
        }
    }

    impl<T> From<T> for Once<T> {
        fn from(data: T) -> Self {
            Self::initialized(data)
        }
    }

    impl<T> Drop for Once<T> {
        fn drop(&mut self) {
            if self.state.load(Ordering::SeqCst) == COMPLETE {
                unsafe {
                    //TODO: Use MaybeUninit::assume_init_drop once stabilised
                    core::ptr::drop_in_place((*self.data.get()).as_mut_ptr());
                }
            }
        }
    }

    struct Finish<'a> {
        state: &'a AtomicUsize,
        panicked: bool,
    }

    impl<'a> Drop for Finish<'a> {
        fn drop(&mut self) {
            if self.panicked {
                self.state.store(PANICKED, Ordering::SeqCst);
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use std::prelude::v1::*;

        use std::sync::mpsc::channel;
        use std::thread;
        use super::Once;

        #[test]
        fn smoke_once() {
            static O: Once<()> = Once::new();
            let mut a = 0;
            O.call_once(|| a += 1);
            assert_eq!(a, 1);
            O.call_once(|| a += 1);
            assert_eq!(a, 1);
        }

        #[test]
        fn smoke_once_value() {
            static O: Once<usize> = Once::new();
            let a = O.call_once(|| 1);
            assert_eq!(*a, 1);
            let b = O.call_once(|| 2);
            assert_eq!(*b, 1);
        }

        #[test]
        fn stampede_once() {
            static O: Once<()> = Once::new();
            static mut RUN: bool = false;

            let (tx, rx) = channel();
            for _ in 0..10 {
                let tx = tx.clone();
                thread::spawn(move|| {
                    for _ in 0..4 { thread::yield_now() }
                    unsafe {
                        O.call_once(|| {
                            assert!(!RUN);
                            RUN = true;
                        });
                        assert!(RUN);
                    }
                    tx.send(()).unwrap();
                });
            }

            unsafe {
                O.call_once(|| {
                    assert!(!RUN);
                    RUN = true;
                });
                assert!(RUN);
            }

            for _ in 0..10 {
                rx.recv().unwrap();
            }
        }

        #[test]
        fn get() {
            static INIT: Once<usize> = Once::new();

            assert!(INIT.get().is_none());
            INIT.call_once(|| 2);
            assert_eq!(INIT.get().map(|r| *r), Some(2));
        }

        #[test]
        fn get_no_wait() {
            static INIT: Once<usize> = Once::new();

            assert!(INIT.get().is_none());
            thread::spawn(move|| {
                INIT.call_once(|| loop { });
            });
            assert!(INIT.get().is_none());
        }

        #[test]
        fn poll() {
            static INIT: Once<usize> = Once::new();

            assert!(INIT.poll().is_none());
            INIT.call_once(|| 3);
            assert_eq!(INIT.poll().map(|r| *r), Some(3));
        }

        #[test]
        fn wait() {
            static INIT: Once<usize> = Once::new();

            std::thread::spawn(|| {
                assert_eq!(*INIT.wait(), 3);
                assert!(INIT.is_completed());
            });

            for _ in 0..4 { thread::yield_now() }

            assert!(INIT.poll().is_none());
            INIT.call_once(|| 3);
        }

        #[test]
        fn panic() {
            use ::std::panic;

            static INIT: Once<()> = Once::new();

            // poison the once
            let t = panic::catch_unwind(|| {
                INIT.call_once(|| panic!());
            });
            assert!(t.is_err());

            // poisoning propagates
            let t = panic::catch_unwind(|| {
                INIT.call_once(|| {});
            });
            assert!(t.is_err());
        }

        #[test]
        fn init_constant() {
            static O: Once<()> = Once::INIT;
            let mut a = 0;
            O.call_once(|| a += 1);
            assert_eq!(a, 1);
            O.call_once(|| a += 1);
            assert_eq!(a, 1);
        }

        static mut CALLED: bool = false;

        struct DropTest {}

        impl Drop for DropTest {
            fn drop(&mut self) {
                unsafe {
                    CALLED = true;
                }
            }
        }

        #[test]
        fn drop() {
            unsafe {
                CALLED = false;
            }

            {
                let once = Once::new();
                once.call_once(|| DropTest {});
            }

            assert!(unsafe {
                CALLED
            });
        }

        #[test]
        fn skip_uninit_drop() {
            unsafe {
                CALLED = false;
            }

            {
                let once = Once::<DropTest>::new();
            }

            assert!(unsafe {
                !CALLED
            });
        }
    }