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    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! A "once initialization" primitive
    //!
    //! This primitive is meant to be used to run one-time initialization. An
    //! example use case would be for initializing an FFI library.

    use isize;
    use sync::atomic::{AtomicIsize, Ordering};
    use sync::StaticMutex;

    /// A synchronization primitive which can be used to run a one-time global
    /// initialization. Useful for one-time initialization for FFI or related
    /// functionality. This type can only be constructed with the `ONCE_INIT`
    /// value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Once, ONCE_INIT};
    ///
    /// static START: Once = ONCE_INIT;
    ///
    /// START.call_once(|| {
    ///     // run initialization here
    /// });
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Once {
        mutex: StaticMutex,
        cnt: AtomicIsize,
        lock_cnt: AtomicIsize,
    }

    /// Initialization value for static `Once` values.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub const ONCE_INIT: Once = Once::new();

    impl Once {
        /// Creates a new `Once` value.
        #[stable(feature = "once_new", since = "1.2.0")]
        pub const fn new() -> Once {
            Once {
                mutex: StaticMutex::new(),
                cnt: AtomicIsize::new(0),
                lock_cnt: AtomicIsize::new(0),
            }
        }

        /// 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). It is also
        /// guaranteed that any memory writes performed by the executed closure can
        /// be reliably observed by other threads at this point (there is a
        /// happens-before relation between the closure and code executing after the
        /// return).
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn call_once<F>(&'static self, f: F) where F: FnOnce() {
            // Optimize common path: load is much cheaper than fetch_add.
            if self.cnt.load(Ordering::SeqCst) < 0 {
                return
            }

            // Implementation-wise, this would seem like a fairly trivial primitive.
            // The stickler part is where our mutexes currently require an
            // allocation, and usage of a `Once` shouldn't leak this allocation.
            //
            // This means that there must be a deterministic destroyer of the mutex
            // contained within (because it's not needed after the initialization
            // has run).
            //
            // The general scheme here is to gate all future threads once
            // initialization has completed with a "very negative" count, and to
            // allow through threads to lock the mutex if they see a non negative
            // count. For all threads grabbing the mutex, exactly one of them should
            // be responsible for unlocking the mutex, and this should only be done
            // once everyone else is done with the mutex.
            //
            // This atomicity is achieved by swapping a very negative value into the
            // shared count when the initialization routine has completed. This will
            // read the number of threads which will at some point attempt to
            // acquire the mutex. This count is then squirreled away in a separate
            // variable, and the last person on the way out of the mutex is then
            // responsible for destroying the mutex.
            //
            // It is crucial that the negative value is swapped in *after* the
            // initialization routine has completed because otherwise new threads
            // calling `call_once` will return immediately before the initialization
            // has completed.

            let prev = self.cnt.fetch_add(1, Ordering::SeqCst);
            if prev < 0 {
                // Make sure we never overflow, we'll never have isize::MIN
                // simultaneous calls to `call_once` to make this value go back to 0
                self.cnt.store(isize::MIN, Ordering::SeqCst);
                return
            }

            // If the count is negative, then someone else finished the job,
            // otherwise we run the job and record how many people will try to grab
            // this lock
            let guard = self.mutex.lock();
            if self.cnt.load(Ordering::SeqCst) > 0 {
                f();
                let prev = self.cnt.swap(isize::MIN, Ordering::SeqCst);
                self.lock_cnt.store(prev, Ordering::SeqCst);
            }
            drop(guard);

            // Last one out cleans up after everyone else, no leaks!
            if self.lock_cnt.fetch_add(-1, Ordering::SeqCst) == 1 {
                unsafe { self.mutex.destroy() }
            }
        }
    }

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

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

        #[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 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();
            }
        }
    }

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