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    //! Implementation of running at_exit routines
    //!
    //! Documentation can be found on the `rt::at_exit` function.

    use boxed::FnBox;
    use ptr;
    use mem;
    use sys_common::mutex::Mutex;

    type Queue = Vec<Box<dyn FnBox()>>;

    // NB these are specifically not types from `std::sync` as they currently rely
    // on poisoning and this module needs to operate at a lower level than requiring
    // the thread infrastructure to be in place (useful on the borders of
    // initialization/destruction).
    // We never call `LOCK.init()`, so it is UB to attempt to
    // acquire this mutex reentrantly!
    static LOCK: Mutex = Mutex::new();
    static mut QUEUE: *mut Queue = ptr::null_mut();

    const DONE: *mut Queue = 1_usize as *mut _;

    // The maximum number of times the cleanup routines will be run. While running
    // the at_exit closures new ones may be registered, and this count is the number
    // of times the new closures will be allowed to register successfully. After
    // this number of iterations all new registrations will return `false`.
    const ITERS: usize = 10;

    unsafe fn init() -> bool {
        if QUEUE.is_null() {
            let state: Box<Queue> = box Vec::new();
            QUEUE = Box::into_raw(state);
        } else if QUEUE == DONE {
            // can't re-init after a cleanup
            return false
        }

        true
    }

    pub fn cleanup() {
        for i in 1..=ITERS {
            unsafe {
                let queue = {
                    let _guard = LOCK.lock();
                    mem::replace(&mut QUEUE, if i == ITERS { DONE } else { ptr::null_mut() })
                };

                // make sure we're not recursively cleaning up
                assert!(queue != DONE);

                // If we never called init, not need to cleanup!
                if !queue.is_null() {
                    let queue: Box<Queue> = Box::from_raw(queue);
                    for to_run in *queue {
                        // We are not holding any lock, so reentrancy is fine.
                        to_run();
                    }
                }
            }
        }
    }

    pub fn push(f: Box<dyn FnBox()>) -> bool {
        unsafe {
            let _guard = LOCK.lock();
            if init() {
                // We are just moving `f` around, not calling it.
                // There is no possibility of reentrancy here.
                (*QUEUE).push(f);
                true
            } else {
                false
            }
        }
    }

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