# imp_std.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/once_cell-1.7.2/src/imp_std.rs`.
Keywords: rust, rustlang, rust-lang

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    // There's a lot of scary concurrent code in this module, but it is copied from
    // `std::sync::Once` with two changes:
    //   * no poisoning
    //   * init function can fail

    use std::{
        cell::{Cell, UnsafeCell},
        hint::unreachable_unchecked,
        marker::PhantomData,
        panic::{RefUnwindSafe, UnwindSafe},
        sync::atomic::{AtomicBool, AtomicUsize, Ordering},
        thread::{self, Thread},
    };

    use crate::take_unchecked;

    #[derive(Debug)]
    pub(crate) struct OnceCell<T> {
        // This `state` word is actually an encoded version of just a pointer to a
        // `Waiter`, so we add the `PhantomData` appropriately.
        state_and_queue: AtomicUsize,
        _marker: PhantomData<*mut Waiter>,
        value: UnsafeCell<Option<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> {}

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

    // Mask to learn about the state. All other bits are the queue of waiters if
    // this is in the RUNNING state.
    const STATE_MASK: usize = 0x3;

    // Representation of a node in the linked list of waiters in the RUNNING state.
    #[repr(align(4))] // Ensure the two lower bits are free to use as state bits.
    struct Waiter {
        thread: Cell<Option<Thread>>,
        signaled: AtomicBool,
        next: *const Waiter,
    }

    // Head of a linked list of waiters.
    // Every node is a struct on the stack of a waiting thread.
    // Will wake up the waiters when it gets dropped, i.e. also on panic.
    struct WaiterQueue<'a> {
        state_and_queue: &'a AtomicUsize,
        set_state_on_drop_to: usize,
    }

    impl<T> OnceCell<T> {
        pub(crate) const fn new() -> OnceCell<T> {
            OnceCell {
                state_and_queue: AtomicUsize::new(INCOMPLETE),
                _marker: PhantomData,
                value: UnsafeCell::new(None),
            }
        }

        /// Safety: synchronizes with store to value via Release/(Acquire|SeqCst).
        #[inline]
        pub(crate) fn is_initialized(&self) -> bool {
            // An `Acquire` load is enough because that makes all the initialization
            // operations visible to us, and, this being a fast path, weaker
            // ordering helps with performance. This `Acquire` synchronizes with
            // `SeqCst` operations on the slow path.
            self.state_and_queue.load(Ordering::Acquire) == COMPLETE
        }

        /// Safety: synchronizes with store to value via SeqCst read from state,
        /// writes value only once because we never get to INCOMPLETE state after a
        /// successful write.
        #[cold]
        pub(crate) fn initialize<F, E>(&self, f: F) -> Result<(), E>
        where
            F: FnOnce() -> Result<T, E>,
        {
            let mut f = Some(f);
            let mut res: Result<(), E> = Ok(());
            let slot: *mut Option<T> = self.value.get();
            initialize_inner(&self.state_and_queue, &mut || {
                let f = unsafe { take_unchecked(&mut f) };
                match f() {
                    Ok(value) => {
                        unsafe { *slot = Some(value) };
                        true
                    }
                    Err(err) => {
                        res = Err(err);
                        false
                    }
                }
            });
            res
        }

        /// 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());
            let slot: &Option<T> = &*self.value.get();
            match slot {
                Some(value) => value,
                // This unsafe does improve performance, see `examples/bench`.
                None => {
                    debug_assert!(false);
                    unreachable_unchecked()
                }
            }
        }

        /// 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> {
            // Safe b/c we have a unique access.
            unsafe { &mut *self.value.get() }.as_mut()
        }

        /// Consumes this `OnceCell`, returning the wrapped value.
        /// Returns `None` if the cell was empty.
        #[inline]
        pub(crate) fn into_inner(self) -> Option<T> {
            // Because `into_inner` takes `self` by value, the compiler statically
            // verifies that it is not currently borrowed.
            // So, it is safe to move out `Option<T>`.
            self.value.into_inner()
        }
    }

    // Corresponds to `std::sync::Once::call_inner`
    // Note: this is intentionally monomorphic
    #[inline(never)]
    fn initialize_inner(my_state_and_queue: &AtomicUsize, init: &mut dyn FnMut() -> bool) -> bool {
        let mut state_and_queue = my_state_and_queue.load(Ordering::Acquire);

        loop {
            match state_and_queue {
                COMPLETE => return true,
                INCOMPLETE => {
                    let exchange = my_state_and_queue.compare_exchange(
                        state_and_queue,
                        RUNNING,
                        Ordering::Acquire,
                        Ordering::Acquire,
                    );
                    if let Err(old) = exchange {
                        state_and_queue = old;
                        continue;
                    }
                    let mut waiter_queue = WaiterQueue {
                        state_and_queue: my_state_and_queue,
                        set_state_on_drop_to: INCOMPLETE, // Difference, std uses `POISONED`
                    };
                    let success = init();

                    // Difference, std always uses `COMPLETE`
                    waiter_queue.set_state_on_drop_to = if success { COMPLETE } else { INCOMPLETE };
                    return success;
                }
                _ => {
                    assert!(state_and_queue & STATE_MASK == RUNNING);
                    wait(&my_state_and_queue, state_and_queue);
                    state_and_queue = my_state_and_queue.load(Ordering::Acquire);
                }
            }
        }
    }

    // Copy-pasted from std exactly.
    fn wait(state_and_queue: &AtomicUsize, mut current_state: usize) {
        loop {
            if current_state & STATE_MASK != RUNNING {
                return;
            }

            let node = Waiter {
                thread: Cell::new(Some(thread::current())),
                signaled: AtomicBool::new(false),
                next: (current_state & !STATE_MASK) as *const Waiter,
            };
            let me = &node as *const Waiter as usize;

            let exchange = state_and_queue.compare_exchange(
                current_state,
                me | RUNNING,
                Ordering::Release,
                Ordering::Relaxed,
            );
            if let Err(old) = exchange {
                current_state = old;
                continue;
            }

            while !node.signaled.load(Ordering::Acquire) {
                thread::park();
            }
            break;
        }
    }

    // Copy-pasted from std exactly.
    impl Drop for WaiterQueue<'_> {
        fn drop(&mut self) {
            let state_and_queue =
                self.state_and_queue.swap(self.set_state_on_drop_to, Ordering::AcqRel);

            assert_eq!(state_and_queue & STATE_MASK, RUNNING);

            unsafe {
                let mut queue = (state_and_queue & !STATE_MASK) as *const Waiter;
                while !queue.is_null() {
                    let next = (*queue).next;
                    let thread = (*queue).thread.replace(None).unwrap();
                    (*queue).signaled.store(true, Ordering::Release);
                    queue = next;
                    thread.unpark();
                }
            }
        }
    }

    // These test are snatched from std as well.
    #[cfg(test)]
    mod tests {
        use std::panic;
        use std::{sync::mpsc::channel, thread};

        use super::OnceCell;

        impl<T> OnceCell<T> {
            fn init(&self, f: impl FnOnce() -> T) {
                enum Void {}
                let _ = self.initialize(|| Ok::<T, Void>(f()));
            }
        }

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

        #[test]
        #[cfg(not(miri))]
        fn stampede_once() {
            static O: OnceCell<()> = OnceCell::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.init(|| {
                            assert!(!RUN);
                            RUN = true;
                        });
                        assert!(RUN);
                    }
                    tx.send(()).unwrap();
                });
            }

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

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

        #[test]
        fn poison_bad() {
            static O: OnceCell<()> = OnceCell::new();

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

            // we can subvert poisoning, however
            let mut called = false;
            O.init(|| {
                called = true;
            });
            assert!(called);

            // once any success happens, we stop propagating the poison
            O.init(|| {});
        }

        #[test]
        fn wait_for_force_to_finish() {
            static O: OnceCell<()> = OnceCell::new();

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

            // make sure someone's waiting inside the once via a force
            let (tx1, rx1) = channel();
            let (tx2, rx2) = channel();
            let t1 = thread::spawn(move || {
                O.init(|| {
                    tx1.send(()).unwrap();
                    rx2.recv().unwrap();
                });
            });

            rx1.recv().unwrap();

            // put another waiter on the once
            let t2 = thread::spawn(|| {
                let mut called = false;
                O.init(|| {
                    called = true;
                });
                assert!(!called);
            });

            tx2.send(()).unwrap();

            assert!(t1.join().is_ok());
            assert!(t2.join().is_ok());
        }

        #[test]
        #[cfg(target_pointer_width = "64")]
        fn test_size() {
            use std::mem::size_of;

            assert_eq!(size_of::<OnceCell<u32>>(), 4 * size_of::<u32>());
        }
    }