conquer-once 0.4.0

//! Synchronized one-time and lazy initialization primitives that use spin-locks
//! in case of concurrent accesses under contention.

use core::{hint, sync::atomic::Ordering};

use crate::{
    cell::{Block, Unblock},
    state::{AtomicOnceState, BlockedState, OnceState::WouldBlock},
    POISON_PANIC_MSG,
};

use self::internal::Spin;

/// A type for lazy initialization of e.g. global static variables, which
/// provides the same functionality as the `lazy_static!` macro.
///
/// This type uses spin-locks if the initialization is contended and is thus
/// `#[no_std]` compatible.
///
/// For the API of this type alias, see the API of the generic
/// [`Lazy`](crate::doc::Lazy) type.
pub type Lazy<T, F = fn() -> T> = crate::lazy::Lazy<T, Spin, F>;

/// An interior mutability cell type which allows synchronized one-time
/// initialization and read-only access exclusively after initialization.
///
/// This type uses spin-locks if the initialization is contended and is thus
/// `#[no_std]` compatible.
///
/// For the API of this type alias, see the generic
/// [`OnceCell`](crate::doc::OnceCell) type.
pub type OnceCell<T> = crate::cell::OnceCell<T, Spin>;

/// A synchronization primitive which can be used to run a one-time global
/// initialization.
///
/// This type uses spin-locks if the initialization is contended and is thus
/// `#[no_std]` compatible.
///
/// For the API of this type alias, see the generic
/// [`OnceCell`](crate::doc::OnceCell) type.
/// This is a specialization with `T = ()`.
pub type Once = OnceCell<()>;

mod internal {
    /// Blocking strategy for blocking threads using spin-locks.
    #[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
    pub struct Spin;
}

impl Unblock for Spin {
    #[inline(always)]
    unsafe fn on_unblock(_: BlockedState) {}
}

unsafe impl Block for Spin {
    /// Spins until the [`OnceCell`] state is set to `READY`, or panics if it
    /// becomes poisoned.
    #[inline]
    fn block(state: &AtomicOnceState) {
        // (spin:1) this acquire load syncs-with the acq-rel swap (guard:2)
        while let WouldBlock(_) = state.load(Ordering::Acquire).expect(POISON_PANIC_MSG) {
            hint::spin_loop();
        }
    }
}

#[cfg(test)]
mod tests {
    generate_tests_non_blocking!();
    generate_tests!();
}