use std::fmt::{self, Debug};
use std::marker::PhantomData;
use std::sync::atomic::{AtomicBool, Ordering};
use once_cell::sync::Lazy;
use thread_local::ThreadLocal;
use self::sealed::Sealed;
mod sealed {
use super::ThreadKey;
pub trait Sealed {}
impl Sealed for ThreadKey {}
impl Sealed for &mut ThreadKey {}
}
static KEY: Lazy<ThreadLocal<AtomicLock>> = Lazy::new(ThreadLocal::new);
/// The key for the current thread.
///
/// Only one of these exist per thread. To get the current thread's key, call
/// [`ThreadKey::lock`]. If the `ThreadKey` is dropped, it can be reobtained.
pub type ThreadKey = Key<'static>;
pub struct Key<'a> {
phantom: PhantomData<*const ()>, // implement !Send and !Sync
lock: &'a AtomicLock,
}
/// Allows the type to be used as a key for a lock
///
/// # Safety
///
/// Only one value which implements this trait may be allowed to exist at a
/// time. Creating a new `Keyable` value requires making any other `Keyable`
/// values invalid.
pub unsafe trait Keyable: Sealed {}
unsafe impl Keyable for ThreadKey {}
unsafe impl Keyable for &mut ThreadKey {}
impl Debug for ThreadKey {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "ThreadKey")
}
}
impl<'a> Drop for Key<'a> {
fn drop(&mut self) {
unsafe { self.lock.force_unlock() }
}
}
impl ThreadKey {
/// Get the current thread's `ThreadKey`, if it's not already taken.
///
/// The first time this is called, it will successfully return a
/// `ThreadKey`. However, future calls to this function will return
/// [`None`], unless the key is dropped or unlocked first.
#[must_use]
pub fn lock() -> Option<Self> {
KEY.get_or_default().try_lock()
}
/// Unlocks the `ThreadKey`.
///
/// After this method is called, a call to [`ThreadKey::lock`] will return
/// this `ThreadKey`.
pub fn unlock(key: Self) {
drop(key);
}
}
/// A dumb lock that's just a wrapper for an [`AtomicBool`].
#[derive(Debug, Default)]
struct AtomicLock {
is_locked: AtomicBool,
}
impl AtomicLock {
/// Attempt to lock the `AtomicLock`.
///
/// If the lock is already locked, then this'll return false. If it is
/// unlocked, it'll return true. If the lock is currently unlocked, then it
/// will be locked after this function is called.
///
/// This is not a fair lock. It is not recommended to call this function
/// repeatedly in a loop.
pub fn try_lock(&self) -> Option<Key> {
// safety: we just acquired the lock
(!self.is_locked.swap(true, Ordering::Acquire)).then_some(Key {
phantom: PhantomData,
lock: self,
})
}
/// Forcibly unlocks the `AtomicLock`.
///
/// # Safety
///
/// This should only be called if the key to the lock has been "lost". That
/// means the program no longer has a reference to the key.
pub unsafe fn force_unlock(&self) {
self.is_locked.store(false, Ordering::Release);
}
}
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