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use std::panic::{RefUnwindSafe, UnwindSafe};
use std::ptr::NonNull;
use super::Epoch;
use super::collectible::DeferredClosure;
use super::collector::Collector;
/// [`Guard`] allows the user to read [`AtomicShared`](super::AtomicShared) and keeps the
/// underlying instance pinned to the thread.
///
/// [`Guard`] internally prevents the global epoch value from passing through the value
/// announced by the current thread, thus keeping reachable instances in the thread from being
/// garbage collected.
#[derive(Debug)]
pub struct Guard {
collector_ptr: NonNull<Collector>,
}
impl Guard {
/// Creates a new [`Guard`].
///
/// # Panics
///
/// The maximum number of [`Guard`] instances in a thread is limited to `u32::MAX`; a
/// thread panics when the number of [`Guard`] instances in the thread exceeds the limit.
///
/// # Examples
///
/// ```
/// use sdd::Guard;
///
/// let guard = Guard::new();
/// ```
#[inline]
#[must_use]
pub fn new() -> Self {
let collector_ptr = Collector::current();
Collector::new_guard(collector_ptr);
Self { collector_ptr }
}
/// Returns the epoch in which the current thread lives.
///
/// This method can be used to check whether a retired memory region is potentially reachable or
/// not. A chunk of memory retired in a witnessed [`Epoch`] can be deallocated after the thread
/// has observed three new epochs. For instance, if the witnessed epoch value is `1` in the
/// current thread where the global epoch value is `2`, and an instance is retired in the same
/// thread, the instance can be dropped when the thread witnesses `0` which is three epochs away
/// from `1`.
///
/// In other words, there can be potential readers of the memory chunk until the current thread
/// witnesses the previous epoch. In the above example, the global epoch can be in `2`
/// while the current thread has only witnessed `1`, and therefore there can a reader of the
/// memory chunk in another thread in epoch `2`. The reader can survive until the global epoch
/// reaches `0`, because the thread being in `2` prevents the global epoch from reaching `0`.
///
/// # Examples
///
/// ```
/// use sdd::{Guard, Owned};
/// use std::sync::atomic::AtomicBool;
/// use std::sync::atomic::Ordering::Relaxed;
///
/// static DROPPED: AtomicBool = AtomicBool::new(false);
///
/// struct D(&'static AtomicBool);
///
/// impl Drop for D {
/// fn drop(&mut self) {
/// self.0.store(true, Relaxed);
/// }
/// }
///
/// let owned = Owned::new(D(&DROPPED));
///
/// let epoch_before = Guard::new().epoch();
///
/// drop(owned);
/// assert!(!DROPPED.load(Relaxed));
///
/// while Guard::new().epoch() == epoch_before {
/// assert!(!DROPPED.load(Relaxed));
/// }
///
/// while Guard::new().epoch() == epoch_before.next() {
/// assert!(!DROPPED.load(Relaxed));
/// }
///
/// while Guard::new().epoch() == epoch_before.next().next() {
/// assert!(!DROPPED.load(Relaxed));
/// }
///
/// assert!(DROPPED.load(Relaxed));
/// assert_eq!(Guard::new().epoch(), epoch_before.next().next().next());
/// ```
#[inline]
#[must_use]
pub fn epoch(&self) -> Epoch {
Collector::current_epoch()
}
/// Returns `true` if the thread local container has garbage.
///
/// # Examples
///
/// ```
/// use sdd::{Guard, Shared};
///
/// let guard = Guard::new();
///
/// assert!(!guard.has_garbage());
///
/// drop(Shared::new(1_usize));
/// assert!(guard.has_garbage());
/// ```
#[inline]
#[must_use]
pub const fn has_garbage(&self) -> bool {
Collector::has_garbage(self.collector_ptr)
}
/// Forces the [`Guard`] to try to start a new epoch when it is dropped.
///
/// # Examples
///
/// ```
/// use sdd::Guard;
///
/// let guard = Guard::new();
///
/// let epoch = guard.epoch();
/// guard.accelerate();
///
/// drop(guard);
///
/// assert_ne!(epoch, Guard::new().epoch());
/// ```
#[inline]
pub const fn accelerate(&self) {
Collector::accelerate(self.collector_ptr);
}
/// Executes the supplied closure at a later point of time.
///
/// It is guaranteed that the closure will be executed after every [`Guard`] at the moment when
/// the method was invoked is dropped, however it is totally non-deterministic when exactly the
/// closure will be executed.
///
/// # Examples
///
/// ```
/// use sdd::Guard;
///
/// let guard = Guard::new();
/// guard.defer_execute(|| println!("deferred"));
/// ```
#[inline]
pub fn defer_execute<F: 'static + FnOnce()>(&self, f: F) {
Collector::collect(
self.collector_ptr,
Box::into_raw(Box::new(DeferredClosure::new(f))),
);
}
}
impl Default for Guard {
#[inline]
fn default() -> Self {
Self::new()
}
}
impl Drop for Guard {
#[inline]
fn drop(&mut self) {
Collector::end_guard(self.collector_ptr);
}
}
impl RefUnwindSafe for Guard {}
impl UnwindSafe for Guard {}