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#![deny(warnings, missing_debug_implementations, missing_docs, rust_2018_idioms)]
use std::future::Future;
use std::mem::ManuallyDrop;
use std::ops::Deref;
use std::pin::Pin;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::SeqCst;
use std::task::{Context, Poll};
use futures_core::ready;
use event_listener::{Event, EventListener};
#[cfg(test)]
mod tests;
#[derive(Debug)]
pub struct AccessQueue<T> {
count: AtomicUsize,
event: Event,
inner: T,
}
impl<T> AccessQueue<T> {
pub fn new(inner: T, count: usize) -> AccessQueue<T> {
AccessQueue {
count: AtomicUsize::new(count),
event: Event::new(),
inner,
}
}
pub fn block(&self, amt: usize) -> bool {
let mut current = self.count.load(SeqCst);
while current >= amt {
match self.count.compare_exchange_weak(current, current - amt, SeqCst, SeqCst) {
Ok(_) => return true,
Err(n) => current = n,
}
}
false
}
pub fn release(&self, amt: usize) {
self.count.fetch_add(amt, SeqCst);
self.event.notify_additional(amt);
}
pub fn access(&self) -> Access<'_, T> {
Access {
queue: self,
listener: None,
}
}
pub fn skip_queue(&self) -> &T {
&self.inner
}
pub fn get_mut(&mut self) -> &mut T {
&mut self.inner
}
}
#[derive(Debug)]
pub struct Access<'a, T> {
queue: &'a AccessQueue<T>,
listener: Option<EventListener>,
}
impl<'a, T> Access<'a, T> {
pub fn skip_queue(&self) -> &T {
self.queue.skip_queue()
}
}
impl<'a, T> Future for Access<'a, T> {
type Output = AccessGuard<'a, T>;
fn poll(mut self: Pin<&mut Self>, ctx: &mut Context<'_>) -> Poll<Self::Output> {
if let Some(listener) = &mut self.listener {
ready!(Pin::new(listener).poll(ctx));
self.listener = None;
}
while !self.queue.block(1) {
match &mut self.listener {
Some(listener) => {
ready!(Pin::new(listener).poll(ctx));
self.listener = None;
}
None => {
let mut listener = self.queue.event.listen();
if let Poll::Pending = Pin::new(&mut listener).poll(ctx) {
self.listener = Some(listener);
return Poll::Pending
}
}
}
}
Poll::Ready(AccessGuard { queue: self.queue })
}
}
#[derive(Debug)]
pub struct AccessGuard<'a, T> {
queue: &'a AccessQueue<T>,
}
impl<'a, T> AccessGuard<'a, T> {
#[inline]
pub fn hold_indefinitely(self) -> &'a T {
ManuallyDrop::new(self).queue.skip_queue()
}
#[inline]
pub fn reenqueue(self) -> Access<'a, T> {
self.queue.release(1);
self.hold_and_reenqueue().1
}
#[inline]
pub fn hold_and_reenqueue(self) -> (&'a T, Access<'a, T>) {
let this = ManuallyDrop::new(self);
(this.queue.skip_queue(), this.queue.access())
}
}
impl<'a, T> Deref for AccessGuard<'a, T> {
type Target = T;
fn deref(&self) -> &T {
self.queue.skip_queue()
}
}
impl<'a, T> Drop for AccessGuard<'a, T> {
fn drop(&mut self) {
self.queue.release(1);
}
}
#[allow(dead_code)]
fn is_send_sync() where AccessQueue<()>: Send + Sync { }