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//! ReverseSemaphore allow waiting for all permits to be released.
use parking_lot::RwLock;
use std::future::Future;
use std::sync::Arc;
use tokio::sync::Notify;
/// When this and all other permits are dropped,
/// futures awaiting `wait_on_zero_permits` will resolve.
pub struct ReverseSemaphorePermit(Arc<RwLock<Inner>>);
/// ReverseSemaphore allow waiting for all permits to be released.
#[derive(Clone)]
pub struct ReverseSemaphore(Arc<RwLock<Inner>>);
impl Default for ReverseSemaphore {
fn default() -> Self {
Self::new()
}
}
impl ReverseSemaphore {
/// Construct a new ReverseSemaphore
pub fn new() -> Self {
Self(Arc::new(RwLock::new(Inner {
n: Arc::new(Notify::new()),
c: 0,
})))
}
/// Acquire a ReverseSemaphorePermit
pub fn acquire(&self) -> ReverseSemaphorePermit {
ReverseSemaphorePermit::new(self.0.clone())
}
/// If no permits are outstanding this future will resolve immediately.
/// If there are permits outstanding, this future will only resolve
/// when all outstanding permits are released.
pub fn wait_on_zero_permits(&self) -> impl Future<Output = ()> + 'static + Send {
let inner = self.0.clone();
async move {
let n;
let fut;
{
let lock = inner.read();
if lock.c == 0 {
// if there are already zero permits, no need to wait
return;
}
n = lock.n.clone();
// make sure to capture this notified before dropping the lock
fut = n.notified();
// make sure to drop the lock before awaiting the notified fut
drop(lock);
}
fut.await;
}
}
}
// -- private -- //
struct Inner {
n: Arc<Notify>,
c: usize,
}
impl ReverseSemaphorePermit {
fn new(inner: Arc<RwLock<Inner>>) -> Self {
inner.write().c += 1;
Self(inner)
}
}
impl Drop for ReverseSemaphorePermit {
fn drop(&mut self) {
let mut lock = self.0.write();
lock.c -= 1;
if lock.c == 0 {
// alas, we need to maintain the lock while we notify,
// otherwise a new permit could be acquired and be notified
// erroneously.
lock.n.notify_waiters();
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[tokio::test(flavor = "multi_thread")]
#[ignore = "flaky due to dependence on sensitive timing"]
async fn test_reverse_semaphore() {
let rs = ReverseSemaphore::new();
let s = tokio::time::Instant::now();
rs.wait_on_zero_permits().await;
let es = s.elapsed().as_secs_f64();
assert!(es < 0.00015);
println!("zero wait, after {} s", es);
let s = tokio::time::Instant::now();
for t in 10..15 {
let permit = rs.acquire();
tokio::task::spawn(async move {
let _permit = permit;
tokio::time::sleep(std::time::Duration::from_millis(t)).await;
});
}
rs.wait_on_zero_permits().await;
let es = s.elapsed().as_secs_f64();
assert!(es > 0.00015);
println!("permit wait, after {} s", es);
}
}