bad_semaphore.rs

/// A basic semaphore that only allows a fixed number of
/// outstanding permits.
pub struct Semaphore {
    remaining_permits: Arc<AtomicU64>,
}

impl Semaphore {
    pub fn new(permits: u64) -> Self {
        Self {
            remaining_permits: Arc::new(AtomicU64::new(permits)),
        }
    }

    /// Acquire a `Permit`, or return `None` if there are no available
    /// permits.
    pub fn try_acquire(&self) -> Option<Permit> {
        // If we still have remaining permits, subtract one
        // and vend a `Permit`.
        if self.remaining_permits.load(Ordering::SeqCst) > 0 {
            self.remaining_permits.fetch_sub(1, Ordering::SeqCst);
            Some(Permit(Arc::clone(&self.remaining_permits)))
        } else {
            None
        }
    }
}

pub struct Permit(Arc<AtomicU64>);

/// Increment the remaining permits count on `drop`
impl Drop for Permit {
    fn drop(&mut self) {
        self.0.fetch_add(1, Ordering::SeqCst);
    }
}