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>
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use super::batch_semaphore as ll; // low level implementation use std::sync::Arc; /// Counting semaphore performing asynchronous permit aquisition. /// /// A semaphore maintains a set of permits. Permits are used to synchronize /// access to a shared resource. A semaphore differs from a mutex in that it /// can allow more than one concurrent caller to access the shared resource at a /// time. /// /// When `acquire` is called and the semaphore has remaining permits, the /// function immediately returns a permit. However, if no remaining permits are /// available, `acquire` (asynchronously) waits until an outstanding permit is /// dropped. At this point, the freed permit is assigned to the caller. #[derive(Debug)] pub struct Semaphore { /// The low level semaphore ll_sem: ll::Semaphore, } /// A permit from the semaphore. /// /// This type is created by the [`acquire`] method. /// /// [`acquire`]: crate::sync::Semaphore::acquire() #[must_use] #[derive(Debug)] pub struct SemaphorePermit<'a> { sem: &'a Semaphore, permits: u16, } /// An owned permit from the semaphore. /// /// This type is created by the [`acquire_owned`] method. /// /// [`acquire_owned`]: crate::sync::Semaphore::acquire_owned() #[must_use] #[derive(Debug)] pub struct OwnedSemaphorePermit { sem: Arc<Semaphore>, permits: u16, } /// Error returned from the [`Semaphore::try_acquire`] function. /// /// A `try_acquire` operation can only fail if the semaphore has no available /// permits. /// /// [`Semaphore::try_acquire`]: Semaphore::try_acquire #[derive(Debug)] pub struct TryAcquireError(()); #[test] #[cfg(not(loom))] fn bounds() { fn check_unpin<T: Unpin>() {} // This has to take a value, since the async fn's return type is unnameable. fn check_send_sync_val<T: Send + Sync>(_t: T) {} fn check_send_sync<T: Send + Sync>() {} check_unpin::<Semaphore>(); check_unpin::<SemaphorePermit<'_>>(); check_send_sync::<Semaphore>(); let semaphore = Semaphore::new(0); check_send_sync_val(semaphore.acquire()); } impl Semaphore { /// Creates a new semaphore with the initial number of permits. pub fn new(permits: usize) -> Self { Self { ll_sem: ll::Semaphore::new(permits), } } /// Returns the current number of available permits. pub fn available_permits(&self) -> usize { self.ll_sem.available_permits() } /// Adds `n` new permits to the semaphore. /// /// The maximum number of permits is `usize::MAX >> 3`, and this function will panic if the limit is exceeded. pub fn add_permits(&self, n: usize) { self.ll_sem.release(n); } /// Acquires permit from the semaphore. pub async fn acquire(&self) -> SemaphorePermit<'_> { self.ll_sem.acquire(1).await.unwrap(); SemaphorePermit { sem: &self, permits: 1, } } /// Tries to acquire a permit from the semaphore. pub fn try_acquire(&self) -> Result<SemaphorePermit<'_>, TryAcquireError> { match self.ll_sem.try_acquire(1) { Ok(_) => Ok(SemaphorePermit { sem: self, permits: 1, }), Err(_) => Err(TryAcquireError(())), } } /// Acquires permit from the semaphore. /// /// The semaphore must be wrapped in an [`Arc`] to call this method. /// /// [`Arc`]: std::sync::Arc pub async fn acquire_owned(self: Arc<Self>) -> OwnedSemaphorePermit { self.ll_sem.acquire(1).await.unwrap(); OwnedSemaphorePermit { sem: self.clone(), permits: 1, } } /// Tries to acquire a permit from the semaphore. /// /// The semaphore must be wrapped in an [`Arc`] to call this method. /// /// [`Arc`]: std::sync::Arc pub fn try_acquire_owned(self: Arc<Self>) -> Result<OwnedSemaphorePermit, TryAcquireError> { match self.ll_sem.try_acquire(1) { Ok(_) => Ok(OwnedSemaphorePermit { sem: self.clone(), permits: 1, }), Err(_) => Err(TryAcquireError(())), } } } impl<'a> SemaphorePermit<'a> { /// Forgets the permit **without** releasing it back to the semaphore. /// This can be used to reduce the amount of permits available from a /// semaphore. pub fn forget(mut self) { self.permits = 0; } } impl OwnedSemaphorePermit { /// Forgets the permit **without** releasing it back to the semaphore. /// This can be used to reduce the amount of permits available from a /// semaphore. pub fn forget(mut self) { self.permits = 0; } } impl<'a> Drop for SemaphorePermit<'_> { fn drop(&mut self) { self.sem.add_permits(self.permits as usize); } } impl Drop for OwnedSemaphorePermit { fn drop(&mut self) { self.sem.add_permits(self.permits as usize); } }