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use super::batch_semaphore as ll; // low level implementation
use super::{AcquireError, TryAcquireError};
#[cfg(all(tokio_unstable, feature = "tracing"))]
use crate::util::trace;
use std::sync::Arc;
/// Counting semaphore performing asynchronous permit acquisition.
///
/// 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.
///
/// This `Semaphore` is fair, which means that permits are given out in the order
/// they were requested. This fairness is also applied when `acquire_many` gets
/// involved, so if a call to `acquire_many` at the front of the queue requests
/// more permits than currently available, this can prevent a call to `acquire`
/// from completing, even if the semaphore has enough permits complete the call
/// to `acquire`.
///
/// To use the `Semaphore` in a poll function, you can use the [`PollSemaphore`]
/// utility.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use tokio::sync::{Semaphore, TryAcquireError};
///
/// #[tokio::main]
/// async fn main() {
/// let semaphore = Semaphore::new(3);
///
/// let a_permit = semaphore.acquire().await.unwrap();
/// let two_permits = semaphore.acquire_many(2).await.unwrap();
///
/// assert_eq!(semaphore.available_permits(), 0);
///
/// let permit_attempt = semaphore.try_acquire();
/// assert_eq!(permit_attempt.err(), Some(TryAcquireError::NoPermits));
/// }
/// ```
///
/// Use [`Semaphore::acquire_owned`] to move permits across tasks:
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::Semaphore;
///
/// #[tokio::main]
/// async fn main() {
/// let semaphore = Arc::new(Semaphore::new(3));
/// let mut join_handles = Vec::new();
///
/// for _ in 0..5 {
/// let permit = semaphore.clone().acquire_owned().await.unwrap();
/// join_handles.push(tokio::spawn(async move {
/// // perform task...
/// // explicitly own `permit` in the task
/// drop(permit);
/// }));
/// }
///
/// for handle in join_handles {
/// handle.await.unwrap();
/// }
/// }
/// ```
///
/// [`Semaphore::acquire_owned`]: crate::sync::Semaphore::acquire_owned
#[derive(Debug)]
pub struct Semaphore {
/// The low level semaphore
ll_sem: ll::Semaphore,
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: tracing::Span,
}
/// A permit from the semaphore.
///
/// This type is created by the [`acquire`] method.
///
/// [`acquire`]: crate::sync::Semaphore::acquire()
#[must_use]
#[clippy::has_significant_drop]
#[derive(Debug)]
pub struct SemaphorePermit<'a> {
sem: &'a Semaphore,
permits: u32,
}
/// An owned permit from the semaphore.
///
/// This type is created by the [`acquire_owned`] method.
///
/// [`acquire_owned`]: crate::sync::Semaphore::acquire_owned()
#[must_use]
#[clippy::has_significant_drop]
#[derive(Debug)]
pub struct OwnedSemaphorePermit {
sem: Arc<Semaphore>,
permits: u32,
}
#[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 {
/// The maximum number of permits which a semaphore can hold. It is `usize::MAX >> 3`.
///
/// Exceeding this limit typically results in a panic.
pub const MAX_PERMITS: usize = super::batch_semaphore::Semaphore::MAX_PERMITS;
/// Creates a new semaphore with the initial number of permits.
///
/// Panics if `permits` exceeds [`Semaphore::MAX_PERMITS`].
#[track_caller]
pub fn new(permits: usize) -> Self {
#[cfg(all(tokio_unstable, feature = "tracing"))]
let resource_span = {
let location = std::panic::Location::caller();
tracing::trace_span!(
"runtime.resource",
concrete_type = "Semaphore",
kind = "Sync",
loc.file = location.file(),
loc.line = location.line(),
loc.col = location.column(),
inherits_child_attrs = true,
)
};
#[cfg(all(tokio_unstable, feature = "tracing"))]
let ll_sem = resource_span.in_scope(|| ll::Semaphore::new(permits));
#[cfg(any(not(tokio_unstable), not(feature = "tracing")))]
let ll_sem = ll::Semaphore::new(permits);
Self {
ll_sem,
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span,
}
}
/// Creates a new semaphore with the initial number of permits.
///
/// # Examples
///
/// ```
/// use tokio::sync::Semaphore;
///
/// static SEM: Semaphore = Semaphore::const_new(10);
/// ```
///
#[cfg(all(feature = "parking_lot", not(all(loom, test))))]
#[cfg_attr(docsrs, doc(cfg(feature = "parking_lot")))]
pub const fn const_new(permits: usize) -> Self {
#[cfg(all(tokio_unstable, feature = "tracing"))]
return Self {
ll_sem: ll::Semaphore::const_new(permits),
resource_span: tracing::Span::none(),
};
#[cfg(any(not(tokio_unstable), not(feature = "tracing")))]
return Self {
ll_sem: ll::Semaphore::const_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 [`Semaphore::MAX_PERMITS`], and this function will panic if the limit is exceeded.
pub fn add_permits(&self, n: usize) {
self.ll_sem.release(n);
}
/// Acquires a permit from the semaphore.
///
/// If the semaphore has been closed, this returns an [`AcquireError`].
/// Otherwise, this returns a [`SemaphorePermit`] representing the
/// acquired permit.
///
/// # Cancel safety
///
/// This method uses a queue to fairly distribute permits in the order they
/// were requested. Cancelling a call to `acquire` makes you lose your place
/// in the queue.
///
/// # Examples
///
/// ```
/// use tokio::sync::Semaphore;
///
/// #[tokio::main]
/// async fn main() {
/// let semaphore = Semaphore::new(2);
///
/// let permit_1 = semaphore.acquire().await.unwrap();
/// assert_eq!(semaphore.available_permits(), 1);
///
/// let permit_2 = semaphore.acquire().await.unwrap();
/// assert_eq!(semaphore.available_permits(), 0);
///
/// drop(permit_1);
/// assert_eq!(semaphore.available_permits(), 1);
/// }
/// ```
///
/// [`AcquireError`]: crate::sync::AcquireError
/// [`SemaphorePermit`]: crate::sync::SemaphorePermit
pub async fn acquire(&self) -> Result<SemaphorePermit<'_>, AcquireError> {
#[cfg(all(tokio_unstable, feature = "tracing"))]
let inner = trace::async_op(
|| self.ll_sem.acquire(1),
self.resource_span.clone(),
"Semaphore::acquire",
"poll",
true,
);
#[cfg(not(all(tokio_unstable, feature = "tracing")))]
let inner = self.ll_sem.acquire(1);
inner.await?;
Ok(SemaphorePermit {
sem: self,
permits: 1,
})
}
/// Acquires `n` permits from the semaphore.
///
/// If the semaphore has been closed, this returns an [`AcquireError`].
/// Otherwise, this returns a [`SemaphorePermit`] representing the
/// acquired permits.
///
/// # Cancel safety
///
/// This method uses a queue to fairly distribute permits in the order they
/// were requested. Cancelling a call to `acquire_many` makes you lose your
/// place in the queue.
///
/// # Examples
///
/// ```
/// use tokio::sync::Semaphore;
///
/// #[tokio::main]
/// async fn main() {
/// let semaphore = Semaphore::new(5);
///
/// let permit = semaphore.acquire_many(3).await.unwrap();
/// assert_eq!(semaphore.available_permits(), 2);
/// }
/// ```
///
/// [`AcquireError`]: crate::sync::AcquireError
/// [`SemaphorePermit`]: crate::sync::SemaphorePermit
pub async fn acquire_many(&self, n: u32) -> Result<SemaphorePermit<'_>, AcquireError> {
#[cfg(all(tokio_unstable, feature = "tracing"))]
trace::async_op(
|| self.ll_sem.acquire(n),
self.resource_span.clone(),
"Semaphore::acquire_many",
"poll",
true,
)
.await?;
#[cfg(not(all(tokio_unstable, feature = "tracing")))]
self.ll_sem.acquire(n).await?;
Ok(SemaphorePermit {
sem: self,
permits: n,
})
}
/// Tries to acquire a permit from the semaphore.
///
/// If the semaphore has been closed, this returns a [`TryAcquireError::Closed`]
/// and a [`TryAcquireError::NoPermits`] if there are no permits left. Otherwise,
/// this returns a [`SemaphorePermit`] representing the acquired permits.
///
/// # Examples
///
/// ```
/// use tokio::sync::{Semaphore, TryAcquireError};
///
/// # fn main() {
/// let semaphore = Semaphore::new(2);
///
/// let permit_1 = semaphore.try_acquire().unwrap();
/// assert_eq!(semaphore.available_permits(), 1);
///
/// let permit_2 = semaphore.try_acquire().unwrap();
/// assert_eq!(semaphore.available_permits(), 0);
///
/// let permit_3 = semaphore.try_acquire();
/// assert_eq!(permit_3.err(), Some(TryAcquireError::NoPermits));
/// # }
/// ```
///
/// [`TryAcquireError::Closed`]: crate::sync::TryAcquireError::Closed
/// [`TryAcquireError::NoPermits`]: crate::sync::TryAcquireError::NoPermits
/// [`SemaphorePermit`]: crate::sync::SemaphorePermit
pub fn try_acquire(&self) -> Result<SemaphorePermit<'_>, TryAcquireError> {
match self.ll_sem.try_acquire(1) {
Ok(_) => Ok(SemaphorePermit {
sem: self,
permits: 1,
}),
Err(e) => Err(e),
}
}
/// Tries to acquire `n` permits from the semaphore.
///
/// If the semaphore has been closed, this returns a [`TryAcquireError::Closed`]
/// and a [`TryAcquireError::NoPermits`] if there are not enough permits left.
/// Otherwise, this returns a [`SemaphorePermit`] representing the acquired permits.
///
/// # Examples
///
/// ```
/// use tokio::sync::{Semaphore, TryAcquireError};
///
/// # fn main() {
/// let semaphore = Semaphore::new(4);
///
/// let permit_1 = semaphore.try_acquire_many(3).unwrap();
/// assert_eq!(semaphore.available_permits(), 1);
///
/// let permit_2 = semaphore.try_acquire_many(2);
/// assert_eq!(permit_2.err(), Some(TryAcquireError::NoPermits));
/// # }
/// ```
///
/// [`TryAcquireError::Closed`]: crate::sync::TryAcquireError::Closed
/// [`TryAcquireError::NoPermits`]: crate::sync::TryAcquireError::NoPermits
/// [`SemaphorePermit`]: crate::sync::SemaphorePermit
pub fn try_acquire_many(&self, n: u32) -> Result<SemaphorePermit<'_>, TryAcquireError> {
match self.ll_sem.try_acquire(n) {
Ok(_) => Ok(SemaphorePermit {
sem: self,
permits: n,
}),
Err(e) => Err(e),
}
}
/// Acquires a permit from the semaphore.
///
/// The semaphore must be wrapped in an [`Arc`] to call this method.
/// If the semaphore has been closed, this returns an [`AcquireError`].
/// Otherwise, this returns a [`OwnedSemaphorePermit`] representing the
/// acquired permit.
///
/// # Cancel safety
///
/// This method uses a queue to fairly distribute permits in the order they
/// were requested. Cancelling a call to `acquire_owned` makes you lose your
/// place in the queue.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::Semaphore;
///
/// #[tokio::main]
/// async fn main() {
/// let semaphore = Arc::new(Semaphore::new(3));
/// let mut join_handles = Vec::new();
///
/// for _ in 0..5 {
/// let permit = semaphore.clone().acquire_owned().await.unwrap();
/// join_handles.push(tokio::spawn(async move {
/// // perform task...
/// // explicitly own `permit` in the task
/// drop(permit);
/// }));
/// }
///
/// for handle in join_handles {
/// handle.await.unwrap();
/// }
/// }
/// ```
///
/// [`Arc`]: std::sync::Arc
/// [`AcquireError`]: crate::sync::AcquireError
/// [`OwnedSemaphorePermit`]: crate::sync::OwnedSemaphorePermit
pub async fn acquire_owned(self: Arc<Self>) -> Result<OwnedSemaphorePermit, AcquireError> {
#[cfg(all(tokio_unstable, feature = "tracing"))]
let inner = trace::async_op(
|| self.ll_sem.acquire(1),
self.resource_span.clone(),
"Semaphore::acquire_owned",
"poll",
true,
);
#[cfg(not(all(tokio_unstable, feature = "tracing")))]
let inner = self.ll_sem.acquire(1);
inner.await?;
Ok(OwnedSemaphorePermit {
sem: self,
permits: 1,
})
}
/// Acquires `n` permits from the semaphore.
///
/// The semaphore must be wrapped in an [`Arc`] to call this method.
/// If the semaphore has been closed, this returns an [`AcquireError`].
/// Otherwise, this returns a [`OwnedSemaphorePermit`] representing the
/// acquired permit.
///
/// # Cancel safety
///
/// This method uses a queue to fairly distribute permits in the order they
/// were requested. Cancelling a call to `acquire_many_owned` makes you lose
/// your place in the queue.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::Semaphore;
///
/// #[tokio::main]
/// async fn main() {
/// let semaphore = Arc::new(Semaphore::new(10));
/// let mut join_handles = Vec::new();
///
/// for _ in 0..5 {
/// let permit = semaphore.clone().acquire_many_owned(2).await.unwrap();
/// join_handles.push(tokio::spawn(async move {
/// // perform task...
/// // explicitly own `permit` in the task
/// drop(permit);
/// }));
/// }
///
/// for handle in join_handles {
/// handle.await.unwrap();
/// }
/// }
/// ```
///
/// [`Arc`]: std::sync::Arc
/// [`AcquireError`]: crate::sync::AcquireError
/// [`OwnedSemaphorePermit`]: crate::sync::OwnedSemaphorePermit
pub async fn acquire_many_owned(
self: Arc<Self>,
n: u32,
) -> Result<OwnedSemaphorePermit, AcquireError> {
#[cfg(all(tokio_unstable, feature = "tracing"))]
let inner = trace::async_op(
|| self.ll_sem.acquire(n),
self.resource_span.clone(),
"Semaphore::acquire_many_owned",
"poll",
true,
);
#[cfg(not(all(tokio_unstable, feature = "tracing")))]
let inner = self.ll_sem.acquire(n);
inner.await?;
Ok(OwnedSemaphorePermit {
sem: self,
permits: n,
})
}
/// Tries to acquire a permit from the semaphore.
///
/// The semaphore must be wrapped in an [`Arc`] to call this method. If
/// the semaphore has been closed, this returns a [`TryAcquireError::Closed`]
/// and a [`TryAcquireError::NoPermits`] if there are no permits left.
/// Otherwise, this returns a [`OwnedSemaphorePermit`] representing the
/// acquired permit.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::{Semaphore, TryAcquireError};
///
/// # fn main() {
/// let semaphore = Arc::new(Semaphore::new(2));
///
/// let permit_1 = Arc::clone(&semaphore).try_acquire_owned().unwrap();
/// assert_eq!(semaphore.available_permits(), 1);
///
/// let permit_2 = Arc::clone(&semaphore).try_acquire_owned().unwrap();
/// assert_eq!(semaphore.available_permits(), 0);
///
/// let permit_3 = semaphore.try_acquire_owned();
/// assert_eq!(permit_3.err(), Some(TryAcquireError::NoPermits));
/// # }
/// ```
///
/// [`Arc`]: std::sync::Arc
/// [`TryAcquireError::Closed`]: crate::sync::TryAcquireError::Closed
/// [`TryAcquireError::NoPermits`]: crate::sync::TryAcquireError::NoPermits
/// [`OwnedSemaphorePermit`]: crate::sync::OwnedSemaphorePermit
pub fn try_acquire_owned(self: Arc<Self>) -> Result<OwnedSemaphorePermit, TryAcquireError> {
match self.ll_sem.try_acquire(1) {
Ok(_) => Ok(OwnedSemaphorePermit {
sem: self,
permits: 1,
}),
Err(e) => Err(e),
}
}
/// Tries to acquire `n` permits from the semaphore.
///
/// The semaphore must be wrapped in an [`Arc`] to call this method. If
/// the semaphore has been closed, this returns a [`TryAcquireError::Closed`]
/// and a [`TryAcquireError::NoPermits`] if there are no permits left.
/// Otherwise, this returns a [`OwnedSemaphorePermit`] representing the
/// acquired permit.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::{Semaphore, TryAcquireError};
///
/// # fn main() {
/// let semaphore = Arc::new(Semaphore::new(4));
///
/// let permit_1 = Arc::clone(&semaphore).try_acquire_many_owned(3).unwrap();
/// assert_eq!(semaphore.available_permits(), 1);
///
/// let permit_2 = semaphore.try_acquire_many_owned(2);
/// assert_eq!(permit_2.err(), Some(TryAcquireError::NoPermits));
/// # }
/// ```
///
/// [`Arc`]: std::sync::Arc
/// [`TryAcquireError::Closed`]: crate::sync::TryAcquireError::Closed
/// [`TryAcquireError::NoPermits`]: crate::sync::TryAcquireError::NoPermits
/// [`OwnedSemaphorePermit`]: crate::sync::OwnedSemaphorePermit
pub fn try_acquire_many_owned(
self: Arc<Self>,
n: u32,
) -> Result<OwnedSemaphorePermit, TryAcquireError> {
match self.ll_sem.try_acquire(n) {
Ok(_) => Ok(OwnedSemaphorePermit {
sem: self,
permits: n,
}),
Err(e) => Err(e),
}
}
/// Closes the semaphore.
///
/// This prevents the semaphore from issuing new permits and notifies all pending waiters.
///
/// # Examples
///
/// ```
/// use tokio::sync::Semaphore;
/// use std::sync::Arc;
/// use tokio::sync::TryAcquireError;
///
/// #[tokio::main]
/// async fn main() {
/// let semaphore = Arc::new(Semaphore::new(1));
/// let semaphore2 = semaphore.clone();
///
/// tokio::spawn(async move {
/// let permit = semaphore.acquire_many(2).await;
/// assert!(permit.is_err());
/// println!("waiter received error");
/// });
///
/// println!("closing semaphore");
/// semaphore2.close();
///
/// // Cannot obtain more permits
/// assert_eq!(semaphore2.try_acquire().err(), Some(TryAcquireError::Closed))
/// }
/// ```
pub fn close(&self) {
self.ll_sem.close();
}
/// Returns true if the semaphore is closed
pub fn is_closed(&self) -> bool {
self.ll_sem.is_closed()
}
}
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;
}
/// Merge two [`SemaphorePermit`] instances together, consuming `other`
/// without releasing the permits it holds.
///
/// Permits held by both `self` and `other` are released when `self` drops.
///
/// # Panics
///
/// This function panics if permits from different [`Semaphore`] instances
/// are merged.
#[track_caller]
pub fn merge(&mut self, mut other: Self) {
assert!(
std::ptr::eq(self.sem, other.sem),
"merging permits from different semaphore instances"
);
self.permits += other.permits;
other.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;
}
/// Merge two [`OwnedSemaphorePermit`] instances together, consuming `other`
/// without releasing the permits it holds.
///
/// Permits held by both `self` and `other` are released when `self` drops.
///
/// # Panics
///
/// This function panics if permits from different [`Semaphore`] instances
/// are merged.
#[track_caller]
pub fn merge(&mut self, mut other: Self) {
assert!(
Arc::ptr_eq(&self.sem, &other.sem),
"merging permits from different semaphore instances"
);
self.permits += other.permits;
other.permits = 0;
}
/// Returns the [`Semaphore`] from which this permit was acquired.
pub fn semaphore(&self) -> &Arc<Semaphore> {
&self.sem
}
}
impl 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);
}
}