Struct glommio::sync::Semaphore

pub struct Semaphore { /* fields omitted */ }

An implementation of semaphore that doesn’t use helper threads, condition variables, and is friendly to single-threaded execution.

Implementations

impl Semaphore

pub fn new(avail: u64) -> Semaphore

Creates a new semaphore with the specified amount of units

Examples

use glommio::sync::Semaphore;

let _ = Semaphore::new(1);

pub fn available(&self) -> u64

Returns the amount of units currently available in this semaphore

Examples

use glommio::sync::Semaphore;

let sem = Semaphore::new(1);
assert_eq!(sem.available(), 1);

pub async fn acquire_permit(&self, units: u64) -> Result<Permit<'_>, ()>

Suspends until a permit can be acquired with the specified amount of units.

Returns Err() if the semaphore is closed during the wait.

Similar to acquire_static_permit, except that it requires the permit never to be passed to contexts that require a static lifetime. As this is cheaper than acquire_static_permit, it is useful in situations where the permit tracks an a asynchronous operation whose lifetime is simple and well-defined.

Examples

use glommio::{sync::Semaphore, LocalExecutor};

let sem = Semaphore::new(1);

let ex = LocalExecutor::default();
ex.run(async move {
    {
        let permit = sem.acquire_permit(1).await.unwrap();
        // once it is dropped it can be acquired again
        // going out of scope will drop
    }
    let _guard = sem.acquire_permit(1).await.unwrap();
});

pub async fn acquire_static_permit(
    self: &Rc<Self>,
    units: u64
) -> Result<StaticPermit, ()>

Suspends until a permit can be acquired with the specified amount of units.

Returns Err() if the semaphore is closed during the wait.

Similar to acquire_permit, except that it requires the semaphore to be contained in an Rc. This is useful in situations where the permit tracks a long-lived asynchronous operation whose lifetime is complex.

Examples

use glommio::{sync::Semaphore, timer::sleep, Local, LocalExecutor};
use std::{rc::Rc, time::Duration};

let sem = Rc::new(Semaphore::new(1));

let ex = LocalExecutor::default();
ex.run(async move {
    {
        let permit = sem.acquire_static_permit(1).await.unwrap();
        Local::local(async move {
            let _guard = permit;
            sleep(Duration::from_secs(1)).await;
        })
        .detach();
        // once it is dropped it can be acquired again
        // going out of scope will drop
    }
    let _guard = sem.acquire_permit(1).await.unwrap();
});

pub async fn acquire(&self, units: u64) -> Result<(), ()>

Acquires the specified amount of units from this semaphore.

The caller is then responsible to release units. Whenever possible, prefer acquire_permit().

Examples

use glommio::{sync::Semaphore, LocalExecutor};

let sem = Semaphore::new(1);

let ex = LocalExecutor::default();
ex.run(async move {
    sem.acquire(1).await.unwrap();
    sem.signal(1); // Has to be signaled explicitly. Be careful
});

pub fn try_acquire(&self, units: u64) -> Result<bool, ()>

Acquires the given number of units, if they are available, and returns immediately, with the value true, reducing the number of available units by the given amount.

If insufficient units are available then this method will return immediately with the value false and the number of available permits is unchanged.

This method does not suspend.

The caller is then responsible to release units. Whenever possible, prefer try_acquire_permit.

Errors

If semaphore is closed Err(io::Error(BrokenPipe)) will be returned.

Examples

use glommio::{sync::Semaphore, LocalExecutor};

let sem = Semaphore::new(1);

let ex = LocalExecutor::default();

ex.run(async move {
    assert!(sem.try_acquire(1).unwrap());
    sem.signal(1); // Has to be signaled explicitly. Be careful
});

pub fn try_acquire_permit(&self, units: u64) -> Result<Permit<'_>, ()>

Acquires the given number of units, if they are available, and returns immediately, with the RAAI guard, reducing the number of available units by the given amount.

If insufficient units are available then this method will return Err and the number of available permits is unchanged.

This method does not suspend.

Errors

If semaphore is closed Err(GlommioError::Closed(ResourceType::Semaphore { .. })) will be returned. If semaphore does not have sufficient amount of units Err(GlommioError::WouldBlock(ResourceType::Semaphore { requested: u64, available: u64 })) will be returned.

Examples

use glommio::{sync::Semaphore, LocalExecutor};

let sem = Semaphore::new(1);

let ex = LocalExecutor::default();

ex.run(async move {
    let permit = sem.acquire_permit(1).await.unwrap();
    // once it is dropped it can be acquired again
    // going out of scope will drop
});

pub fn signal(&self, units: u64)

Signals the semaphore to release the specified amount of units.

This needs to be paired with a call to acquire(). You should not call this if the units were acquired with acquire_permit().

Examples

use glommio::{sync::Semaphore, LocalExecutor};

let sem = Semaphore::new(0);

let ex = LocalExecutor::default();
ex.run(async move {
    // Note that we can signal to expand to more units than the original capacity had.
    sem.signal(1);
    sem.acquire(1).await.unwrap();
});

pub fn close(&self)

Closes the semaphore

All existing waiters will return Err(), and no new waiters are allowed.

Examples

use glommio::{sync::Semaphore, LocalExecutor};

let sem = Semaphore::new(0);

let ex = LocalExecutor::default();
ex.run(async move {
    // Note that we can signal to expand to more units than the original capacity had.
    sem.close();
    if let Ok(_) = sem.acquire(1).await {
        panic!("a closed semaphore should have errored");
    }
});

Trait Implementations

impl Debug for Semaphore

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Drop for Semaphore

fn drop(&mut self)

Executes the destructor for this type.