Struct futures::task::AtomicWaker

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pub struct AtomicWaker { /* private fields */ }
Expand description

A synchronization primitive for task wakeup.

Sometimes the task interested in a given event will change over time. An AtomicWaker can coordinate concurrent notifications with the consumer potentially “updating” the underlying task to wake up. This is useful in scenarios where a computation completes in another thread and wants to notify the consumer, but the consumer is in the process of being migrated to a new logical task.

Consumers should call register before checking the result of a computation and producers should call wake after producing the computation (this differs from the usual thread::park pattern). It is also permitted for wake to be called before register. This results in a no-op.

A single AtomicWaker may be reused for any number of calls to register or wake.

Memory ordering

Calling register “acquires” all memory “released” by calls to wake before the call to register. Later calls to wake will wake the registered waker (on contention this wake might be triggered in register).

For concurrent calls to register (should be avoided) the ordering is only guaranteed for the winning call.

Examples

Here is a simple example providing a Flag that can be signalled manually when it is ready.

use futures::future::Future;
use futures::task::{Context, Poll, AtomicWaker};
use std::sync::Arc;
use std::sync::atomic::AtomicBool;
use std::sync::atomic::Ordering::Relaxed;
use std::pin::Pin;

struct Inner {
    waker: AtomicWaker,
    set: AtomicBool,
}

#[derive(Clone)]
struct Flag(Arc<Inner>);

impl Flag {
    pub fn new() -> Self {
        Self(Arc::new(Inner {
            waker: AtomicWaker::new(),
            set: AtomicBool::new(false),
        }))
    }

    pub fn signal(&self) {
        self.0.set.store(true, Relaxed);
        self.0.waker.wake();
    }
}

impl Future for Flag {
    type Output = ();

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        // quick check to avoid registration if already done.
        if self.0.set.load(Relaxed) {
            return Poll::Ready(());
        }

        self.0.waker.register(cx.waker());

        // Need to check condition **after** `register` to avoid a race
        // condition that would result in lost notifications.
        if self.0.set.load(Relaxed) {
            Poll::Ready(())
        } else {
            Poll::Pending
        }
    }
}

Implementations§

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impl AtomicWaker

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pub const fn new() -> AtomicWaker

Create an AtomicWaker.

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pub fn register(&self, waker: &Waker)

Registers the waker to be notified on calls to wake.

The new task will take place of any previous tasks that were registered by previous calls to register. Any calls to wake that happen after a call to register (as defined by the memory ordering rules), will notify the register caller’s task and deregister the waker from future notifications. Because of this, callers should ensure register gets invoked with a new Waker each time they require a wakeup.

It is safe to call register with multiple other threads concurrently calling wake. This will result in the register caller’s current task being notified once.

This function is safe to call concurrently, but this is generally a bad idea. Concurrent calls to register will attempt to register different tasks to be notified. One of the callers will win and have its task set, but there is no guarantee as to which caller will succeed.

Examples

Here is how register is used when implementing a flag.

use futures::future::Future;
use futures::task::{Context, Poll, AtomicWaker};
use std::sync::atomic::AtomicBool;
use std::sync::atomic::Ordering::Relaxed;
use std::pin::Pin;

struct Flag {
    waker: AtomicWaker,
    set: AtomicBool,
}

impl Future for Flag {
    type Output = ();

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        // Register **before** checking `set` to avoid a race condition
        // that would result in lost notifications.
        self.waker.register(cx.waker());

        if self.set.load(Relaxed) {
            Poll::Ready(())
        } else {
            Poll::Pending
        }
    }
}
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pub fn wake(&self)

Calls wake on the last Waker passed to register.

If register has not been called yet, then this does nothing.

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pub fn take(&self) -> Option<Waker>

Returns the last Waker passed to register, so that the user can wake it.

Sometimes, just waking the AtomicWaker is not fine grained enough. This allows the user to take the waker and then wake it separately, rather than performing both steps in one atomic action.

If a waker has not been registered, this returns None.

Trait Implementations§

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impl Debug for AtomicWaker

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

Formats the value using the given formatter. Read more
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impl Default for AtomicWaker

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fn default() -> AtomicWaker

Returns the “default value” for a type. Read more
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impl Send for AtomicWaker

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impl Sync for AtomicWaker

Auto Trait Implementations§

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impl<T> Any for Twhere T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for Twhere T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for Twhere U: From<T>,

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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.