yield_now.rs

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

/// Yields execution back to the Tokio runtime.
///
/// A task yields by awaiting on `yield_now()`, and may resume when that future
/// completes (with no output.) The current task will be re-added as a pending
/// task at the _back_ of the pending queue. Any other pending tasks will be
/// scheduled. No other waking is required for the task to continue.
///
/// See also the usage example in the [task module](index.html#yield_now).
///
/// ## Non-guarantees
///
/// This function may not yield all the way up to the executor if there are any
/// special combinators above it in the call stack. For example, if a
/// [`tokio::select!`] has another branch complete during the same poll as the
/// `yield_now()`, then the yield is not propagated all the way up to the
/// runtime.
///
/// It is generally not guaranteed that the runtime behaves like you expect it
/// to when deciding which task to schedule next after a call to `yield_now()`.
/// In particular, the runtime may choose to poll the task that just ran
/// `yield_now()` again immediately without polling any other tasks first. For
/// example, the runtime will not drive the IO driver between every poll of a
/// task, and this could result in the runtime polling the current task again
/// immediately even if there is another task that could make progress if that
/// other task is waiting for a notification from the IO driver.
///
/// In general, changes to the order in which the runtime polls tasks is not
/// considered a breaking change, and your program should be correct no matter
/// which order the runtime polls your tasks in.
///
/// [`tokio::select!`]: macro@crate::select
#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
pub async fn yield_now() {
    /// Yield implementation
    struct YieldNow {
        yielded: bool,
    }

    impl Future for YieldNow {
        type Output = ();

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
            if self.yielded {
                return Poll::Ready(());
            }

            self.yielded = true;
            cx.waker().wake_by_ref();
            Poll::Pending
        }
    }

    YieldNow { yielded: false }.await
}

/// Yields execution back to the Tokio runtime *if* the task went out ouf budget.
///
/// The task will only yield if it ran out of its coop budget.
/// It can be used in order to insert optional yield points into long
/// computations that do not use Tokio resources like sockets or semaphores,
/// without redundantly yielding to runtime each time.
///
/// See also the usage example in the [task module](index.html#yield_now).
#[must_use = "maybe_yield_now does nothing unless polled/`await`-ed"]
pub async fn maybe_yield_now() {
    struct MaybeYieldNow {
        status: Poll<()>,
    }

    impl Future for MaybeYieldNow {
        type Output = ();

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
            if self.status.is_ready() {
                return self.status;
            }
            self.status = match crate::coop::poll_proceed(cx) {
                Poll::Ready(restore) => {
                    restore.made_progress();
                    Poll::Ready(())
                },
                Poll::Pending => {
                    cx.waker().wake_by_ref();
                    Poll::Pending
                },
            };
            self.status
        }
    }

    MaybeYieldNow { status: Poll::Pending }.await
}