use crate::runtime::context;

use std::future::poll_fn;
use std::task::{ready, 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() {
    let mut yielded = false;
    poll_fn(|cx| {
        ready!(crate::trace::trace_leaf());

        if yielded {
            return Poll::Ready(());
        }

        yielded = true;

        // Don't wake the task immediately, as that would push it right back
        // onto the run queue and it could be polled again before other tasks
        // or the IO/timer driver get a chance to run. Instead, hand the waker
        // to the scheduler, which wakes deferred tasks only after it has run
        // out of ready tasks and polled the driver. When polled from outside
        // a Tokio runtime, the waker is woken immediately.
        context::defer(cx.waker());

        Poll::Pending
    })
    .await
}
