1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103

//! Asynchronous values.

use core::cell::Cell;
use core::marker::Unpin;
use core::pin::Pin;
use core::option::Option;
use core::ptr::NonNull;
use core::task::{Waker, Poll};
use core::ops::{Drop, Generator, GeneratorState};

#[doc(inline)]
pub use core::future::*;

/// Wrap a generator in a future.
///
/// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
/// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
#[unstable(feature = "gen_future", issue = "50547")]
pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
    GenFuture(x)
}

/// A wrapper around generators used to implement `Future` for `async`/`await` code.
#[unstable(feature = "gen_future", issue = "50547")]
#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
struct GenFuture<T: Generator<Yield = ()>>(T);

// We rely on the fact that async/await futures are immovable in order to create
// self-referential borrows in the underlying generator.
impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {}

#[unstable(feature = "gen_future", issue = "50547")]
impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
    type Output = T::Return;
    fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output> {
        // Safe because we're !Unpin + !Drop mapping to a ?Unpin value
        let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
        set_task_waker(waker, || match gen.resume() {
            GeneratorState::Yielded(()) => Poll::Pending,
            GeneratorState::Complete(x) => Poll::Ready(x),
        })
    }
}

thread_local! {
    static TLS_WAKER: Cell<Option<NonNull<Waker>>> = Cell::new(None);
}

struct SetOnDrop(Option<NonNull<Waker>>);

impl Drop for SetOnDrop {
    fn drop(&mut self) {
        TLS_WAKER.with(|tls_waker| {
            tls_waker.set(self.0.take());
        });
    }
}

#[unstable(feature = "gen_future", issue = "50547")]
/// Sets the thread-local task context used by async/await futures.
pub fn set_task_waker<F, R>(waker: &Waker, f: F) -> R
where
    F: FnOnce() -> R
{
    let old_waker = TLS_WAKER.with(|tls_waker| {
        tls_waker.replace(Some(NonNull::from(waker)))
    });
    let _reset_waker = SetOnDrop(old_waker);
    f()
}

#[unstable(feature = "gen_future", issue = "50547")]
/// Retrieves the thread-local task waker used by async/await futures.
///
/// This function acquires exclusive access to the task waker.
///
/// Panics if no waker has been set or if the waker has already been
/// retrieved by a surrounding call to get_task_waker.
pub fn get_task_waker<F, R>(f: F) -> R
where
    F: FnOnce(&Waker) -> R
{
    let waker_ptr = TLS_WAKER.with(|tls_waker| {
        // Clear the entry so that nested `get_task_waker` calls
        // will fail or set their own value.
        tls_waker.replace(None)
    });
    let _reset_waker = SetOnDrop(waker_ptr);

    let waker_ptr = waker_ptr.expect(
        "TLS Waker not set. This is a rustc bug. \
        Please file an issue on https://github.com/rust-lang/rust.");
    unsafe { f(waker_ptr.as_ref()) }
}

#[unstable(feature = "gen_future", issue = "50547")]
/// Polls a future in the current thread-local task waker.
pub fn poll_with_tls_waker<F>(f: Pin<&mut F>) -> Poll<F::Output>
where
    F: Future
{
    get_task_waker(|waker| F::poll(f, waker))
}