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
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
#![stable(feature = "wake_trait", since = "1.51.0")]

//! Types and Traits for working with asynchronous tasks.
//!
//! **Note**: This module is only available on platforms that support atomic
//! loads and stores of pointers. This may be detected at compile time using
//! `#[cfg(target_has_atomic = "ptr")]`.

use core::mem::ManuallyDrop;
use core::task::{RawWaker, RawWakerVTable, Waker};

use crate::sync::Arc;

/// The implementation of waking a task on an executor.
///
/// This trait can be used to create a [`Waker`]. An executor can define an
/// implementation of this trait, and use that to construct a Waker to pass
/// to the tasks that are executed on that executor.
///
/// This trait is a memory-safe and ergonomic alternative to constructing a
/// [`RawWaker`]. It supports the common executor design in which the data used
/// to wake up a task is stored in an [`Arc`]. Some executors (especially
/// those for embedded systems) cannot use this API, which is why [`RawWaker`]
/// exists as an alternative for those systems.
///
/// [arc]: ../../std/sync/struct.Arc.html
///
/// # Examples
///
/// A basic `block_on` function that takes a future and runs it to completion on
/// the current thread.
///
/// **Note:** This example trades correctness for simplicity. In order to prevent
/// deadlocks, production-grade implementations will also need to handle
/// intermediate calls to `thread::unpark` as well as nested invocations.
///
/// ```rust
/// use std::future::Future;
/// use std::sync::Arc;
/// use std::task::{Context, Poll, Wake};
/// use std::thread::{self, Thread};
/// use core::pin::pin;
///
/// /// A waker that wakes up the current thread when called.
/// struct ThreadWaker(Thread);
///
/// impl Wake for ThreadWaker {
///     fn wake(self: Arc<Self>) {
///         self.0.unpark();
///     }
/// }
///
/// /// Run a future to completion on the current thread.
/// fn block_on<T>(fut: impl Future<Output = T>) -> T {
///     // Pin the future so it can be polled.
///     let mut fut = pin!(fut);
///
///     // Create a new context to be passed to the future.
///     let t = thread::current();
///     let waker = Arc::new(ThreadWaker(t)).into();
///     let mut cx = Context::from_waker(&waker);
///
///     // Run the future to completion.
///     loop {
///         match fut.as_mut().poll(&mut cx) {
///             Poll::Ready(res) => return res,
///             Poll::Pending => thread::park(),
///         }
///     }
/// }
///
/// block_on(async {
///     println!("Hi from inside a future!");
/// });
/// ```
#[stable(feature = "wake_trait", since = "1.51.0")]
pub trait Wake {
    /// Wake this task.
    #[stable(feature = "wake_trait", since = "1.51.0")]
    fn wake(self: Arc<Self>);

    /// Wake this task without consuming the waker.
    ///
    /// If an executor supports a cheaper way to wake without consuming the
    /// waker, it should override this method. By default, it clones the
    /// [`Arc`] and calls [`wake`] on the clone.
    ///
    /// [`wake`]: Wake::wake
    #[stable(feature = "wake_trait", since = "1.51.0")]
    fn wake_by_ref(self: &Arc<Self>) {
        self.clone().wake();
    }
}

#[stable(feature = "wake_trait", since = "1.51.0")]
impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for Waker {
    /// Use a `Wake`-able type as a `Waker`.
    ///
    /// No heap allocations or atomic operations are used for this conversion.
    fn from(waker: Arc<W>) -> Waker {
        // SAFETY: This is safe because raw_waker safely constructs
        // a RawWaker from Arc<W>.
        unsafe { Waker::from_raw(raw_waker(waker)) }
    }
}

#[stable(feature = "wake_trait", since = "1.51.0")]
impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for RawWaker {
    /// Use a `Wake`-able type as a `RawWaker`.
    ///
    /// No heap allocations or atomic operations are used for this conversion.
    fn from(waker: Arc<W>) -> RawWaker {
        raw_waker(waker)
    }
}

// NB: This private function for constructing a RawWaker is used, rather than
// inlining this into the `From<Arc<W>> for RawWaker` impl, to ensure that
// the safety of `From<Arc<W>> for Waker` does not depend on the correct
// trait dispatch - instead both impls call this function directly and
// explicitly.
#[inline(always)]
fn raw_waker<W: Wake + Send + Sync + 'static>(waker: Arc<W>) -> RawWaker {
    // Increment the reference count of the arc to clone it.
    unsafe fn clone_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) -> RawWaker {
        unsafe { Arc::increment_strong_count(waker as *const W) };
        RawWaker::new(
            waker as *const (),
            &RawWakerVTable::new(clone_waker::<W>, wake::<W>, wake_by_ref::<W>, drop_waker::<W>),
        )
    }

    // Wake by value, moving the Arc into the Wake::wake function
    unsafe fn wake<W: Wake + Send + Sync + 'static>(waker: *const ()) {
        let waker = unsafe { Arc::from_raw(waker as *const W) };
        <W as Wake>::wake(waker);
    }

    // Wake by reference, wrap the waker in ManuallyDrop to avoid dropping it
    unsafe fn wake_by_ref<W: Wake + Send + Sync + 'static>(waker: *const ()) {
        let waker = unsafe { ManuallyDrop::new(Arc::from_raw(waker as *const W)) };
        <W as Wake>::wake_by_ref(&waker);
    }

    // Decrement the reference count of the Arc on drop
    unsafe fn drop_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) {
        unsafe { Arc::decrement_strong_count(waker as *const W) };
    }

    RawWaker::new(
        Arc::into_raw(waker) as *const (),
        &RawWakerVTable::new(clone_waker::<W>, wake::<W>, wake_by_ref::<W>, drop_waker::<W>),
    )
}