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
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178

use core::{
    cell::Cell,
    fmt,
    future::Future,
    marker::PhantomData,
    mem::ManuallyDrop,
    ops::Deref,
    sync::atomic::{AtomicBool, Ordering},
    task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
};
use std::{
    sync::Arc,
    thread::{self, Thread},
};

#[macro_export]
macro_rules! pin_mut {
    ($($x:ident),* $(,)?) => { $(
        let mut $x = $x;
        #[allow(unused_mut)]
        let mut $x = unsafe {
            core::pin::Pin::new_unchecked(&mut $x)
        };
    )* }
}

trait ArcWake: Send + Sync {
    fn wake(self: Arc<Self>) {
        Self::wake_by_ref(&self)
    }
    fn wake_by_ref(arc_self: &Arc<Self>);
}

unsafe fn increase_refcount<T: ArcWake>(data: *const ()) {
    let arc = core::mem::ManuallyDrop::new(Arc::<T>::from_raw(data.cast::<T>()));
    let _arc_clone: core::mem::ManuallyDrop<_> = arc.clone();
}

unsafe fn clone_arc_raw<T: ArcWake>(data: *const ()) -> RawWaker {
    increase_refcount::<T>(data);
    RawWaker::new(data, waker_vtable::<T>())
}

unsafe fn wake_arc_raw<T: ArcWake>(data: *const ()) {
    let arc: Arc<T> = Arc::from_raw(data.cast::<T>());
    ArcWake::wake(arc);
}

unsafe fn wake_by_ref_arc_raw<T: ArcWake>(data: *const ()) {
    let arc = core::mem::ManuallyDrop::new(Arc::<T>::from_raw(data.cast::<T>()));
    ArcWake::wake_by_ref(&arc);
}

unsafe fn drop_arc_raw<T: ArcWake>(data: *const ()) {
    drop(Arc::<T>::from_raw(data.cast::<T>()))
}

fn waker_vtable<W: ArcWake>() -> &'static RawWakerVTable {
    &RawWakerVTable::new(
        clone_arc_raw::<W>,
        wake_arc_raw::<W>,
        wake_by_ref_arc_raw::<W>,
        drop_arc_raw::<W>,
    )
}

struct ThreadNotify {
    thread: Thread,
    unparked: AtomicBool,
}

impl ArcWake for ThreadNotify {
    fn wake_by_ref(arc_self: &Arc<Self>) {
        let unparked = arc_self.unparked.swap(true, Ordering::Release);
        if !unparked {
            arc_self.thread.unpark();
        }
    }
}

thread_local! {
    static CURRENT_THREAD_NOTIFY: Arc<ThreadNotify> = Arc::new(ThreadNotify {
        thread: thread::current(),
        unparked: AtomicBool::new(false),
    });
}

thread_local!(static ENTERED: Cell<bool> = Cell::new(false));

struct Enter {
    _priv: (),
}
struct EnterError {
    _priv: (),
}

impl fmt::Debug for EnterError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("EnterError").finish()
    }
}

impl fmt::Display for EnterError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "an execution scope has already been entered")
    }
}

impl std::error::Error for EnterError {}

fn enter() -> Result<Enter, EnterError> {
    ENTERED.with(|c| {
        if c.get() {
            Err(EnterError { _priv: () })
        } else {
            c.set(true);

            Ok(Enter { _priv: () })
        }
    })
}

fn run_executor<T, F: FnMut(&mut Context<'_>) -> Poll<T>>(mut f: F) -> T {
    let _enter = enter().expect(
        "cannot execute `LocalPool` executor from within \
         another executor",
    );

    CURRENT_THREAD_NOTIFY.with(|thread_notify| {
        let waker = waker_ref(thread_notify);
        let mut cx = Context::from_waker(&waker);
        loop {
            if let Poll::Ready(t) = f(&mut cx) {
                return t;
            }
            while !thread_notify.unparked.swap(false, Ordering::Acquire) {
                thread::park();
            }
        }
    })
}
#[derive(Debug)]
struct WakerRef<'a> {
    waker: ManuallyDrop<Waker>,
    _marker: PhantomData<&'a ()>,
}
impl<'a> WakerRef<'a> {
    #[inline]
    fn new_unowned(waker: ManuallyDrop<Waker>) -> Self {
        Self {
            waker,
            _marker: PhantomData,
        }
    }
}
impl Deref for WakerRef<'_> {
    type Target = Waker;

    #[inline]
    fn deref(&self) -> &Waker {
        &self.waker
    }
}
fn waker_ref<W>(wake: &Arc<W>) -> WakerRef<'_>
where
    W: ArcWake,
{
    let ptr = Arc::as_ptr(wake).cast::<()>();

    let waker =
        ManuallyDrop::new(unsafe { Waker::from_raw(RawWaker::new(ptr, waker_vtable::<W>())) });
    WakerRef::new_unowned(waker)
}

pub(crate) fn block_on<F: Future>(f: F) -> F::Output {
    pin_mut!(f);
    run_executor(|cx| f.as_mut().poll(cx))
}