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

#![forbid(unsafe_code)]

use core::future::Future;
use core::ops::{Deref, DerefMut};
use core::pin::Pin;
use core::task::{Context, Poll};
use std::collections::VecDeque;
use std::sync::TryLockError;
use std::task::Waker;

/// An [RAII](https://doc.rust-lang.org/rust-by-example/scope/raii.html)
/// scoped lock of a [`Mutex`](struct.Mutex.html).
/// It automatically unlocks the mutex when dropped (falls out of scope).
///
/// You can access the data in the mutex through this guard's
/// [`Deref`](https://doc.rust-lang.org/stable/std/ops/trait.Deref.html)
/// and
/// [`DerefMut`](https://doc.rust-lang.org/stable/std/ops/trait.DerefMut.html)
/// implementations.
///
/// The struct is not
/// [`Send`](https://doc.rust-lang.org/stable/std/marker/trait.Send.html)
/// so you cannot await while holding it.
///
/// If a task panics while holding the struct, the underlying mutex becomes
/// ["poisoned"](https://doc.rust-lang.org/stable/std/sync/struct.Mutex.html#poisoning)
/// and subsequent calls to `lock` will panic.
#[allow(clippy::module_name_repetitions)]
pub struct MutexGuard<'a, T> {
    mutex: &'a Mutex<T>,
    value_guard: Option<std::sync::MutexGuard<'a, T>>,
}
impl<'a, T> MutexGuard<'a, T> {
    fn new(mutex: &'a Mutex<T>, value_guard: std::sync::MutexGuard<'a, T>) -> MutexGuard<'a, T> {
        let mut inner_guard = mutex.inner.lock().unwrap();
        assert!(!inner_guard.locked);
        inner_guard.locked = true;
        MutexGuard {
            mutex,
            value_guard: Some(value_guard),
        }
    }
}
impl<'a, T> Drop for MutexGuard<'a, T> {
    fn drop(&mut self) {
        let mut wakers = VecDeque::new();
        {
            let mut inner_guard = self.mutex.inner.lock().unwrap();
            assert!(inner_guard.locked);
            inner_guard.locked = false;
            std::mem::swap(&mut inner_guard.wakers, &mut wakers);
        }
        self.value_guard.take();
        for waker in wakers {
            waker.wake();
        }
    }
}
impl<'a, T> Deref for MutexGuard<'a, T> {
    type Target = T;
    fn deref(&self) -> &Self::Target {
        &*self.value_guard.as_ref().unwrap()
    }
}
impl<'a, T> DerefMut for MutexGuard<'a, T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut *self.value_guard.as_mut().unwrap()
    }
}

#[doc(hidden)]
pub struct LockFuture<'a, T> {
    mutex: &'a Mutex<T>,
}
impl<'a, T> Future for LockFuture<'a, T> {
    type Output = MutexGuard<'a, T>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        loop {
            match self.mutex.value.try_lock() {
                Ok(guard) => return Poll::Ready(MutexGuard::new(self.mutex, guard)),
                Err(TryLockError::Poisoned(e)) => panic!("{}", e),
                Err(TryLockError::WouldBlock) => {}
            }
            let mut guard = self.mutex.inner.lock().unwrap();
            if guard.locked {
                // Mutex is locked.  Add our waker and sleep.
                guard.wakers.push_back(cx.waker().clone());
                return Poll::Pending;
            }
            // Mutex is now unlocked.  Try to acquire it again.
        }
    }
}

struct Inner {
    wakers: VecDeque<Waker>,
    locked: bool,
}

/// A wrapper around
/// [`std::sync::Mutex`](https://doc.rust-lang.org/stable/std/sync/struct.Mutex.html)
/// with an async [`lock`](#method.lock) method.
pub struct Mutex<T> {
    inner: std::sync::Mutex<Inner>,
    value: std::sync::Mutex<T>,
}

impl<T> Mutex<T> {
    pub fn new(value: T) -> Mutex<T> {
        Self {
            inner: std::sync::Mutex::new(Inner {
                wakers: VecDeque::new(),
                locked: false,
            }),
            value: std::sync::Mutex::new(value),
        }
    }

    /// Acquires the mutex, sleeping the current task until it is able to do so.
    ///
    /// After this function returns, is the only task holding the lock.
    ///
    /// The returned guard is an
    /// [RAII](https://doc.rust-lang.org/rust-by-example/scope/raii.html)
    /// scoped lock.
    /// It automatically unlocks the mutex when dropped (falls out of scope).
    ///
    /// The returned guard is not
    /// [`Send`](https://doc.rust-lang.org/stable/std/marker/trait.Send.html)
    /// so you cannot `await` while holding it.
    ///
    /// If a task panics while holding the guard, the underlying mutex becomes
    /// ["poisoned"](https://doc.rust-lang.org/stable/std/sync/struct.Mutex.html#poisoning)
    /// and subsequent calls to `lock` will panic.
    pub async fn lock(&self) -> MutexGuard<'_, T> {
        LockFuture { mutex: self }.await
    }
}