futures_channel/
lock.rs

1//! A "mutex" which only supports `try_lock`
2//!
3//! As a futures library the eventual call to an event loop should be the only
4//! thing that ever blocks, so this is assisted with a fast user-space
5//! implementation of a lock that can only have a `try_lock` operation.
6
7use core::cell::UnsafeCell;
8use core::ops::{Deref, DerefMut};
9use core::sync::atomic::AtomicBool;
10use core::sync::atomic::Ordering::SeqCst;
11
12/// A "mutex" around a value, similar to `std::sync::Mutex<T>`.
13///
14/// This lock only supports the `try_lock` operation, however, and does not
15/// implement poisoning.
16#[derive(Debug)]
17pub(crate) struct Lock<T> {
18    locked: AtomicBool,
19    data: UnsafeCell<T>,
20}
21
22/// Sentinel representing an acquired lock through which the data can be
23/// accessed.
24pub(crate) struct TryLock<'a, T> {
25    __ptr: &'a Lock<T>,
26}
27
28// The `Lock` structure is basically just a `Mutex<T>`, and these two impls are
29// intended to mirror the standard library's corresponding impls for `Mutex<T>`.
30//
31// If a `T` is sendable across threads, so is the lock, and `T` must be sendable
32// across threads to be `Sync` because it allows mutable access from multiple
33// threads.
34unsafe impl<T: Send> Send for Lock<T> {}
35unsafe impl<T: Send> Sync for Lock<T> {}
36
37impl<T> Lock<T> {
38    /// Creates a new lock around the given value.
39    pub(crate) fn new(t: T) -> Self {
40        Self { locked: AtomicBool::new(false), data: UnsafeCell::new(t) }
41    }
42
43    /// Attempts to acquire this lock, returning whether the lock was acquired or
44    /// not.
45    ///
46    /// If `Some` is returned then the data this lock protects can be accessed
47    /// through the sentinel. This sentinel allows both mutable and immutable
48    /// access.
49    ///
50    /// If `None` is returned then the lock is already locked, either elsewhere
51    /// on this thread or on another thread.
52    pub(crate) fn try_lock(&self) -> Option<TryLock<'_, T>> {
53        if !self.locked.swap(true, SeqCst) {
54            Some(TryLock { __ptr: self })
55        } else {
56            None
57        }
58    }
59}
60
61impl<T> Deref for TryLock<'_, T> {
62    type Target = T;
63    fn deref(&self) -> &T {
64        // The existence of `TryLock` represents that we own the lock, so we
65        // can safely access the data here.
66        unsafe { &*self.__ptr.data.get() }
67    }
68}
69
70impl<T> DerefMut for TryLock<'_, T> {
71    fn deref_mut(&mut self) -> &mut T {
72        // The existence of `TryLock` represents that we own the lock, so we
73        // can safely access the data here.
74        //
75        // Additionally, we're the *only* `TryLock` in existence so mutable
76        // access should be ok.
77        unsafe { &mut *self.__ptr.data.get() }
78    }
79}
80
81impl<T> Drop for TryLock<'_, T> {
82    fn drop(&mut self) {
83        self.__ptr.locked.store(false, SeqCst);
84    }
85}
86
87#[cfg(test)]
88mod tests {
89    use super::Lock;
90
91    #[test]
92    fn smoke() {
93        let a = Lock::new(1);
94        let mut a1 = a.try_lock().unwrap();
95        assert!(a.try_lock().is_none());
96        assert_eq!(*a1, 1);
97        *a1 = 2;
98        drop(a1);
99        assert_eq!(*a.try_lock().unwrap(), 2);
100        assert_eq!(*a.try_lock().unwrap(), 2);
101    }
102}
futures_channel/lock.rs

futures_channel/
lock.rs
