summaryrefslogtreecommitdiffstats
path: root/third_party/rust/try-lock/src/lib.rs
blob: 80a240efe198d3f499cb4b86ce99faced20a86bc (plain)
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
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
#![doc(html_root_url = "https://docs.rs/try-lock/0.2.3")]
#![deny(missing_docs)]
#![deny(missing_debug_implementations)]
#![deny(warnings)]
#![cfg_attr(not(test), no_std)]

//! A light-weight lock guarded by an atomic boolean.
//!
//! Most efficient when contention is low, acquiring the lock is a single
//! atomic swap, and releasing it just 1 more atomic swap.
//!
//! # Example
//!
//! ```
//! use std::sync::Arc;
//! use try_lock::TryLock;
//!
//! // a thing we want to share
//! struct Widget {
//!     name: String,
//! }
//!
//! // lock it up!
//! let widget1 = Arc::new(TryLock::new(Widget {
//!     name: "Spanner".into(),
//! }));
//!
//! let widget2 = widget1.clone();
//!
//!
//! // mutate the widget
//! let mut locked = widget1.try_lock().expect("example isn't locked yet");
//! locked.name.push_str(" Bundle");
//!
//! // hands off, buddy
//! let not_locked = widget2.try_lock();
//! assert!(not_locked.is_none(), "widget1 has the lock");
//!
//! // ok, you can have it
//! drop(locked);
//!
//! let locked2 = widget2.try_lock().expect("widget1 lock is released");
//!
//! assert_eq!(locked2.name, "Spanner Bundle");
//! ```

#[cfg(test)]
extern crate core;

use core::cell::UnsafeCell;
use core::fmt;
use core::ops::{Deref, DerefMut};
use core::sync::atomic::{AtomicBool, Ordering};
use core::marker::PhantomData;

/// A light-weight lock guarded by an atomic boolean.
///
/// Most efficient when contention is low, acquiring the lock is a single
/// atomic swap, and releasing it just 1 more atomic swap.
///
/// It is only possible to try to acquire the lock, it is not possible to
/// wait for the lock to become ready, like with a `Mutex`.
#[derive(Default)]
pub struct TryLock<T> {
    is_locked: AtomicBool,
    value: UnsafeCell<T>,
}

impl<T> TryLock<T> {
    /// Create a `TryLock` around the value.
    #[inline]
    pub fn new(val: T) -> TryLock<T> {
        TryLock {
            is_locked: AtomicBool::new(false),
            value: UnsafeCell::new(val),
        }
    }

    /// Try to acquire the lock of this value.
    ///
    /// If the lock is already acquired by someone else, this returns
    /// `None`. You can try to acquire again whenever you want, perhaps
    /// by spinning a few times, or by using some other means of
    /// notification.
    ///
    /// # Note
    ///
    /// The default memory ordering is to use `Acquire` to lock, and `Release`
    /// to unlock. If different ordering is required, use
    /// [`try_lock_explicit`](TryLock::try_lock_explicit) or
    /// [`try_lock_explicit_unchecked`](TryLock::try_lock_explicit_unchecked).
    #[inline]
    pub fn try_lock(&self) -> Option<Locked<T>> {
        unsafe {
            self.try_lock_explicit_unchecked(Ordering::Acquire, Ordering::Release)
        }
    }

    /// Try to acquire the lock of this value using the lock and unlock orderings.
    ///
    /// If the lock is already acquired by someone else, this returns
    /// `None`. You can try to acquire again whenever you want, perhaps
    /// by spinning a few times, or by using some other means of
    /// notification.
    #[inline]
    #[deprecated(
        since = "0.2.3",
        note = "This method is actually unsafe because it unsafely allows \
        the use of weaker memory ordering. Please use try_lock_explicit instead"
    )]
    pub fn try_lock_order(&self, lock_order: Ordering, unlock_order: Ordering) -> Option<Locked<T>> {
        unsafe {
            self.try_lock_explicit_unchecked(lock_order, unlock_order)
        }
    }

    /// Try to acquire the lock of this value using the specified lock and
    /// unlock orderings.
    ///
    /// If the lock is already acquired by someone else, this returns
    /// `None`. You can try to acquire again whenever you want, perhaps
    /// by spinning a few times, or by using some other means of
    /// notification.
    ///
    /// # Panic
    ///
    /// This method panics if `lock_order` is not any of `Acquire`, `AcqRel`,
    /// and `SeqCst`, or `unlock_order` is not any of `Release` and `SeqCst`.
    #[inline]
    pub fn try_lock_explicit(&self, lock_order: Ordering, unlock_order: Ordering) -> Option<Locked<T>> {
        match lock_order {
            Ordering::Acquire |
            Ordering::AcqRel |
            Ordering::SeqCst => {}
            _ => panic!("lock ordering must be `Acquire`, `AcqRel`, or `SeqCst`"),
        }

        match unlock_order {
            Ordering::Release |
            Ordering::SeqCst => {}
            _ => panic!("unlock ordering must be `Release` or `SeqCst`"),
        }

        unsafe {
            self.try_lock_explicit_unchecked(lock_order, unlock_order)
        }
    }

    /// Try to acquire the lock of this value using the specified lock and
    /// unlock orderings without checking that the specified orderings are
    /// strong enough to be safe.
    ///
    /// If the lock is already acquired by someone else, this returns
    /// `None`. You can try to acquire again whenever you want, perhaps
    /// by spinning a few times, or by using some other means of
    /// notification.
    ///
    /// # Safety
    ///
    /// Unlike [`try_lock_explicit`], this method is unsafe because it does not
    /// check that the given memory orderings are strong enough to prevent data
    /// race.
    ///
    /// [`try_lock_explicit`]: Self::try_lock_explicit
    #[inline]
    pub unsafe fn try_lock_explicit_unchecked(&self, lock_order: Ordering, unlock_order: Ordering) -> Option<Locked<T>> {
        if !self.is_locked.swap(true, lock_order) {
            Some(Locked {
                lock: self,
                order: unlock_order,
                _p: PhantomData,
            })
        } else {
            None
        }
    }

    /// Take the value back out of the lock when this is the sole owner.
    #[inline]
    pub fn into_inner(self) -> T {
        debug_assert!(!self.is_locked.load(Ordering::Relaxed), "TryLock was mem::forgotten");
        // Since the compiler can statically determine this is the only owner,
        // it's safe to take the value out. In fact, in newer versions of Rust,
        // `UnsafeCell::into_inner` has been marked safe.
        //
        // To support older version (1.21), the unsafe block is still here.
        #[allow(unused_unsafe)]
        unsafe {
            self.value.into_inner()
        }
    }
}

unsafe impl<T: Send> Send for TryLock<T> {}
unsafe impl<T: Send> Sync for TryLock<T> {}

impl<T: fmt::Debug> fmt::Debug for TryLock<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

        // Used if the TryLock cannot acquire the lock.
        struct LockedPlaceholder;

        impl fmt::Debug for LockedPlaceholder {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                f.write_str("<locked>")
            }
        }

        let mut builder = f.debug_struct("TryLock");
        if let Some(locked) = self.try_lock() {
            builder.field("value", &*locked);
        } else {
            builder.field("value", &LockedPlaceholder);
        }
        builder.finish()
    }
}

/// A locked value acquired from a `TryLock`.
///
/// The type represents an exclusive view at the underlying value. The lock is
/// released when this type is dropped.
///
/// This type derefs to the underlying value.
#[must_use = "TryLock will immediately unlock if not used"]
pub struct Locked<'a, T: 'a> {
    lock: &'a TryLock<T>,
    order: Ordering,
    /// Suppresses Send and Sync autotraits for `struct Locked`.
    _p: PhantomData<*mut T>,
}

impl<'a, T> Deref for Locked<'a, T> {
    type Target = T;
    #[inline]
    fn deref(&self) -> &T {
        unsafe { &*self.lock.value.get() }
    }
}

impl<'a, T> DerefMut for Locked<'a, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.lock.value.get() }
    }
}

impl<'a, T> Drop for Locked<'a, T> {
    #[inline]
    fn drop(&mut self) {
        self.lock.is_locked.store(false, self.order);
    }
}

impl<'a, T: fmt::Debug> fmt::Debug for Locked<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

#[cfg(test)]
mod tests {
    use super::TryLock;

    #[test]
    fn fmt_debug() {
        let lock = TryLock::new(5);
        assert_eq!(format!("{:?}", lock), "TryLock { value: 5 }");

        let locked = lock.try_lock().unwrap();
        assert_eq!(format!("{:?}", locked), "5");

        assert_eq!(format!("{:?}", lock), "TryLock { value: <locked> }");
    }
}