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
#![doc(html_root_url = "https://docs.rs/try-lock/0.1.0")]
#![deny(missing_docs)]
#![deny(missing_debug_implementations)]
#![deny(warnings)]

//! 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");
//! ```

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

/// 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`.
pub struct TryLock<T> {
    is_locked: AtomicBool,
    value: UnsafeCell<T>,
}

impl<T> TryLock<T> {
    /// Create a `TryLock` around the value.
    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.
    pub fn try_lock(&self) -> Option<Locked<T>> {
        if !self.is_locked.swap(true, Ordering::SeqCst) {
            Some(Locked { lock: self })
        } else {
            None
        }
    }
}

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.
pub struct Locked<'a, T: 'a> {
    lock: &'a TryLock<T>,
}

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

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

impl<'a, T> Drop for Locked<'a, T> {
    fn drop(&mut self) {
        self.lock.is_locked.store(false, Ordering::SeqCst);
    }
}

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> }");
    }
}