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

// Necessary for using `Mutex<usize>` for conditional variables
#![allow(clippy::mutex_atomic)]

use crate::primitive::sync::{Arc, Condvar, Mutex};
use std::fmt;

/// Enables threads to synchronize the beginning or end of some computation.
///
/// # Wait groups vs barriers
///
/// `WaitGroup` is very similar to [`Barrier`], but there are a few differences:
///
/// * [`Barrier`] needs to know the number of threads at construction, while `WaitGroup` is cloned to
///   register more threads.
///
/// * A [`Barrier`] can be reused even after all threads have synchronized, while a `WaitGroup`
///   synchronizes threads only once.
///
/// * All threads wait for others to reach the [`Barrier`]. With `WaitGroup`, each thread can choose
///   to either wait for other threads or to continue without blocking.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::WaitGroup;
/// use std::thread;
///
/// // Create a new wait group.
/// let wg = WaitGroup::new();
///
/// for _ in 0..4 {
///     // Create another reference to the wait group.
///     let wg = wg.clone();
///
///     thread::spawn(move || {
///         // Do some work.
///
///         // Drop the reference to the wait group.
///         drop(wg);
///     });
/// }
///
/// // Block until all threads have finished their work.
/// wg.wait();
/// ```
///
/// [`Barrier`]: std::sync::Barrier
pub struct WaitGroup {
    inner: Arc<Inner>,
}

/// Inner state of a `WaitGroup`.
struct Inner {
    cvar: Condvar,
    count: Mutex<usize>,
}

impl Default for WaitGroup {
    fn default() -> Self {
        Self {
            inner: Arc::new(Inner {
                cvar: Condvar::new(),
                count: Mutex::new(1),
            }),
        }
    }
}

impl WaitGroup {
    /// Creates a new wait group and returns the single reference to it.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_utils::sync::WaitGroup;
    ///
    /// let wg = WaitGroup::new();
    /// ```
    pub fn new() -> Self {
        Self::default()
    }

    /// Drops this reference and waits until all other references are dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_utils::sync::WaitGroup;
    /// use std::thread;
    ///
    /// let wg = WaitGroup::new();
    ///
    /// thread::spawn({
    ///     let wg = wg.clone();
    ///     move || {
    ///         // Block until both threads have reached `wait()`.
    ///         wg.wait();
    ///     }
    /// });
    ///
    /// // Block until both threads have reached `wait()`.
    /// wg.wait();
    /// ```
    pub fn wait(self) {
        if *self.inner.count.lock().unwrap() == 1 {
            return;
        }

        let inner = self.inner.clone();
        drop(self);

        let mut count = inner.count.lock().unwrap();
        while *count > 0 {
            count = inner.cvar.wait(count).unwrap();
        }
    }
}

impl Drop for WaitGroup {
    fn drop(&mut self) {
        let mut count = self.inner.count.lock().unwrap();
        *count -= 1;

        if *count == 0 {
            self.inner.cvar.notify_all();
        }
    }
}

impl Clone for WaitGroup {
    fn clone(&self) -> WaitGroup {
        let mut count = self.inner.count.lock().unwrap();
        *count += 1;

        WaitGroup {
            inner: self.inner.clone(),
        }
    }
}

impl fmt::Debug for WaitGroup {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let count: &usize = &*self.inner.count.lock().unwrap();
        f.debug_struct("WaitGroup").field("count", count).finish()
    }
}