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

use crate::sync::watch;

use std::sync::Mutex;

/// A barrier enables multiple threads to synchronize the beginning of some computation.
///
/// ```
/// # #[tokio::main]
/// # async fn main() {
/// use tokio::sync::Barrier;
/// use std::sync::Arc;
///
/// let mut handles = Vec::with_capacity(10);
/// let barrier = Arc::new(Barrier::new(10));
/// for _ in 0..10 {
///     let c = barrier.clone();
///     // The same messages will be printed together.
///     // You will NOT see any interleaving.
///     handles.push(tokio::spawn(async move {
///         println!("before wait");
///         let wait_result = c.wait().await;
///         println!("after wait");
///         wait_result
///     }));
/// }
///
/// // Will not resolve until all "after wait" messages have been printed
/// let mut num_leaders = 0;
/// for handle in handles {
///     let wait_result = handle.await.unwrap();
///     if wait_result.is_leader() {
///         num_leaders += 1;
///     }
/// }
///
/// // Exactly one barrier will resolve as the "leader"
/// assert_eq!(num_leaders, 1);
/// # }
/// ```
#[derive(Debug)]
pub struct Barrier {
    state: Mutex<BarrierState>,
    wait: watch::Receiver<usize>,
    n: usize,
}

#[derive(Debug)]
struct BarrierState {
    waker: watch::Sender<usize>,
    arrived: usize,
    generation: usize,
}

impl Barrier {
    /// Creates a new barrier that can block a given number of threads.
    ///
    /// A barrier will block `n`-1 threads which call [`Barrier::wait`] and then wake up all
    /// threads at once when the `n`th thread calls `wait`.
    pub fn new(mut n: usize) -> Barrier {
        let (waker, wait) = crate::sync::watch::channel(0);

        if n == 0 {
            // if n is 0, it's not clear what behavior the user wants.
            // in std::sync::Barrier, an n of 0 exhibits the same behavior as n == 1, where every
            // .wait() immediately unblocks, so we adopt that here as well.
            n = 1;
        }

        Barrier {
            state: Mutex::new(BarrierState {
                waker,
                arrived: 0,
                generation: 1,
            }),
            n,
            wait,
        }
    }

    /// Does not resolve until all tasks have rendezvoused here.
    ///
    /// Barriers are re-usable after all threads have rendezvoused once, and can
    /// be used continuously.
    ///
    /// A single (arbitrary) future will receive a [`BarrierWaitResult`] that returns `true` from
    /// [`BarrierWaitResult::is_leader`] when returning from this function, and all other threads
    /// will receive a result that will return `false` from `is_leader`.
    pub async fn wait(&self) -> BarrierWaitResult {
        // NOTE: we are taking a _synchronous_ lock here.
        // It is okay to do so because the critical section is fast and never yields, so it cannot
        // deadlock even if another future is concurrently holding the lock.
        // It is _desireable_ to do so as synchronous Mutexes are, at least in theory, faster than
        // the asynchronous counter-parts, so we should use them where possible [citation needed].
        // NOTE: the extra scope here is so that the compiler doesn't think `state` is held across
        // a yield point, and thus marks the returned future as !Send.
        let generation = {
            let mut state = self.state.lock().unwrap();
            let generation = state.generation;
            state.arrived += 1;
            if state.arrived == self.n {
                // we are the leader for this generation
                // wake everyone, increment the generation, and return
                state
                    .waker
                    .send(state.generation)
                    .expect("there is at least one receiver");
                state.arrived = 0;
                state.generation += 1;
                return BarrierWaitResult(true);
            }

            generation
        };

        // we're going to have to wait for the last of the generation to arrive
        let mut wait = self.wait.clone();

        loop {
            let _ = wait.changed().await;

            // note that the first time through the loop, this _will_ yield a generation
            // immediately, since we cloned a receiver that has never seen any values.
            if *wait.borrow() >= generation {
                break;
            }
        }

        BarrierWaitResult(false)
    }
}

/// A `BarrierWaitResult` is returned by `wait` when all threads in the `Barrier` have rendezvoused.
#[derive(Debug, Clone)]
pub struct BarrierWaitResult(bool);

impl BarrierWaitResult {
    /// Returns `true` if this thread from wait is the "leader thread".
    ///
    /// Only one thread will have `true` returned from their result, all other threads will have
    /// `false` returned.
    pub fn is_leader(&self) -> bool {
        self.0
    }
}