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use crate::loom::sync::Mutex;
use crate::sync::watch;
/// A barrier enables multiple tasks 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 tasks.
///
/// A barrier will block `n`-1 tasks which call [`Barrier::wait`] and then wake up all
/// tasks at once when the `n`th task 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 tasks 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 tasks
/// 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();
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 tasks in the `Barrier` have rendezvoused.
#[derive(Debug, Clone)]
pub struct BarrierWaitResult(bool);
impl BarrierWaitResult {
/// Returns `true` if this task from wait is the "leader task".
///
/// Only one task will have `true` returned from their result, all other tasks will have
/// `false` returned.
pub fn is_leader(&self) -> bool {
self.0
}
}
|
