feat: implement sync::Barrier

Based on the implementation in https://github.com/tokio-rs/tokio/pull/1571

Cargo.toml

@@ -42,6 +42,7 @@ num_cpus = "1.10.1"
 pin-utils = "0.1.0-alpha.4"
 slab = "0.4.2"
 kv-log-macro = "1.0.4"
+broadcaster = "0.2.4"
 
 [dev-dependencies]
 femme = "1.2.0"

src/sync/barrier.rs

@@ -0,0 +1,174 @@
+use broadcaster::BroadcastChannel;
+
+use crate::sync::Mutex;
+
+/// A barrier enables multiple tasks to synchronize the beginning
+/// of some computation.
+///
+/// ```
+/// # fn main() { async_std::task::block_on(async {
+/// #
+/// use std::sync::Arc;
+/// use async_std::sync::Barrier;
+/// use async_std::task;
+///
+/// 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(task::spawn(async move {
+///         println!("before wait");
+///         let wr = c.wait().await;
+///         println!("after wait");
+///         wr
+///     }));
+/// }
+/// // Wait for the other futures to finish.
+/// for handle in handles {
+///   handle.await;
+/// }
+/// # });
+/// # }
+/// ```
+#[derive(Debug)]
+pub struct Barrier {
+    state: Mutex<BarrierState>,
+    wait: BroadcastChannel<(usize, usize)>,
+    n: usize,
+}
+
+// The inner state of a double barrier
+#[derive(Debug)]
+struct BarrierState {
+    waker: BroadcastChannel<(usize, usize)>,
+    count: usize,
+    generation_id: usize,
+}
+
+/// A `BarrierWaitResult` is returned by `wait` when all threads in the `Barrier` have rendezvoused.
+///
+/// [`wait`]: struct.Barrier.html#method.wait
+/// [`Barrier`]: struct.Barrier.html
+///
+/// # Examples
+///
+/// ```
+/// use async_std::sync::Barrier;
+///
+/// let barrier = Barrier::new(1);
+/// let barrier_wait_result = barrier.wait();
+/// ```
+#[derive(Debug, Clone)]
+pub struct BarrierWaitResult(bool);
+
+impl Barrier {
+    /// Creates a new barrier that can block a given number of tasks.
+    ///
+    /// A barrier will block `n`-1 tasks which call [`wait`] and then wake up
+    /// all tasks at once when the `n`th task calls [`wait`].
+    ///
+    /// [`wait`]: #method.wait
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Barrier;
+    ///
+    /// let barrier = Barrier::new(10);
+    /// ```
+    pub fn new(mut n: usize) -> Barrier {
+        let waker = BroadcastChannel::new();
+        let wait = waker.clone();
+
+        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,
+                count: 0,
+                generation_id: 1,
+            }),
+            n,
+            wait,
+        }
+    }
+
+    /// Blocks the current task until all tasks have rendezvoused here.
+    ///
+    /// Barriers are re-usable after all tasks have rendezvoused once, and can
+    /// be used continuously.
+    ///
+    /// A single (arbitrary) task will receive a [`BarrierWaitResult`] that
+    /// returns `true` from [`is_leader`] when returning from this function, and
+    /// all other tasks will receive a result that will return `false` from
+    /// [`is_leader`].
+    ///
+    /// [`BarrierWaitResult`]: struct.BarrierWaitResult.html
+    /// [`is_leader`]: struct.BarrierWaitResult.html#method.is_leader
+    pub async fn wait(&self) -> BarrierWaitResult {
+        let mut lock = self.state.lock().await;
+        let local_gen = lock.generation_id;
+
+        lock.count += 1;
+
+        if lock.count < self.n {
+            let mut wait = self.wait.clone();
+
+            let mut generation_id = lock.generation_id;
+            let mut count = lock.count;
+
+            drop(lock);
+
+            while local_gen == generation_id && count < self.n {
+                let (g, c) = wait.recv().await.expect("sender hasn not been closed");
+                generation_id = g;
+                count = c;
+            }
+
+            BarrierWaitResult(false)
+        } else {
+            lock.count = 0;
+            lock.generation_id = lock.generation_id.wrapping_add(1);
+
+            lock.waker
+                .send(&(lock.generation_id, lock.count))
+                .await
+                .expect("there should be at least one receiver");
+
+            BarrierWaitResult(true)
+        }
+    }
+}
+
+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.
+    ///
+    /// [`wait`]: struct.Barrier.html#method.wait
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # fn main() { async_std::task::block_on(async {
+    /// #
+    /// use async_std::sync::Barrier;
+    ///
+    /// let barrier = Barrier::new(1);
+    /// let barrier_wait_result = barrier.wait().await;
+    /// println!("{:?}", barrier_wait_result.is_leader());
+    /// # });
+    /// # }
+    /// ```
+    pub fn is_leader(&self) -> bool {
+        self.0
+    }
+}

src/sync/mod.rs

@@ -32,8 +32,10 @@
 #[doc(inline)]
 pub use std::sync::{Arc, Weak};
 
+pub use barrier::{Barrier, BarrierWaitResult};
 pub use mutex::{Mutex, MutexGuard};
 pub use rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 
+mod barrier;
 mod mutex;
 mod rwlock;

tests/barrier.rs

@@ -0,0 +1,52 @@
+use std::sync::Arc;
+
+use futures_channel::mpsc::unbounded;
+use futures_util::sink::SinkExt;
+use futures_util::stream::StreamExt;
+
+use async_std::sync::Barrier;
+use async_std::task;
+
+#[test]
+fn test_barrier() {
+    // Based on the test in std, I was seeing some race conditions, so running it in a loop to make sure
+    // things are solid.
+
+    for _ in 0..1_000 {
+        task::block_on(async move {
+            const N: usize = 10;
+
+            let barrier = Arc::new(Barrier::new(N));
+            let (tx, mut rx) = unbounded();
+
+            for _ in 0..N - 1 {
+                let c = barrier.clone();
+                let mut tx = tx.clone();
+                task::spawn(async move {
+                    let res = c.wait().await;
+
+                    tx.send(res.is_leader()).await.unwrap();
+                });
+            }
+
+            // At this point, all spawned threads should be blocked,
+            // so we shouldn't get anything from the port
+            let res = rx.try_next();
+            assert!(match res {
+                Err(_err) => true,
+                _ => false,
+            });
+
+            let mut leader_found = barrier.wait().await.is_leader();
+
+            // Now, the barrier is cleared and we should get data.
+            for _ in 0..N - 1 {
+                if rx.next().await.unwrap() {
+                    assert!(!leader_found);
+                    leader_found = true;
+                }
+            }
+            assert!(leader_found);
+        });
+    }
+}