1 files changed, 116 insertions, 0 deletions
diff --git a/src/libstd/sync/barrier.rs b/src/libstd/sync/barrier.rs
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+++ b/src/libstd/sync/barrier.rs
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+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use sync::{Mutex, Condvar};
+
+/// A barrier enables multiple tasks to synchronize the beginning
+/// of some computation.
+///
+/// ```rust
+/// use std::sync::{Arc, Barrier};
+///
+/// let barrier = Arc::new(Barrier::new(10));
+/// for _ in range(0u, 10) {
+///     let c = barrier.clone();
+///     // The same messages will be printed together.
+///     // You will NOT see any interleaving.
+///     spawn(proc() {
+///         println!("before wait");
+///         c.wait();
+///         println!("after wait");
+///     });
+/// }
+/// ```
+pub struct Barrier {
+    lock: Mutex<BarrierState>,
+    cvar: Condvar,
+    num_threads: uint,
+}
+
+// The inner state of a double barrier
+struct BarrierState {
+    count: uint,
+    generation_id: uint,
+}
+
+impl Barrier {
+    /// Create a new barrier that can block a given number of threads.
+    ///
+    /// A barrier will block `n`-1 threads which call `wait` and then wake up
+    /// all threads at once when the `n`th thread calls `wait`.
+    pub fn new(n: uint) -> Barrier {
+        Barrier {
+            lock: Mutex::new(BarrierState {
+                count: 0,
+                generation_id: 0,
+            }),
+            cvar: Condvar::new(),
+            num_threads: n,
+        }
+    }
+
+    /// Block the current thread until all threads has rendezvoused here.
+    ///
+    /// Barriers are re-usable after all threads have rendezvoused once, and can
+    /// be used continuously.
+    pub fn wait(&self) {
+        let mut lock = self.lock.lock();
+        let local_gen = lock.generation_id;
+        lock.count += 1;
+        if lock.count < self.num_threads {
+            // We need a while loop to guard against spurious wakeups.
+            // http://en.wikipedia.org/wiki/Spurious_wakeup
+            while local_gen == lock.generation_id &&
+                  lock.count < self.num_threads {
+                self.cvar.wait(&lock);
+            }
+        } else {
+            lock.count = 0;
+            lock.generation_id += 1;
+            self.cvar.notify_all();
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use prelude::*;
+
+    use sync::{Arc, Barrier};
+    use comm::Empty;
+
+    #[test]
+    fn test_barrier() {
+        let barrier = Arc::new(Barrier::new(10));
+        let (tx, rx) = channel();
+
+        for _ in range(0u, 9) {
+            let c = barrier.clone();
+            let tx = tx.clone();
+            spawn(proc() {
+                c.wait();
+                tx.send(true);
+            });
+        }
+
+        // At this point, all spawned tasks should be blocked,
+        // so we shouldn't get anything from the port
+        assert!(match rx.try_recv() {
+            Err(Empty) => true,
+            _ => false,
+        });
+
+        barrier.wait();
+        // Now, the barrier is cleared and we should get data.
+        for _ in range(0u, 9) {
+            rx.recv();
+        }
+    }
+}