auto merge of #11725 : derekchiang/rust/add-barrier, r=alexcrichton

src/libextra/sync.rs

@@ -27,6 +27,8 @@
 use std::util;
 use std::util::NonCopyable;
 
+use arc::MutexArc;
+
 /****************************************************************************
  * Internals
  ****************************************************************************/
@@ -682,6 +684,67 @@ impl<'a> RWLockReadMode<'a> {
     pub fn read<U>(&self, blk: || -> U) -> U { blk() }
 }
 
+/// A barrier enables multiple tasks to synchronize the beginning
+/// of some computation.
+/// ```rust
+/// use extra::sync::Barrier;
+///
+/// let barrier = Barrier::new(10);
+/// 10.times(|| {
+///     let c = barrier.clone();
+///     // The same messages will be printed together.
+///     // You will NOT see any interleaving.
+///     do spawn {
+///         println!("before wait");
+///         c.wait();
+///         println!("after wait");
+///     }
+/// });
+/// ```
+#[deriving(Clone)]
+pub struct Barrier {
+    priv arc: MutexArc<BarrierState>,
+    priv num_tasks: uint,
+}
+
+// The inner state of a double barrier
+struct BarrierState {
+    priv count: uint,
+    priv generation_id: uint,
+}
+
+impl Barrier {
+    /// Create a new barrier that can block a given number of tasks.
+    pub fn new(num_tasks: uint) -> Barrier {
+        Barrier {
+            arc: MutexArc::new(BarrierState {
+                count: 0,
+                generation_id: 0,
+            }),
+            num_tasks: num_tasks,
+        }
+    }
+
+    /// Block the current task until a certain number of tasks is waiting.
+    pub fn wait(&self) {
+        self.arc.access_cond(|state, cond| {
+            let local_gen = state.generation_id;
+            state.count += 1;
+            if state.count < self.num_tasks {
+                // We need a while loop to guard against spurious wakeups.
+                // http://en.wikipedia.org/wiki/Spurious_wakeup
+                while local_gen == state.generation_id && state.count < self.num_tasks {
+                    cond.wait();
+                }
+            } else {
+                state.count = 0;
+                state.generation_id += 1;
+                cond.broadcast();
+            }
+        });
+    }
+}
+
 /****************************************************************************
  * Tests
  ****************************************************************************/
@@ -693,6 +756,7 @@ mod tests {
     use std::cast;
     use std::result;
     use std::task;
+    use std::comm::{SharedChan, Empty};
 
     /************************************************************************
      * Semaphore tests
@@ -1315,4 +1379,35 @@ fn test_rwlock_downgrade_cant_swap() {
             })
         })
     }
+
+    /************************************************************************
+     * Barrier tests
+     ************************************************************************/
+    #[test]
+    fn test_barrier() {
+        let barrier = Barrier::new(10);
+        let (port, chan) = SharedChan::new();
+
+        9.times(|| {
+            let c = barrier.clone();
+            let chan = chan.clone();
+            do spawn {
+                c.wait();
+                chan.send(true);
+            }
+        });
+
+        // At this point, all spawned tasks should be blocked,
+        // so we shouldn't get anything from the port
+        assert!(match port.try_recv() {
+            Empty => true,
+            _ => false,
+        });
+
+        barrier.wait();
+        // Now, the barrier is cleared and we should get data.
+        9.times(|| {
+            port.recv();
+        });
+    }
 }