barrier.rs.html -- source
Description: Source to the Rust file `src/libstd/sync/barrier.rs`.
Keywords: rust, rustlang, rust-lang

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    use crate::fmt;
    use crate::sync::{Mutex, Condvar};

    /// A barrier enables multiple threads to synchronize the beginning
    /// of some computation.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Barrier};
    /// use std::thread;
    ///
    /// 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(thread::spawn(move|| {
    ///         println!("before wait");
    ///         c.wait();
    ///         println!("after wait");
    ///     }));
    /// }
    /// // Wait for other threads to finish.
    /// for handle in handles {
    ///     handle.join().unwrap();
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Barrier {
        lock: Mutex<BarrierState>,
        cvar: Condvar,
        num_threads: usize,
    }

    // The inner state of a double barrier
    struct BarrierState {
        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 std::sync::Barrier;
    ///
    /// let barrier = Barrier::new(1);
    /// let barrier_wait_result = barrier.wait();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BarrierWaitResult(bool);

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl fmt::Debug for Barrier {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("Barrier { .. }")
        }
    }

    impl Barrier {
        /// Creates 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`].
        ///
        /// [`wait`]: #method.wait
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Barrier;
        ///
        /// let barrier = Barrier::new(10);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(n: usize) -> Barrier {
            Barrier {
                lock: Mutex::new(BarrierState {
                    count: 0,
                    generation_id: 0,
                }),
                cvar: Condvar::new(),
                num_threads: n,
            }
        }

        /// Blocks the current thread until all threads have rendezvoused here.
        ///
        /// Barriers are re-usable after all threads have rendezvoused once, and can
        /// be used continuously.
        ///
        /// A single (arbitrary) thread will receive a [`BarrierWaitResult`] that
        /// returns `true` from [`is_leader`] when returning from this function, and
        /// all other threads will receive a result that will return `false` from
        /// [`is_leader`].
        ///
        /// [`BarrierWaitResult`]: struct.BarrierWaitResult.html
        /// [`is_leader`]: struct.BarrierWaitResult.html#method.is_leader
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::{Arc, Barrier};
        /// use std::thread;
        ///
        /// 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(thread::spawn(move|| {
        ///         println!("before wait");
        ///         c.wait();
        ///         println!("after wait");
        ///     }));
        /// }
        /// // Wait for other threads to finish.
        /// for handle in handles {
        ///     handle.join().unwrap();
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn wait(&self) -> BarrierWaitResult {
            let mut lock = self.lock.lock().unwrap();
            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 {
                    lock = self.cvar.wait(lock).unwrap();
                }
                BarrierWaitResult(false)
            } else {
                lock.count = 0;
                lock.generation_id = lock.generation_id.wrapping_add(1);
                self.cvar.notify_all();
                BarrierWaitResult(true)
            }
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl fmt::Debug for BarrierWaitResult {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("BarrierWaitResult")
                .field("is_leader", &self.is_leader())
                .finish()
        }
    }

    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.
        ///
        /// [`wait`]: struct.Barrier.html#method.wait
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Barrier;
        ///
        /// let barrier = Barrier::new(1);
        /// let barrier_wait_result = barrier.wait();
        /// println!("{:?}", barrier_wait_result.is_leader());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_leader(&self) -> bool { self.0 }
    }

    #[cfg(test)]
    mod tests {
        use crate::sync::{Arc, Barrier};
        use crate::sync::mpsc::{channel, TryRecvError};
        use crate::thread;

        #[test]
        #[cfg_attr(target_os = "emscripten", ignore)]
        fn test_barrier() {
            const N: usize = 10;

            let barrier = Arc::new(Barrier::new(N));
            let (tx, rx) = channel();

            for _ in 0..N - 1 {
                let c = barrier.clone();
                let tx = tx.clone();
                thread::spawn(move|| {
                    tx.send(c.wait().is_leader()).unwrap();
                });
            }

            // At this point, all spawned threads should be blocked,
            // so we shouldn't get anything from the port
            assert!(match rx.try_recv() {
                Err(TryRecvError::Empty) => true,
                _ => false,
            });

            let mut leader_found = barrier.wait().is_leader();

            // Now, the barrier is cleared and we should get data.
            for _ in 0..N - 1 {
                if rx.recv().unwrap() {
                    assert!(!leader_found);
                    leader_found = true;
                }
            }
            assert!(leader_found);
        }
    }