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

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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 fmt;
    use sync::atomic::{AtomicUsize, Ordering};
    use sync::{mutex, MutexGuard, PoisonError};
    use sys_common::condvar as sys;
    use sys_common::mutex as sys_mutex;
    use sys_common::poison::{self, LockResult};
    use time::Duration;

    /// A type indicating whether a timed wait on a condition variable returned
    /// due to a time out or not.
    #[derive(Debug, PartialEq, Eq, Copy, Clone)]
    #[stable(feature = "wait_timeout", since = "1.5.0")]
    pub struct WaitTimeoutResult(bool);

    impl WaitTimeoutResult {
        /// Returns whether the wait was known to have timed out.
        #[stable(feature = "wait_timeout", since = "1.5.0")]
        pub fn timed_out(&self) -> bool {
            self.0
        }
    }

    /// A Condition Variable
    ///
    /// Condition variables represent the ability to block a thread such that it
    /// consumes no CPU time while waiting for an event to occur. Condition
    /// variables are typically associated with a boolean predicate (a condition)
    /// and a mutex. The predicate is always verified inside of the mutex before
    /// determining that thread must block.
    ///
    /// Functions in this module will block the current **thread** of execution and
    /// are bindings to system-provided condition variables where possible. Note
    /// that this module places one additional restriction over the system condition
    /// variables: each condvar can be used with precisely one mutex at runtime. Any
    /// attempt to use multiple mutexes on the same condition variable will result
    /// in a runtime panic. If this is not desired, then the unsafe primitives in
    /// `sys` do not have this restriction but may result in undefined behavior.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Mutex, Condvar};
    /// use std::thread;
    ///
    /// let pair = Arc::new((Mutex::new(false), Condvar::new()));
    /// let pair2 = pair.clone();
    ///
    /// // Inside of our lock, spawn a new thread, and then wait for it to start
    /// thread::spawn(move|| {
    ///     let &(ref lock, ref cvar) = &*pair2;
    ///     let mut started = lock.lock().unwrap();
    ///     *started = true;
    ///     cvar.notify_one();
    /// });
    ///
    /// // wait for the thread to start up
    /// let &(ref lock, ref cvar) = &*pair;
    /// let mut started = lock.lock().unwrap();
    /// while !*started {
    ///     started = cvar.wait(started).unwrap();
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Condvar {
        inner: Box<sys::Condvar>,
        mutex: AtomicUsize,
    }

    impl Condvar {
        /// Creates a new condition variable which is ready to be waited on and
        /// notified.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new() -> Condvar {
            let mut c = Condvar {
                inner: box sys::Condvar::new(),
                mutex: AtomicUsize::new(0),
            };
            unsafe {
                c.inner.init();
            }
            c
        }

        /// Blocks the current thread until this condition variable receives a
        /// notification.
        ///
        /// This function will atomically unlock the mutex specified (represented by
        /// `mutex_guard`) and block the current thread. This means that any calls
        /// to `notify_*()` which happen logically after the mutex is unlocked are
        /// candidates to wake this thread up. When this function call returns, the
        /// lock specified will have been re-acquired.
        ///
        /// Note that this function is susceptible to spurious wakeups. Condition
        /// variables normally have a boolean predicate associated with them, and
        /// the predicate must always be checked each time this function returns to
        /// protect against spurious wakeups.
        ///
        /// # Errors
        ///
        /// This function will return an error if the mutex being waited on is
        /// poisoned when this thread re-acquires the lock. For more information,
        /// see information about poisoning on the Mutex type.
        ///
        /// # Panics
        ///
        /// This function will `panic!()` if it is used with more than one mutex
        /// over time. Each condition variable is dynamically bound to exactly one
        /// mutex to ensure defined behavior across platforms. If this functionality
        /// is not desired, then unsafe primitives in `sys` are provided.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn wait<'a, T>(&self, guard: MutexGuard<'a, T>)
                           -> LockResult<MutexGuard<'a, T>> {
            let poisoned = unsafe {
                let lock = mutex::guard_lock(&guard);
                self.verify(lock);
                self.inner.wait(lock);
                mutex::guard_poison(&guard).get()
            };
            if poisoned {
                Err(PoisonError::new(guard))
            } else {
                Ok(guard)
            }
        }

        /// Waits on this condition variable for a notification, timing out after a
        /// specified duration.
        ///
        /// The semantics of this function are equivalent to `wait()`
        /// except that the thread will be blocked for roughly no longer
        /// than `ms` milliseconds. This method should not be used for
        /// precise timing due to anomalies such as preemption or platform
        /// differences that may not cause the maximum amount of time
        /// waited to be precisely `ms`.
        ///
        /// Note that the best effort is made to ensure that the time waited is
        /// measured with a monotonic clock, and not affected by the changes made to
        /// the system time.
        ///
        /// The returned boolean is `false` only if the timeout is known
        /// to have elapsed.
        ///
        /// Like `wait`, the lock specified will be re-acquired when this function
        /// returns, regardless of whether the timeout elapsed or not.
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_deprecated(since = "1.6.0", reason = "replaced by `std::sync::Condvar::wait_timeout`")]
        pub fn wait_timeout_ms<'a, T>(&self, guard: MutexGuard<'a, T>, ms: u32)
                                      -> LockResult<(MutexGuard<'a, T>, bool)> {
            let res = self.wait_timeout(guard, Duration::from_millis(ms as u64));
            poison::map_result(res, |(a, b)| {
                (a, !b.timed_out())
            })
        }

        /// Waits on this condition variable for a notification, timing out after a
        /// specified duration.
        ///
        /// The semantics of this function are equivalent to `wait()` except that
        /// the thread will be blocked for roughly no longer than `dur`. This
        /// method should not be used for precise timing due to anomalies such as
        /// preemption or platform differences that may not cause the maximum
        /// amount of time waited to be precisely `dur`.
        ///
        /// Note that the best effort is made to ensure that the time waited is
        /// measured with a monotonic clock, and not affected by the changes made to
        /// the system time.
        ///
        /// The returned `WaitTimeoutResult` value indicates if the timeout is
        /// known to have elapsed.
        ///
        /// Like `wait`, the lock specified will be re-acquired when this function
        /// returns, regardless of whether the timeout elapsed or not.
        #[stable(feature = "wait_timeout", since = "1.5.0")]
        pub fn wait_timeout<'a, T>(&self, guard: MutexGuard<'a, T>,
                                   dur: Duration)
                                   -> LockResult<(MutexGuard<'a, T>, WaitTimeoutResult)> {
            let (poisoned, result) = unsafe {
                let lock = mutex::guard_lock(&guard);
                self.verify(lock);
                let success = self.inner.wait_timeout(lock, dur);
                (mutex::guard_poison(&guard).get(), WaitTimeoutResult(!success))
            };
            if poisoned {
                Err(PoisonError::new((guard, result)))
            } else {
                Ok((guard, result))
            }
        }

        /// Wakes up one blocked thread on this condvar.
        ///
        /// If there is a blocked thread on this condition variable, then it will
        /// be woken up from its call to `wait` or `wait_timeout`. Calls to
        /// `notify_one` are not buffered in any way.
        ///
        /// To wake up all threads, see `notify_all()`.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn notify_one(&self) {
            unsafe { self.inner.notify_one() }
        }

        /// Wakes up all blocked threads on this condvar.
        ///
        /// This method will ensure that any current waiters on the condition
        /// variable are awoken. Calls to `notify_all()` are not buffered in any
        /// way.
        ///
        /// To wake up only one thread, see `notify_one()`.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn notify_all(&self) {
            unsafe { self.inner.notify_all() }
        }

        fn verify(&self, mutex: &sys_mutex::Mutex) {
            let addr = mutex as *const _ as usize;
            match self.mutex.compare_and_swap(0, addr, Ordering::SeqCst) {
                // If we got out 0, then we have successfully bound the mutex to
                // this cvar.
                0 => {}

                // If we get out a value that's the same as `addr`, then someone
                // already beat us to the punch.
                n if n == addr => {}

                // Anything else and we're using more than one mutex on this cvar,
                // which is currently disallowed.
                _ => panic!("attempted to use a condition variable with two \
                             mutexes"),
            }
        }
    }

    #[stable(feature = "std_debug", since = "1.15.0")]
    impl fmt::Debug for Condvar {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Condvar { .. }")
        }
    }

    #[stable(feature = "condvar_default", since = "1.9.0")]
    impl Default for Condvar {
        /// Creates a `Condvar` which is ready to be waited on and notified.
        fn default() -> Condvar {
            Condvar::new()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Drop for Condvar {
        fn drop(&mut self) {
            unsafe { self.inner.destroy() }
        }
    }

    #[cfg(test)]
    mod tests {
        use sync::mpsc::channel;
        use sync::{Condvar, Mutex, Arc};
        use thread;
        use time::Duration;
        use u32;

        #[test]
        fn smoke() {
            let c = Condvar::new();
            c.notify_one();
            c.notify_all();
        }

        #[test]
        #[cfg_attr(target_os = "emscripten", ignore)]
        fn notify_one() {
            let m = Arc::new(Mutex::new(()));
            let m2 = m.clone();
            let c = Arc::new(Condvar::new());
            let c2 = c.clone();

            let g = m.lock().unwrap();
            let _t = thread::spawn(move|| {
                let _g = m2.lock().unwrap();
                c2.notify_one();
            });
            let g = c.wait(g).unwrap();
            drop(g);
        }

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

            let data = Arc::new((Mutex::new(0), Condvar::new()));
            let (tx, rx) = channel();
            for _ in 0..N {
                let data = data.clone();
                let tx = tx.clone();
                thread::spawn(move|| {
                    let &(ref lock, ref cond) = &*data;
                    let mut cnt = lock.lock().unwrap();
                    *cnt += 1;
                    if *cnt == N {
                        tx.send(()).unwrap();
                    }
                    while *cnt != 0 {
                        cnt = cond.wait(cnt).unwrap();
                    }
                    tx.send(()).unwrap();
                });
            }
            drop(tx);

            let &(ref lock, ref cond) = &*data;
            rx.recv().unwrap();
            let mut cnt = lock.lock().unwrap();
            *cnt = 0;
            cond.notify_all();
            drop(cnt);

            for _ in 0..N {
                rx.recv().unwrap();
            }
        }

        #[test]
        #[cfg_attr(target_os = "emscripten", ignore)]
        fn wait_timeout_ms() {
            let m = Arc::new(Mutex::new(()));
            let m2 = m.clone();
            let c = Arc::new(Condvar::new());
            let c2 = c.clone();

            let g = m.lock().unwrap();
            let (g, _no_timeout) = c.wait_timeout(g, Duration::from_millis(1)).unwrap();
            // spurious wakeups mean this isn't necessarily true
            // assert!(!no_timeout);
            let _t = thread::spawn(move || {
                let _g = m2.lock().unwrap();
                c2.notify_one();
            });
            let (g, timeout_res) = c.wait_timeout(g, Duration::from_millis(u32::MAX as u64)).unwrap();
            assert!(!timeout_res.timed_out());
            drop(g);
        }

        #[test]
        #[should_panic]
        #[cfg_attr(target_os = "emscripten", ignore)]
        fn two_mutexes() {
            let m = Arc::new(Mutex::new(()));
            let m2 = m.clone();
            let c = Arc::new(Condvar::new());
            let c2 = c.clone();

            let mut g = m.lock().unwrap();
            let _t = thread::spawn(move|| {
                let _g = m2.lock().unwrap();
                c2.notify_one();
            });
            g = c.wait(g).unwrap();
            drop(g);

            let m = Mutex::new(());
            let _ = c.wait(m.lock().unwrap()).unwrap();
        }
    }

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