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//! A collection of synchronization primitives that build on the primitives available in the //! standard library. //! //! This library contains the following special-purpose synchronization primitives: //! //! * [`CountdownEvent`], a primitive that keeps a counter and allows a thread to wait until the //! counter reaches zero. //! * [`SignalEvent`], a primitive that allows one or more threads to wait on a signal from another //! thread. //! * [`WriterReaderPhaser`], a primitive that allows multiple wait-free "writer critical sections" //! against a "reader phase flip" that waits for currently-active writers to finish. //! //! [`CountdownEvent`]: struct.CountdownEvent.html //! [`SignalEvent`]: struct.SignalEvent.html //! [`WriterReaderPhaser`]: struct.WriterReaderPhaser.html #![deny(warnings, missing_docs)] //Name source: http://bulbapedia.bulbagarden.net/wiki/Synchronoise_(move) extern crate crossbeam; use std::sync::{Arc, Mutex, LockResult, MutexGuard}; use std::sync::atomic::{AtomicBool, AtomicUsize, AtomicIsize, Ordering}; use std::isize::MIN as ISIZE_MIN; use std::time::Duration; use std::thread; use crossbeam::sync::MsQueue; /// A synchronization primitive that signals when its count reaches zero. /// /// With a `CountdownEvent`, it's possible to cause one thread to wait on a set of computations /// occurring in other threads by making the other threads interact with the counter as they perform /// their work. /// /// The main limitation of a CountdownEvent is that once its counter reaches zero (even by starting /// there), any attempts to update the counter will return `CountdownError::AlreadySet` until the /// counter is reset by calling `reset` or `reset_to_count`. /// /// `CountdownEvent` is a port of [System.Threading.CountdownEvent][src-link] from .NET (also /// called `CountDownLatch` in Java). /// /// [src-link]: https://msdn.microsoft.com/en-us/library/system.threading.countdownevent(v=vs.110).aspx /// /// # Example /// /// This example uses a `CountdownEvent` to make the "coordinator" thread sleep until all of its /// "worker" threads have finished. Each thread calls `signal.decrement()` to signal to the Event /// that its work has completed. When the last thread does this (and brings the counter to zero), /// the "coordinator" thread wakes up and prints `all done!`. /// /// ``` /// use synchronoise::CountdownEvent; /// use std::sync::Arc; /// use std::thread; /// use std::time::Duration; /// /// let thread_count = 5; /// let counter = Arc::new(CountdownEvent::new(thread_count)); /// /// for i in 0..thread_count { /// let signal = counter.clone(); /// thread::spawn(move || { /// thread::sleep(Duration::from_secs(i as u64)); /// println!("thread {} activated!", i); /// signal.decrement().unwrap(); /// }); /// } /// /// counter.wait(); /// /// println!("all done!"); /// ``` pub struct CountdownEvent { initial: usize, counter: AtomicUsize, waiting: MsQueue<thread::Thread>, } ///The collection of errors that can be returned by [`CountdownEvent`] methods. /// ///See [`CountdownEvent`] for more details. /// ///[`CountdownEvent`]: struct.CountdownEvent.html #[derive(Debug, Copy, Clone, PartialEq, Eq)] pub enum CountdownError { ///Returned when adding to a counter would have caused it to overflow. SaturatedCounter, ///Returned when attempting to signal would have caused the counter to go below zero. TooManySignals, ///Returned when attempting to modify the counter after it has reached zero. AlreadySet, } impl CountdownEvent { ///Creates a new `CountdownEvent`, initialized to the given count. /// ///Remember that once the counter reaches zero, calls to `add` or `signal` will fail, so ///passing zero to this function will create a `CountdownEvent` that is permanently signaled. pub fn new(count: usize) -> CountdownEvent { CountdownEvent { initial: count, counter: AtomicUsize::new(count), waiting: MsQueue::new(), } } ///Resets the counter to the count given to `new`. /// ///This function is safe because the `&mut self` enforces that no other references or locks ///exist. pub fn reset(&mut self) { self.counter = AtomicUsize::new(self.initial); //there shouldn't be any remaining thread handles in here, but let's clear it out anyway while let Some(thread) = self.waiting.try_pop() { thread.unpark(); } } ///Resets the counter to the given count. /// ///This function is safe because the `&mut self` enforces that no other references or locks ///exist. pub fn reset_to_count(&mut self, count: usize) { self.initial = count; self.reset(); } ///Returns the current counter value. pub fn count(&self) -> usize { self.counter.load(Ordering::SeqCst) } ///Adds the given count to the counter. /// ///# Errors /// ///If the counter is already at zero, this function will return `CountdownError::AlreadySet`. /// ///If the given count would cause the counter to overflow `usize`, this function will return ///`CountdownError::SaturatedCounter`. pub fn add(&self, count: usize) -> Result<(), CountdownError> { let mut current = self.count(); loop { if current == 0 { return Err(CountdownError::AlreadySet); } if let Some(new_count) = current.checked_add(count) { let last_count = self.counter.compare_and_swap(current, new_count, Ordering::SeqCst); if last_count == current { return Ok(()); } else { current = last_count; } } else { return Err(CountdownError::SaturatedCounter); } } } ///Subtracts the given count to the counter, and returns whether this caused any waiting ///threads to wake up. /// ///# Errors /// ///If the counter is already at zero, this function will return `CountdownError::AlreadySet`. /// ///If the given count would cause the counter to go *below* zero (instead of reaching zero), ///this function will return `CountdownError::TooManySignals`. pub fn signal(&self, count: usize) -> Result<bool, CountdownError> { let mut current = self.count(); loop { if current == 0 { return Err(CountdownError::AlreadySet); } if let Some(new_count) = current.checked_sub(count) { let last_count = self.counter.compare_and_swap(current, new_count, Ordering::SeqCst); if last_count == current { current = new_count; break; } else { current = last_count; } } else { return Err(CountdownError::TooManySignals); } } if current == 0 { while let Some(thread) = self.waiting.try_pop() { thread.unpark(); } Ok(true) } else { Ok(false) } } ///Adds one to the count. /// ///# Errors /// ///See [`add`] for the situations where this function will return an error. /// ///[`add`]: #method.add pub fn increment(&self) -> Result<(), CountdownError> { self.add(1) } ///Subtracts one from the counter, and returns whether this caused any waiting threads to wake ///up. /// ///# Errors /// ///See [`signal`] for the situations where this function will return an error. /// ///[`signal`]: #method.signal pub fn decrement(&self) -> Result<bool, CountdownError> { self.signal(1) } /// Increments the counter, then returns a guard object that will decrement the counter upon /// drop. /// /// # Errors /// /// This function will return the same errors as `add`. If the event has already signaled by the /// time the guard is dropped (and would cause its `decrement` call to return an error), then /// the error will be silently ignored. /// /// # Example /// /// Here's the sample from the main docs, using `CountdownGuard`s instead of manually /// decrementing: /// /// ``` /// use synchronoise::CountdownEvent; /// use std::sync::Arc; /// use std::thread; /// use std::time::Duration; /// /// let thread_count = 5; /// //counter can't start from zero, but the guard increments on its own, so start at one and /// //just decrement once when we're ready to wait /// let counter = Arc::new(CountdownEvent::new(1)); /// /// for i in 0..thread_count { /// let signal = counter.clone(); /// thread::spawn(move || { /// let _guard = signal.guard().unwrap(); /// thread::sleep(Duration::from_secs(i)); /// println!("thread {} activated!", i); /// }); /// } /// /// //give all the threads time to increment the counter before continuing /// thread::sleep(Duration::from_millis(100)); /// counter.decrement().unwrap(); /// counter.wait(); /// /// println!("all done!"); /// ``` pub fn guard(&self) -> Result<CountdownGuard, CountdownError> { CountdownGuard::new(self) } ///Blocks the current thread until the counter reaches zero. /// ///This function will block indefinitely until the counter reaches zero. It will return ///immediately if it is already at zero. pub fn wait(&self) { //see SignalEvent::wait for why we push first even if the count is already set self.waiting.push(thread::current()); let mut first = true; while self.count() > 0 { if first { first = false; } else { self.waiting.push(thread::current()); } thread::park(); } } ///Blocks the current thread until the timer reaches zero, or until the given timeout elapses, ///returning the count at the time of wakeup. /// ///This function will return immediately if the counter was already at zero. Otherwise, it will ///block for roughly no longer than `timeout`, or when the counter reaches zero, whichever ///comes first. pub fn wait_timeout(&self, timeout: Duration) -> usize { use std::time::Instant; //see SignalEvent::wait for why we push first even if the count is already set self.waiting.push(thread::current()); let begin = Instant::now(); let mut first = true; let mut remaining = timeout; loop { let current = self.count(); if current == 0 { return 0; } if first { first = false; } else { let elapsed = begin.elapsed(); if elapsed >= timeout { return current; } else { remaining = timeout - elapsed; } self.waiting.push(thread::current()); } thread::park_timeout(remaining); } } } ///An opaque guard struct that decrements the count of a borrowed `CountdownEvent` on drop. /// ///See [`CountdownEvent::guard`] for more information about this struct. /// ///[`CountdownEvent::guard`]: struct.CountdownEvent.html#method.guard pub struct CountdownGuard<'a> { event: &'a CountdownEvent, } impl<'a> CountdownGuard<'a> { fn new(event: &'a CountdownEvent) -> Result<CountdownGuard<'a>, CountdownError> { try!(event.increment()); Ok(CountdownGuard { event: event, }) } } ///Upon drop, this guard will decrement the counter of its parent `CountdownEvent`. If this would ///cause an error (see [`CountdownEvent::signal`] for details), the error is silently ignored. /// ///[`CountdownEvent::signal`]: struct.CountdownEvent.html#method.signal impl<'a> Drop for CountdownGuard<'a> { fn drop(&mut self) { //if decrement() returns an error, then the event has already been signaled somehow. i'm //not gonna care about it tho self.event.decrement().ok(); } } ///Determines the reset behavior of a [`SignalEvent`]. /// ///See [`SignalEvent`] for more information. /// ///[`SignalEvent`]: struct.SignalEvent.html #[derive(Debug, PartialEq, Copy, Clone)] pub enum SignalKind { ///An activated `SignalEvent` automatically resets when a thread is resumed. /// ///`SignalEvent`s with this kind will only resume one thread at a time. Auto, ///An activated `SignalEvent` must be manually reset to block threads again. /// ///`SignalEvent`s with this kind will signal every waiting thread to continue at once. Manual, } /// A synchronization primitive that allows one or more threads to wait on a signal from another /// thread. /// /// With a `SignalEvent`, it's possible to have one or more threads gate on a signal from another /// thread. The behavior for what happens when an event is signaled depends on the value of the /// `signal_kind` parameter given to `new`: /// /// * A value of `SignalKind::Auto` will automatically reset the signal when a thread is resumed by /// this event. If more than one thread is waiting on the event when it is signaled, only one /// will be resumed. /// * A value of `SignalKind::Manual` will remain signaled until it is manually reset. If more than /// one thread is waiting on the event when it is signaled, all of them will be resumed. Any /// other thread that tries to wait on the signal before it is reset will not be blocked at all. /// /// `SignalEvent` is a port of [System.Threading.EventWaitHandle][src-link] from .NET. /// /// [src-link]: https://msdn.microsoft.com/en-us/library/system.threading.eventwaithandle(v=vs.110).aspx /// /// # Example /// /// The following example uses two `SignalEvent`s: /// /// * `start_signal` is used as a kind of `std::sync::Barrier`, that keeps all the threads inside /// the loop from starting until they all have been spawned. All the `start.wait()` calls resume /// when `start_signal.signal()` is called after the initial loop. /// * Note that because the "coordinator" doesn't wait for each thread to be scheduled before /// signaling, it's possible that some later threads may not have had a chance to enter /// `start.wait()` before the signal is set. In this case they won't block in the first place, /// and immediately return. /// * `stop_signal` is used to wake up the "coordinator" thread when each "worker" thread is /// finished with its work. This allows it to keep a count of the number of threads yet to /// finish, so it can exit its final loop when all the threads have stopped. /// /// ``` /// use synchronoise::{SignalEvent, SignalKind}; /// use std::sync::Arc; /// use std::thread; /// use std::time::Duration; /// /// let start_signal = Arc::new(SignalEvent::new(false, SignalKind::Manual)); /// let stop_signal = Arc::new(SignalEvent::new(false, SignalKind::Auto)); /// let mut thread_count = 5; /// /// for i in 0..thread_count { /// let start = start_signal.clone(); /// let stop = stop_signal.clone(); /// thread::spawn(move || { /// //as a Manual-reset signal, all the threads will start at the same time /// start.wait(); /// thread::sleep(Duration::from_secs(i)); /// println!("thread {} activated!", i); /// stop.signal(); /// }); /// } /// /// start_signal.signal(); /// /// while thread_count > 0 { /// //as an Auto-reset signal, this will automatically reset when resuming /// //so when the loop comes back, we don't have to reset before blocking again /// stop_signal.wait(); /// thread_count -= 1; /// } /// /// println!("all done!"); /// ``` pub struct SignalEvent { reset: SignalKind, signal: AtomicBool, waiting: MsQueue<thread::Thread>, } impl SignalEvent { ///Creates a new `SignalEvent` with the given starting state and reset behavior. /// ///If `init_state` is `true`, then this `SignalEvent` will start with the signal already set, ///so that threads that wait will immediately unblock. pub fn new(init_state: bool, signal_kind: SignalKind) -> SignalEvent { SignalEvent { reset: signal_kind, signal: AtomicBool::new(init_state), waiting: MsQueue::new(), } } ///Returns the current signal status of the `SignalEvent`. pub fn status(&self) -> bool { self.signal.load(Ordering::SeqCst) } ///Sets the signal on this `SignalEvent`, potentially waking up one or all threads waiting on ///it. /// ///If more than one thread is waiting on the event, the behavior is different depending on the ///`SignalKind` passed to the event when it was created. For a value of `Auto`, one thread will ///be resumed. For a value of `Manual`, all waiting threads will be resumed. /// ///If no thread is currently waiting on the event, its state will be set regardless. Any future ///attempts to wait on the event will unblock immediately, except for a `SignalKind` of Auto, ///which will immediately unblock the first thread only. pub fn signal(&self) { self.signal.store(true, Ordering::SeqCst); match self.reset { //there may be duplicate handles in the queue due to spurious wakeups, so just loop //until we know the signal got reset - any that got woken up wrongly will also observe //the reset signal and push their handle back in SignalKind::Auto => while self.signal.load(Ordering::SeqCst) { if let Some(thread) = self.waiting.try_pop() { thread.unpark(); } else { break; } }, //for manual resets, just unilaterally drain the queue SignalKind::Manual => while let Some(thread) = self.waiting.try_pop() { thread.unpark(); } } } ///Resets the signal on this `SignalEvent`, allowing threads that wait on it to block. pub fn reset(&self) { self.signal.store(false, Ordering::SeqCst); } ///Blocks this thread until another thread calls `signal`. /// ///If this event is already set, then this function will immediately return without blocking. ///For events with a `SignalKind` of `Auto`, this will reset the signal so that the next thread ///to wait will block. pub fn wait(&self) { //Push first, regardless, because in SignalEvent's doctest there's a thorny race condition //where (1) the waiting thread will see an unset signal, (2) the signalling thread will set //the signal and drain the queue, and only then (3) the waiting thread will push its //handle. Having erroneous handles is ultimately harmless from a correctness standpoint //because signal loops properly anyway, and if the park handle is already set when a thread //tries to wait it will just immediately unpark, see that the signal is still unset, and //park again. Shame about those spent cycles dealing with it though. self.waiting.push(thread::current()); //loop on the park in case we spuriously wake up let mut first = true; while !self.check_signal() { //push every time in case there's a race between `signal` and this, since on //`SignalKind::Auto` it will loop until someone turns it off - but only one will //actually exit this loop, because `check_signal` does a CAS if first { first = false; } else { self.waiting.push(thread::current()); } thread::park(); } } ///Blocks this thread until either another thread calls `signal`, or until the timeout elapses. /// ///This function returns the status of the signal when it woke up. If this function exits ///because the signal was set, and this event has a `SignalKind` of `Auto`, the signal will be ///reset so that the next thread to wait will block. pub fn wait_timeout(&self, timeout: Duration) -> bool { use std::time::Instant; //see SignalEvent::wait for why we push first even if the signal is already set self.waiting.push(thread::current()); let begin = Instant::now(); let mut first = true; let mut remaining = timeout; loop { if self.check_signal() { return true; } if first { first = false; } else { let elapsed = begin.elapsed(); if elapsed >= timeout { return self.status(); } else { remaining = timeout - elapsed; } self.waiting.push(thread::current()); } thread::park_timeout(remaining); } } ///Perfoms an atomic compare-and-swap on the signal, resetting it if (1) it was set, and (2) ///this `SignalEvent` was configured with `SignalKind::Auto`. Returns whether the signal was ///previously set. fn check_signal(&self) -> bool { self.signal.compare_and_swap(true, self.reset == SignalKind::Manual, Ordering::SeqCst) } } /// A synchronization primitive that allows for multiple concurrent wait-free "writer critical /// sections" and a "reader phase flip" that can wait for all currently-active writers to finish. /// /// The basic interaction setup for a `WriterReaderPhaser` is as follows: /// /// * Any number of writers can open and close a "writer critical section" with no waiting. /// * Zero or one readers can be active at one time, by holding a "read lock". Any reader who /// wishes to open a "read lock" while another one is active is blocked until the previous one /// finishes. /// * The holder of a read lock may request a "phase flip", which causes the reader to wait until /// all current writer critical sections are finished before continuing. /// /// `WriterReaderPhaser` is a port of the primitive of the same name from `HdrHistogram`. For a /// summary of the rationale behind its design, see [this post by its author][wrp-blog]. Part of /// its assumptions is that this primitive is synchronizing access to a double-buffered set of /// counters, and the readers are expected to swap the buffers while holding a read lock but before /// flipping the phase. This allows them to access a stable sample to read and perform calculations /// from, while writers still have wait-free synchronization. /// /// [wrp-blog]: https://stuff-gil-says.blogspot.com/2014/11/writerreaderphaser-story-about-new.html /// /// "Writer critical sections" and "read locks" are represented by guard structs that allow /// scope-based resource management of the counters and locks. /// /// * The `PhaserCriticalSection` atomically increments and decrements the phase counters upon /// creation and drop. These operations use `std::sync::atomic::AtomicIsize` from the standard /// library, and provide no-wait handling for platforms with atomic addition instructions. /// * The `PhaserReadLock` is kept in the `WriterReaderPhaser` as a Mutex, enforcing the mutual /// exclusion of the read lock. The "phase flip" operation is defined on the read lock guard /// itself, enforcing that only the holder of a read lock can execute one. pub struct WriterReaderPhaser { start_epoch: Arc<AtomicIsize>, even_end_epoch: Arc<AtomicIsize>, odd_end_epoch: Arc<AtomicIsize>, read_lock: Mutex<PhaserReadLock>, } /// Guard struct that represents a "writer critical section" for a `WriterReaderPhaser`. /// /// `PhaserCriticalSection` is a scope-based guard to signal the beginning and end of a "writer /// critical section" to the phaser. Upon calling `writer_critical_section`, the phaser atomically /// increments a counter, and when the returned `PhaserCriticalSection` drops, the `drop` call /// atomically increments another counter. On platforms with atomic increment instructions, this /// should result in wait-free synchronization. /// /// # Example /// /// ``` /// # let phaser = synchronoise::WriterReaderPhaser::new(); /// { /// let _guard = phaser.writer_critical_section(); /// // perform writes /// } // _guard drops, signaling the end of the section /// ``` pub struct PhaserCriticalSection { end_epoch: Arc<AtomicIsize>, } ///Upon drop, a `PhaserCriticalSection` will signal its parent `WriterReaderPhaser` that the ///critical section has ended. impl Drop for PhaserCriticalSection { fn drop(&mut self) { self.end_epoch.fetch_add(1, Ordering::Release); } } /// Guard struct for a `WriterReaderPhaser` that allows a reader to perform a "phase flip". /// /// The `PhaserReadLock` struct allows one to perform a "phase flip" on its parent /// `WriterReaderPhaser`. It is held in a `std::sync::Mutex` in its parent phaser, enforcing that /// only one reader may be active at once. /// /// The `flip_phase` call performs a spin-wait while waiting the the currently-active writers to /// finish. A sleep time may be added between checks by calling `flip_with_sleep` instead. /// /// # Example /// /// ``` /// # let phaser = synchronoise::WriterReaderPhaser::new(); /// { /// let lock = phaser.read_lock().unwrap(); /// // swap buffers /// lock.flip_phase(); /// // reader now has access to a stable snapshot /// } // lock drops, relinquishing the read lock and allowing another reader to lock /// ``` pub struct PhaserReadLock { start_epoch: Arc<AtomicIsize>, even_end_epoch: Arc<AtomicIsize>, odd_end_epoch: Arc<AtomicIsize>, } impl WriterReaderPhaser { ///Creates a new `WriterReaderPhaser`. pub fn new() -> WriterReaderPhaser { let start = Arc::new(AtomicIsize::new(0)); let even = Arc::new(AtomicIsize::new(0)); let odd = Arc::new(AtomicIsize::new(ISIZE_MIN)); let read_lock = PhaserReadLock { start_epoch: start.clone(), even_end_epoch: even.clone(), odd_end_epoch: odd.clone(), }; WriterReaderPhaser { start_epoch: start, even_end_epoch: even, odd_end_epoch: odd, read_lock: Mutex::new(read_lock), } } ///Enters a writer critical section, returning a guard object that signals the end of the ///critical section upon drop. pub fn writer_critical_section(&self) -> PhaserCriticalSection { let flag = self.start_epoch.fetch_add(1, Ordering::Release); if flag < 0 { PhaserCriticalSection { end_epoch: self.odd_end_epoch.clone(), } } else { PhaserCriticalSection { end_epoch: self.even_end_epoch.clone(), } } } ///Enter a reader criticial section, potentially blocking until a currently active read section ///finishes. Returns a guard object that allows the user to flip the phase of the ///`WriterReaderPhaser`, and unlocks the read lock upon drop. /// ///# Errors /// ///If another reader critical section panicked while holding the read lock, this call will ///return an error once the lock is acquired. See the documentation for ///`std::sync::Mutex::lock` for details. pub fn read_lock(&self) -> LockResult<MutexGuard<PhaserReadLock>> { self.read_lock.lock() } } impl PhaserReadLock { ///Wait until all currently-active writer critical sections have completed. pub fn flip_phase(&self) { self.flip_with_sleep(Duration::default()); } ///Wait until all currently-active writer critical sections have completed. While waiting, ///sleep with the given duration. pub fn flip_with_sleep(&self, sleep_time: Duration) { let next_phase_even = self.start_epoch.load(Ordering::Relaxed) < 0; let start_value = if next_phase_even { let tmp = 0; self.even_end_epoch.store(tmp, Ordering::Relaxed); tmp } else { let tmp = ISIZE_MIN; self.odd_end_epoch.store(tmp, Ordering::Relaxed); tmp }; let value_at_flip = self.start_epoch.swap(start_value, Ordering::AcqRel); let end_epoch = if next_phase_even { self.odd_end_epoch.clone() } else { self.even_end_epoch.clone() }; while end_epoch.load(Ordering::Relaxed) != value_at_flip { if sleep_time == Duration::default() { thread::yield_now(); } else { thread::sleep(sleep_time); } } } }