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//! Notify async tasks or threads. //! //! This is a synchronization primitive similar to [eventcounts] invented by Dmitry Vyukov. //! //! You can use this crate to turn non-blocking data structures into async or blocking data //! structures. See a [simple mutex] implementation that exposes an async and a blocking interface //! for acquiring locks. //! //! [eventcounts]: http://www.1024cores.net/home/lock-free-algorithms/eventcounts //! [simple mutex]: https://github.com/stjepang/event-listener/blob/master/examples/mutex.rs //! //! # Examples //! //! Wait until another thread sets a boolean flag: //! //! ``` //! use std::sync::atomic::{AtomicBool, Ordering}; //! use std::sync::Arc; //! use std::thread; //! use std::time::Duration; //! use event_listener::Event; //! //! let flag = Arc::new(AtomicBool::new(false)); //! let event = Arc::new(Event::new()); //! //! // Spawn a thread that will set the flag after 1 second. //! thread::spawn({ //! let flag = flag.clone(); //! let event = event.clone(); //! move || { //! // Wait for a second. //! thread::sleep(Duration::from_secs(1)); //! //! // Set the flag. //! flag.store(true, Ordering::SeqCst); //! //! // Notify all listeners that the flag has been set. //! event.notify_all(); //! } //! }); //! //! // Wait until the flag is set. //! loop { //! // Check the flag. //! if flag.load(Ordering::SeqCst) { //! break; //! } //! //! // Start listening for events. //! let listener = event.listen(); //! //! // Check the flag again after creating the listener. //! if flag.load(Ordering::SeqCst) { //! break; //! } //! //! // Wait for a notification and continue the loop. //! listener.wait(); //! } //! ``` #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)] use std::cell::Cell; use std::fmt; use std::future::Future; use std::mem::{self, ManuallyDrop}; use std::ops::{Deref, DerefMut}; use std::pin::Pin; use std::ptr::NonNull; use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering}; use std::sync::{Arc, Mutex, MutexGuard}; use std::task::{Context, Poll, Waker}; use std::thread::{self, Thread}; use std::time::{Duration, Instant}; /// Inner state of [`Event`]. struct Inner { /// The number of notified entries, or `usize::MAX` if all of them have been notified. /// /// If there are no entries, this value is set to `usize::MAX`. notified: AtomicUsize, /// A linked list holding registered listeners. list: Mutex<List>, } impl Inner { /// Locks the list. fn lock(&self) -> ListGuard<'_> { ListGuard { inner: self, guard: self.list.lock().unwrap(), } } } /// A synchronization primitive for notifying async tasks and threads. /// /// Listeners can be registered using [`Event::listen()`]. There are threifour ways of notifying /// listeners: /// /// 1. [`Event::notify_one()`] notifies one listener. /// 2. [`Event::notify_all()`] notifies all listeners. /// 3. [`Event::notify()`] notifies an arbitrary number of listeners. /// 4. [`Event::notify_additional()`] notifies an arbitrary number of unnotified listeners. /// /// If there are no active listeners at the time a notification is sent, it simply gets lost. /// /// Note that [`Event::notify_one()`] does not notify one *additional* listener - it only makes /// sure *at least* one listener among the active ones is notified. Similarly, [`Event::notify()`] /// makes sure a number of active listeners are notified. If you need to notify additional /// listeners, use [`Event::notify_additional()`]. /// /// There are two ways for a listener to wait for a notification: /// /// 1. In an asynchronous manner using `.await`. /// 2. In a blocking manner by calling [`EventListener::wait()`] on it. /// /// If a notified listener is dropped without receiving a notification, dropping will notify /// another active listener. /// /// Listeners are registered and notified in the first-in first-out fashion, ensuring fairness. pub struct Event { /// A pointer to heap-allocated inner state. /// /// This pointer is initially null and gets lazily initialized on first use. Semantically, it /// is an `Arc<Inner>` so it's important to keep in mind that it contributes to the [`Arc`]'s /// reference count. inner: AtomicPtr<Inner>, } unsafe impl Send for Event {} unsafe impl Sync for Event {} impl Event { /// Creates a new [`Event`]. /// /// # Examples /// /// ``` /// use event_listener::Event; /// /// let event = Event::new(); /// ``` #[inline] pub fn new() -> Event { Event { inner: AtomicPtr::default(), } } /// Returns a guard listening for a notification. /// /// # Examples /// /// ``` /// use event_listener::Event; /// /// let event = Event::new(); /// let listener = event.listen(); /// ``` #[cold] pub fn listen(&self) -> EventListener { let inner = self.inner(); let listener = EventListener { inner: unsafe { Arc::clone(&ManuallyDrop::new(Arc::from_raw(inner))) }, entry: Some(inner.lock().insert()), }; // Make sure the listener is registered before whatever happens next. full_fence(); listener } /// Notifies a number of active listeners. /// /// The number is allowed to be zero or exceed the current number of listeners. /// /// Note that this does not notify `n` *additional* listeners - it only makes sure *at least* /// `n` listeners among the active ones are notified. /// /// # Examples /// /// ``` /// use event_listener::Event; /// /// let event = Event::new(); /// /// // This notification gets lost because there are no listeners. /// event.notify(1); /// /// let listener1 = event.listen(); /// let listener2 = event.listen(); /// let listener3 = event.listen(); /// /// // Notifies two listeners. /// // /// // Listener queueing is fair, which means `listener1` and `listener2` /// // get notified here since they start listening before `listener3`. /// event.notify_one(); /// ``` #[inline] pub fn notify(&self, n: usize) { let inner = self.inner(); // Make sure the notification comes after whatever triggered it. full_fence(); // Notify if there is at least one unnotified listener and the number of notified listeners // is less than `n`. if inner.notified.load(Ordering::Acquire) < n { inner.lock().notify(n); } } /// Notifies a number of active and still unnotified listeners. /// /// The number is allowed to be zero or exceed the current number of listeners. /// /// In contrast to [`Event::notify()`], this method will notify `n` *additional* listeners that /// were previously unnotified. /// /// # Examples /// /// ``` /// use event_listener::Event; /// /// let event = Event::new(); /// /// // This notification gets lost because there are no listeners. /// event.notify(1); /// /// let listener1 = event.listen(); /// let listener2 = event.listen(); /// let listener3 = event.listen(); /// /// // Notifies two listeners. /// // /// // Listener queueing is fair, which means `listener1` and `listener2` /// // get notified here since they start listening before `listener3`. /// event.notify_one(); /// ``` #[inline] pub fn notify_additional(&self, n: usize) { let inner = self.inner(); // Make sure the notification comes after whatever triggered it. full_fence(); // Notify if there is at least one unnotified listener. if inner.notified.load(Ordering::Acquire) < usize::MAX { let mut inner = inner.lock(); let n = n.saturating_add(inner.notified); inner.notify(n); } } /// Notifies a single active listener. /// /// Note that this does not notify one *additional* listener - it only makes sure *at least* /// one listener among the active ones is notified. /// /// This method is equivalent to `self.notify(1)`. /// /// # Examples /// /// ``` /// use event_listener::Event; /// /// let event = Event::new(); /// /// // This notification gets lost because there are no listeners. /// event.notify_one(); /// /// let listener1 = event.listen(); /// let listener2 = event.listen(); /// /// // Notifies just one of `listener1` and `listener2`. /// // /// // Listener queueing is fair, which means `listener1` gets notified /// // here since it was the first to start listening. /// event.notify_one(); /// ``` #[inline] pub fn notify_one(&self) { self.notify(1); } /// Notifies all active listeners. /// /// This method is equivalent to `self.notify(usize::MAX)`. /// /// # Examples /// /// ``` /// use event_listener::Event; /// /// let event = Event::new(); /// /// // This notification gets lost because there are no listeners. /// event.notify_all(); /// /// let listener1 = event.listen(); /// let listener2 = event.listen(); /// /// // Both `listener1` and `listener2` get notified. /// event.notify_all(); /// ``` #[inline] pub fn notify_all(&self) { self.notify(usize::MAX); } /// Returns a reference to the inner state. fn inner(&self) -> &Inner { let mut inner = self.inner.load(Ordering::Acquire); // Initialize the state if this is its first use. if inner.is_null() { // Allocate on the heap. let new = Arc::new(Inner { notified: AtomicUsize::new(usize::MAX), list: Mutex::new(List { head: None, tail: None, start: None, len: 0, notified: 0, }), }); // Convert the heap-allocated state into a raw pointer. let new = Arc::into_raw(new) as *mut Inner; // Attempt to replace the null-pointer with the new state pointer. inner = self.inner.compare_and_swap(inner, new, Ordering::AcqRel); // Check if the old pointer value was indeed null. if inner.is_null() { // If yes, then use the new state pointer. inner = new; } else { // If not, that means a concurrent operation has initialized the state. // In that case, use the old pointer and deallocate the new one. unsafe { drop(Arc::from_raw(new)); } } } unsafe { &*inner } } } impl Drop for Event { #[inline] fn drop(&mut self) { let inner: *mut Inner = *self.inner.get_mut(); // If the state pointer has been initialized, deallocate it. if !inner.is_null() { unsafe { drop(Arc::from_raw(inner)); } } } } impl fmt::Debug for Event { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.pad("Event { .. }") } } impl Default for Event { fn default() -> Event { Event::new() } } /// A guard waiting for a notification from an [`Event`]. /// /// There are two ways for a listener to wait for a notification: /// /// 1. In an asynchronous manner using `.await`. /// 2. In a blocking manner by calling [`EventListener::wait()`] on it. /// /// If a notified listener is dropped without receiving a notification, dropping will notify /// another active listener. pub struct EventListener { /// A reference to [`Event`]'s inner state. inner: Arc<Inner>, /// A pointer to this listener's entry in the linked list. entry: Option<NonNull<Entry>>, } unsafe impl Send for EventListener {} unsafe impl Sync for EventListener {} impl EventListener { /// Blocks until a notification is received. /// /// # Examples /// /// ``` /// use event_listener::Event; /// /// let event = Event::new(); /// let listener = event.listen(); /// /// // Notify `listener`. /// event.notify_one(); /// /// // Receive the notification. /// listener.wait(); /// ``` pub fn wait(self) { self.wait_internal(None); } /// Blocks until a notification is received or a timeout is reached. /// /// Returns `true` if a notification was received. /// /// # Examples /// /// ``` /// use std::time::Duration; /// use event_listener::Event; /// /// let event = Event::new(); /// let listener = event.listen(); /// /// // There are no notification so this times out. /// assert!(!listener.wait_timeout(Duration::from_secs(1))); /// ``` pub fn wait_timeout(self, timeout: Duration) -> bool { self.wait_internal(Some(Instant::now() + timeout)) } /// Blocks until a notification is received or a deadline is reached. /// /// Returns `true` if a notification was received. /// /// # Examples /// /// ``` /// use std::time::{Duration, Instant}; /// use event_listener::Event; /// /// let event = Event::new(); /// let listener = event.listen(); /// /// // There are no notification so this times out. /// assert!(!listener.wait_deadline(Instant::now() + Duration::from_secs(1))); /// ``` pub fn wait_deadline(self, deadline: Instant) -> bool { self.wait_internal(Some(deadline)) } fn wait_internal(mut self, deadline: Option<Instant>) -> bool { // Take out the entry pointer and set it to `None`. let entry = match self.entry.take() { None => unreachable!("cannot wait twice on an `EventListener`"), Some(entry) => entry, }; // Set this listener's state to `Waiting`. { let mut list = self.inner.lock(); let e = unsafe { entry.as_ref() }; // Do a dummy replace operation in order to take out the state. match e.state.replace(State::Notified) { State::Notified => { // If this listener has been notified, remove it from the list and return. list.remove(entry); return true; } // Otherwise, set the state to `Waiting`. _ => e.state.set(State::Waiting(thread::current())), } } // Wait until a notification is received or the timeout is reached. loop { match deadline { None => thread::park(), Some(deadline) => { // Check for timeout. let now = Instant::now(); if now >= deadline { // Remove the entry and check if notified. if self.inner.lock().remove(entry).is_notified() { return true; } else { return false; } } // Park until the deadline. thread::park_timeout(deadline - now); } } let mut list = self.inner.lock(); let e = unsafe { entry.as_ref() }; // Do a dummy replace operation in order to take out the state. match e.state.replace(State::Notified) { State::Notified => { // If this listener has been notified, remove it from the list and return. list.remove(entry); return true; } // Otherwise, set the state back to `Waiting`. state => e.state.set(state), } } } } impl fmt::Debug for EventListener { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.pad("EventListener { .. }") } } impl Future for EventListener { type Output = (); fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { let mut list = self.inner.lock(); let entry = match self.entry { None => unreachable!("cannot poll a completed `EventListener` future"), Some(entry) => entry, }; let state = unsafe { &entry.as_ref().state }; // Do a dummy replace operation in order to take out the state. match state.replace(State::Notified) { State::Notified => { // If this listener has been notified, remove it from the list and return. list.remove(entry); drop(list); self.entry = None; return Poll::Ready(()); } State::Created => { // If the listener was just created, put it in the `Polling` state. state.set(State::Polling(cx.waker().clone())); } State::Polling(w) => { // If the listener was in the `Pooling` state, keep it. state.set(State::Polling(w)); } State::Waiting(_) => { unreachable!("cannot poll and wait on `EventListener` at the same time") } } Poll::Pending } } impl Drop for EventListener { fn drop(&mut self) { // If this listener has never picked up a notification... if let Some(entry) = self.entry.take() { let mut list = self.inner.lock(); // But if a notification was delivered to it... if list.remove(entry).is_notified() { // Then pass it on to another active listener. list.notify(1); } } } } /// A guard holding the linked list locked. struct ListGuard<'a> { /// A reference to [`Event`]'s inner state. inner: &'a Inner, /// The actual guard that acquired the linked list. guard: MutexGuard<'a, List>, } impl Drop for ListGuard<'_> { #[inline] fn drop(&mut self) { let list = &mut **self; // Update the atomic `notified` counter. let notified = if list.notified < list.len { list.notified } else { usize::MAX }; self.inner.notified.store(notified, Ordering::Release); } } impl Deref for ListGuard<'_> { type Target = List; #[inline] fn deref(&self) -> &List { &*self.guard } } impl DerefMut for ListGuard<'_> { #[inline] fn deref_mut(&mut self) -> &mut List { &mut *self.guard } } /// The state of a listener. enum State { /// It has just been created. Created, /// It has received a notification. Notified, /// An async task is polling it. Polling(Waker), /// A thread is blocked on it. Waiting(Thread), } impl State { /// Returns `true` if this is the `Notified` state. #[inline] fn is_notified(&self) -> bool { match self { State::Notified => true, State::Created | State::Polling(_) | State::Waiting(_) => false, } } } /// An entry representing a registered listener. struct Entry { /// THe state of this listener. state: Cell<State>, /// Previous entry in the linked list. prev: Cell<Option<NonNull<Entry>>>, /// Next entry in the linked list. next: Cell<Option<NonNull<Entry>>>, } /// A linked list of entries. struct List { /// First entry in the list. head: Option<NonNull<Entry>>, /// Last entry in the list. tail: Option<NonNull<Entry>>, /// The first unnotified entry in the list. start: Option<NonNull<Entry>>, /// Total number of entries in the list. len: usize, /// The number of notified entries in the list. notified: usize, } impl List { /// Inserts a new entry into the list. fn insert(&mut self) -> NonNull<Entry> { unsafe { // Allocate an entry that is going to become the new tail. let entry = NonNull::new_unchecked(Box::into_raw(Box::new(Entry { state: Cell::new(State::Created), prev: Cell::new(self.tail), next: Cell::new(None), }))); // Replace the tail with the new entry. match mem::replace(&mut self.tail, Some(entry)) { None => self.head = Some(entry), Some(t) => t.as_ref().next.set(Some(entry)), } // If there were no unnotified entries, this one is the first now. if self.start.is_none() { self.start = self.tail; } // Bump the entry count. self.len += 1; entry } } /// Removes an entry from the list and returns its state. fn remove(&mut self, entry: NonNull<Entry>) -> State { unsafe { let prev = entry.as_ref().prev.get(); let next = entry.as_ref().next.get(); // Unlink from the previous entry. match prev { None => self.head = next, Some(p) => p.as_ref().next.set(next), } // Unlink from the next entry. match next { None => self.tail = prev, Some(n) => n.as_ref().prev.set(prev), } // If this was the first unnotified entry, move the pointer to the next one. if self.start == Some(entry) { self.start = next; } // Deallocate and extract the state. let entry = Box::from_raw(entry.as_ptr()); let state = entry.state.into_inner(); // Update the counters. if state.is_notified() { self.notified -= 1; } self.len -= 1; state } } /// Notifies a number of entries. #[cold] fn notify(&mut self, mut n: usize) { if n <= self.notified { return; } n -= self.notified; while n > 0 { n -= 1; // Notify the first unnotified entry. match self.start { None => break, Some(e) => { // Get the entry and move the pointer forward. let e = unsafe { e.as_ref() }; self.start = e.next.get(); // Set the state of this entry to `Notified` and notify. match e.state.replace(State::Notified) { State::Notified => {} State::Created => {} State::Polling(w) => w.wake(), State::Waiting(t) => t.unpark(), } // Update the counter. self.notified += 1; } } } } } /// Equivalent to `atomic::fence(Ordering::SeqCst)`, but in some cases faster. #[inline] fn full_fence() { if cfg!(any(target_arch = "x86", target_arch = "x86_64")) { // HACK(stjepang): On x86 architectures there are two different ways of executing // a `SeqCst` fence. // // 1. `atomic::fence(SeqCst)`, which compiles into a `mfence` instruction. // 2. `_.compare_and_swap(_, _, SeqCst)`, which compiles into a `lock cmpxchg` instruction. // // Both instructions have the effect of a full barrier, but empirical benchmarks have shown // that the second one is sometimes a bit faster. // // The ideal solution here would be to use inline assembly, but we're instead creating a // temporary atomic variable and compare-and-exchanging its value. No sane compiler to // x86 platforms is going to optimize this away. let a = AtomicUsize::new(0); a.compare_and_swap(0, 1, Ordering::SeqCst); } else { atomic::fence(Ordering::SeqCst); } }