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//! Thread parking and unparking. //! //! This is a copy of the //! [`park()`][`std::thread::park()`]/[`unpark()`][`std::thread::Thread::unpark()`] mechanism from //! the standard library. //! //! # What is parking //! //! Conceptually, each [`Parker`] has a token which is initially not present: //! //! * The [`Parker::park()`] method blocks the current thread unless or until the token is //! available, at which point it automatically consumes the token. It may also return //! *spuriously*, without consuming the token. //! //! * The [`Parker::park_timeout()`] method works the same as [`Parker::park()`], but blocks until //! a timeout is reached. //! //! * The [`Parker::park_deadline()`] method works the same as [`Parker::park()`], but blocks until //! a deadline is reached. //! //! * The [`Parker::unpark()`] and [`Unparker::unpark()`] methods atomically make the token //! available if it wasn't already. Because the token is initially absent, [`Unparker::unpark()`] //! followed by [`Parker::park()`] will result in the second call returning immediately. //! //! # Analogy with channels //! //! Another way of thinking about [`Parker`] is as a bounded //! [channel][`std::sync::mpsc::sync_channel()`] with capacity of 1. //! //! Then, [`Parker::park()`] is equivalent to blocking on a //! [receive][`std::sync::mpsc::Receiver::recv()`] operation, and [`Unparker::unpark()`] is //! equivalent to a non-blocking [send][`std::sync::mpsc::SyncSender::try_send()`] operation. #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)] use std::cell::Cell; use std::fmt; use std::marker::PhantomData; use std::sync::atomic::AtomicUsize; use std::sync::atomic::Ordering::SeqCst; use std::sync::{Arc, Condvar, Mutex}; use std::time::{Duration, Instant}; /// Parks a thread. /// /// # Examples /// /// ``` /// use std::thread; /// use std::time::Duration; /// use parking::Parker; /// /// let p = Parker::new(); /// let u = p.unparker(); /// /// // Make the token available. /// u.unpark(); /// // Wakes up immediately and consumes the token. /// p.park(); /// /// thread::spawn(move || { /// thread::sleep(Duration::from_millis(500)); /// u.unpark(); /// }); /// /// // Wakes up when `u.unpark()` provides the token, but may also wake up /// // spuriously before that without consuming the token. /// p.park(); /// ``` pub struct Parker { unparker: Unparker, _marker: PhantomData<Cell<()>>, } impl Parker { /// Creates a new [`Parker`]. /// /// # Examples /// /// ``` /// use parking::Parker; /// /// let p = Parker::new(); /// ``` /// pub fn new() -> Parker { Parker { unparker: Unparker { inner: Arc::new(Inner { state: AtomicUsize::new(EMPTY), lock: Mutex::new(()), cvar: Condvar::new(), }), }, _marker: PhantomData, } } /// Blocks the current thread until the token is made available. /// /// This method may wake up spuriously without consuming the token, and callers should be /// prepared for this possibility. /// /// # Examples /// /// ``` /// use parking::Parker; /// /// let p = Parker::new(); /// let u = p.unparker(); /// /// // Make the token available. /// u.unpark(); /// /// // Wakes up immediately and consumes the token. /// p.park(); /// ``` pub fn park(&self) { self.unparker.inner.park(None); } /// Blocks the current thread until the token is made available or the timeout is reached. /// /// Returns `true` if the token was received before the timeout. /// /// This method may wake up spuriously without consuming the token, and callers should be /// prepared for this possibility. /// /// # Examples /// /// ``` /// use std::time::Duration; /// use parking::Parker; /// /// let p = Parker::new(); /// /// // Waits for the token to become available, but will not wait longer than 500 ms. /// p.park_timeout(Duration::from_millis(500)); /// ``` pub fn park_timeout(&self, timeout: Duration) -> bool { self.unparker.inner.park(Some(timeout)) } /// Blocks the current thread until the token is made available or the deadline is reached. /// /// Returns `true` if the token was received before the deadline. /// /// This method may wake up spuriously without consuming the token, and callers should be /// prepared for this possibility. /// /// # Examples /// /// ``` /// use std::time::{Duration, Instant}; /// use parking::Parker; /// /// let p = Parker::new(); /// /// // Waits for the token to become available, but will not wait longer than 500 ms. /// p.park_deadline(Instant::now() + Duration::from_millis(500)); /// ``` pub fn park_deadline(&self, deadline: Instant) -> bool { self.unparker .inner .park(Some(deadline.saturating_duration_since(Instant::now()))) } /// Atomically makes the token available if it is not already. /// /// The next time a thread blocks on this [`Parker`], it will wake up immediately. /// /// # Examples /// /// ``` /// use std::thread; /// use std::time::Duration; /// use parking::Parker; /// /// let p = Parker::new(); /// let u = p.unparker(); /// /// thread::spawn(move || { /// thread::sleep(Duration::from_millis(500)); /// u.unpark(); /// }); /// /// // Wakes up when `u.unpark()` provides the token, but may also wake up /// // spuriously before that without consuming the token. /// p.park(); /// ``` pub fn unpark(&self) { self.unparker.unpark() } /// Returns a handle for unparking. /// /// The returned [`Unparker`] handle can be cloned and shared among threads. /// /// # Examples /// /// ``` /// use parking::Parker; /// /// let p = Parker::new(); /// let u = p.unparker(); /// /// // Make the token available. /// u.unpark(); /// // Wakes up immediately and consumes the token. /// p.park(); /// ``` pub fn unparker(&self) -> Unparker { self.unparker.clone() } } impl fmt::Debug for Parker { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.pad("Parker { .. }") } } /// Unparks a thread. pub struct Unparker { inner: Arc<Inner>, } impl Unparker { /// Atomically makes the token available if it is not already. /// /// This method will wake up the thread blocked on [`Parker::park()`], /// [`Parker::park_timeout()`], or [`Parker::park_deadline()`], if there is one. /// /// # Examples /// /// ``` /// use std::thread; /// use std::time::Duration; /// use parking::Parker; /// /// let p = Parker::new(); /// let u = p.unparker(); /// /// thread::spawn(move || { /// thread::sleep(Duration::from_millis(500)); /// u.unpark(); /// }); /// /// // Wakes up when `u.unpark()` provides the token, but may also wake up /// // spuriously before that without consuming the token. /// p.park(); /// ``` pub fn unpark(&self) { self.inner.unpark() } } impl fmt::Debug for Unparker { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.pad("Unparker { .. }") } } impl Clone for Unparker { fn clone(&self) -> Unparker { Unparker { inner: self.inner.clone(), } } } const EMPTY: usize = 0; const PARKED: usize = 1; const NOTIFIED: usize = 2; struct Inner { state: AtomicUsize, lock: Mutex<()>, cvar: Condvar, } impl Inner { fn park(&self, timeout: Option<Duration>) -> bool { // If we were previously notified then we consume this notification and return quickly. if self .state .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) .is_ok() { return true; } // If the timeout is zero, then there is no need to actually block. if let Some(dur) = timeout { if dur == Duration::from_millis(0) { return false; } } // Otherwise we need to coordinate going to sleep. let mut m = self.lock.lock().unwrap(); match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) { Ok(_) => {} // Consume this notification to avoid spurious wakeups in the next park. Err(NOTIFIED) => { // We must read `state` here, even though we know it will be `NOTIFIED`. This is // because `unpark` may have been called again since we read `NOTIFIED` in the // `compare_exchange` above. We must perform an acquire operation that synchronizes // with that `unpark` to observe any writes it made before the call to `unpark`. To // do that we must read from the write it made to `state`. let old = self.state.swap(EMPTY, SeqCst); assert_eq!(old, NOTIFIED, "park state changed unexpectedly"); return true; } Err(n) => panic!("inconsistent park_timeout state: {}", n), } match timeout { None => { loop { // Block the current thread on the conditional variable. m = self.cvar.wait(m).unwrap(); match self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) { Ok(_) => return true, // got a notification Err(_) => {} // spurious wakeup, go back to sleep } } } Some(timeout) => { // Wait with a timeout, and if we spuriously wake up or otherwise wake up from a // notification we just want to unconditionally set `state` back to `EMPTY`, either // consuming a notification or un-flagging ourselves as parked. let (_m, _result) = self.cvar.wait_timeout(m, timeout).unwrap(); match self.state.swap(EMPTY, SeqCst) { NOTIFIED => true, // got a notification PARKED => false, // no notification n => panic!("inconsistent park_timeout state: {}", n), } } } } pub fn unpark(&self) { // To ensure the unparked thread will observe any writes we made before this call, we must // perform a release operation that `park` can synchronize with. To do that we must write // `NOTIFIED` even if `state` is already `NOTIFIED`. That is why this must be a swap rather // than a compare-and-swap that returns if it reads `NOTIFIED` on failure. match self.state.swap(NOTIFIED, SeqCst) { EMPTY => return, // no one was waiting NOTIFIED => return, // already unparked PARKED => {} // gotta go wake someone up _ => panic!("inconsistent state in unpark"), } // There is a period between when the parked thread sets `state` to `PARKED` (or last // checked `state` in the case of a spurious wakeup) and when it actually waits on `cvar`. // If we were to notify during this period it would be ignored and then when the parked // thread went to sleep it would never wake up. Fortunately, it has `lock` locked at this // stage so we can acquire `lock` to wait until it is ready to receive the notification. // // Releasing `lock` before the call to `notify_one` means that when the parked thread wakes // it doesn't get woken only to have to wait for us to release `lock`. drop(self.lock.lock().unwrap()); self.cvar.notify_one(); } }