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//! Triggers for one time events between tasks and threads. //! //! The mechanism consists of two types, the [`Trigger`] and the [`Listener`]. They come together //! as a pair. Much like the sender/receiver pair of a channel. The trigger half has a //! [`Trigger::trigger`] method that will make all tasks/threads waiting on //! a listener continue executing. //! The listener both has a sync [`Listener::wait`] method, and it also implements //! `Future<Output = ()>` for async support. //! //! Both the [`Trigger`] and [`Listener`] can be cloned. So any number of trigger instances can //! trigger any number of waiting listeners. When any one trigger instance belonging to the pair is //! triggered, all the waiting listeners will be unblocked. Waiting on a listener whose //! trigger already went off will return instantly. So each trigger/listener pair can only be fired //! once. //! //! This crate does not use any `unsafe` code. //! //! # Examples //! //! A trivial example showing the basic usage: //! //! ``` //! #[tokio::main] //! async fn main() { //! let (trigger, listener) = triggered::trigger(); //! //! let task = tokio::spawn(async { //! // Blocks until `trigger.trigger()` below //! listener.await; //! //! println!("Triggered async task"); //! }); //! //! // This will make any thread blocked in `Listener::wait()` or async task awaiting the //! // listener continue execution again. //! trigger.trigger(); //! //! let _ = task.await; //! } //! ``` //! //! An example showing a trigger/listener pair being used to gracefully shut down some async //! server instances on a Ctrl-C event, where only an immutable `Fn` closure is accepted: //! //! ``` //! # use std::future::Future; //! # type Error = Box<dyn std::error::Error>; //! # struct SomeServer; //! # impl SomeServer { //! # fn new() -> Self { SomeServer } //! # async fn serve_with_shutdown_signal(self, s: impl Future<Output = ()>) -> Result<(), Error> {Ok(())} //! # async fn serve(self) -> Result<(), Error> {Ok(())} //! # } //! //! #[tokio::main] //! async fn main() -> Result<(), Error> { //! let (shutdown_trigger, shutdown_signal1) = triggered::trigger(); //! //! // A sync `Fn` closure will trigger the trigger when the user hits Ctrl-C //! ctrlc::set_handler(move || { //! shutdown_trigger.trigger(); //! }).expect("Error setting Ctrl-C handler"); //! //! // If the server library has support for something like a shutdown signal: //! let shutdown_signal2 = shutdown_signal1.clone(); //! let server1_task = tokio::spawn(async move { //! SomeServer::new().serve_with_shutdown_signal(shutdown_signal1).await; //! }); //! //! // Or just select between the long running future and the signal to abort it //! tokio::select! { //! server_result = SomeServer::new().serve() => { //! eprintln!("Server error: {:?}", server_result); //! } //! _ = shutdown_signal2 => {} //! } //! //! let _ = server1_task.await; //! Ok(()) //! } //! ``` //! //! # Rust Compatibility //! //! Will work with at least the two latest stable Rust releases. This gives users at least six //! weeks to upgrade their Rust toolchain after a new stable is released. //! //! The current MSRV can be seen in `travis.yml`. Any change to the MSRV will be considered a //! breaking change and listed in the changelog. //! //! # Comparison with similar primitives //! //! ## Channels //! //! The event triggering primitives in this library is somewhat similar to channels. The main //! difference and why I developed this library is that //! //! The listener is somewhat similar to a `futures::channel::oneshot::Receiver<()>`. But it: //! * Is not fallible - Implements `Future<Output = ()>` instead of //! `Future<Output = Result<T, Canceled>>` //! * Implements `Clone` - Any number of listeners can wait for the same event //! * Has a sync [`Listener::wait`] - Both synchronous threads, and asynchronous tasks can wait //! at the same time. //! //! The trigger, when compared to a `futures::channel::oneshot::Sender<()>` has the differences //! that it: //! * Is not fallible - The trigger does not care if there are any listeners left //! * Does not consume itself on send, instead takes `&self` - So can be used //! in situations where it is not owned or not mutable. For example in `Drop` implementations //! or callback closures that are limited to `Fn` or `FnMut`. //! //! ## `futures::future::Abortable` //! //! One use case of these triggers is to abort futures when some event happens. See examples above. //! The differences include: //! * A single handle can abort any number of futures //! * Some futures are not properly cleaned up when just dropped the way `Abortable` does it. //! These libraries sometimes allows creating their futures with a shutdown signal that triggers //! a clean abort. Something like `serve_with_shutdown(signal: impl Future<Output = ()>)`. #![deny(unsafe_code)] #![deny(rust_2018_idioms)] use std::mem; use std::pin::Pin; use std::sync::{ atomic::{AtomicBool, Ordering}, Arc, Condvar, Mutex, }; use std::task::{Context, Poll, Waker}; /// Returns a [`Trigger`] and [`Listener`] pair bound to each other. /// /// The [`Listener`] is used to wait for the trigger to fire. It can be waited on both sync /// and async. pub fn trigger() -> (Trigger, Listener) { let inner = Arc::new(Inner { complete: AtomicBool::new(false), tasks: Mutex::new(Vec::new()), condvar: Condvar::new(), }); let trigger = Trigger { inner: inner.clone(), }; let listener = Listener { inner }; (trigger, listener) } /// A struct used to trigger [`Listener`]s it is paired with. /// /// Can be cloned to create multiple instances that all trigger the same listeners. #[derive(Clone, Debug)] pub struct Trigger { inner: Arc<Inner>, } /// A struct used to wait for a trigger event from a [`Trigger`]. /// /// Can be waited on synchronously via [`Listener::wait`] or asynchronously thanks to the struct /// implementing `Future`. /// /// The listener can be cloned and any amount of threads and tasks can wait for the same trigger /// at the same time. #[derive(Clone, Debug)] pub struct Listener { inner: Arc<Inner>, } #[derive(Debug)] struct Inner { complete: AtomicBool, tasks: Mutex<Vec<Waker>>, condvar: Condvar, } impl Unpin for Trigger {} impl Unpin for Listener {} impl Trigger { /// Trigger all [`Listener`]s paired with this trigger. /// /// Makes all listeners currently blocked in [`Listener::wait`] return, /// and all that is being `await`ed finish. /// /// Calling this method only does anything the first time. Any subsequent trigger call to /// the same instance or a clone thereof does nothing, it has already been triggered. /// Any listener waiting on the trigger after it has been triggered will just return /// instantly. /// /// This method is safe to call from both async and sync code. It's not an async function, /// but it always finishes very fast. pub fn trigger(&self) { if self.inner.complete.swap(true, Ordering::SeqCst) { return; } // This code will only be executed once per trigger instance. No matter the amount of // `Trigger` clones or calls to `trigger()`, thanks to the atomic swap above. let mut tasks_guard = self .inner .tasks .lock() .expect("Some Trigger/Listener has panicked"); let tasks = mem::replace(&mut *tasks_guard, Vec::new()); mem::drop(tasks_guard); for task in tasks { task.wake(); } self.inner.condvar.notify_all(); } /// Returns true if this trigger has been triggered. pub fn is_triggered(&self) -> bool { self.inner.complete.load(Ordering::SeqCst) } } impl std::future::Future for Listener { type Output = (); fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { if self.inner.complete.load(Ordering::SeqCst) { return Poll::Ready(()); } let mut tasks = self .inner .tasks .lock() .expect("Some Trigger/Listener has panicked"); // If the trigger completed while we waited for the lock, skip adding our waker to the list // of tasks. if self.inner.complete.load(Ordering::SeqCst) { Poll::Ready(()) } else { tasks.push(cx.waker().clone()); Poll::Pending } } } impl Listener { /// Wait for this trigger synchronously. /// /// Blocks the current thread until the corresponding [`Trigger`] is triggered. /// If the trigger has already been triggered at least once, this returns immediately. pub fn wait(&self) { if self.inner.complete.load(Ordering::SeqCst) { return; } let mut guard = self .inner .tasks .lock() .expect("Some Trigger/Listener has panicked"); while !self.inner.complete.load(Ordering::SeqCst) { guard = self .inner .condvar .wait(guard) .expect("Some Trigger/Listener has panicked"); } } /// Returns true if this trigger has been triggered. pub fn is_triggered(&self) -> bool { self.inner.complete.load(Ordering::SeqCst) } }