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// Copyright (c) 2018 Nuclear Furnace // // Permission is hereby granted, free of charge, to any person obtaining a copy // of this software and associated documentation files (the "Software"), to deal // in the Software without restriction, including without limitation the rights // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell // copies of the Software, and to permit persons to whom the Software is // furnished to do so, subject to the following conditions: // // The above copyright notice and this permission notice shall be included in all // copies or substantial portions of the Software. // // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE // SOFTWARE. //! Turnstyles are a way to provide futures-aware gated access in a sequential fashion. //! //! Callers will "join" the queue, and receive a future that will complete once they make it //! through the turnstyle. The turnstyle is controlled externally by some coordinator, and can //! "turn" it to let the next waiter in line through. Thus, you can have multiple waiters, which //! can join the line at any time, and allow the coordinator to continually admit them through. //! //! This can be used to synchronize access to a resource, or to provide a synchronization point //! to repeatedly calculation. //! //! One example is a network daemon that reloads its configuration and reestablishes all of its //! listening sockets. Normally, you might join (using `select`) both the listening socket //! future and a close future so that you can shutdown the socket when its no longer required. //! //! With a turnstyle, you can join the queue every time you reload the configuration, and then //! share that future to all of the newly created listeners. Once the new listeners are ready, you //! also do one turn on the turnstyle, which signals the last waiter in line -- a future shared //! with all of the old listeners -- that they can now shutdown. That same turnstyle can perform //! this over and over without issue. //! //! Turnstyles internally protect themselves via a `Mutex` but are fast enough in normal cases that //! you can `join` or `turn` from within a future without fear of stalling the executor. If you're //! joining at an extremely high frequency, you could potentially cause performance degradation. extern crate futures; use futures::{prelude::*, sync::oneshot}; use std::{ collections::VecDeque, sync::{Arc, Mutex}, }; /// A future that waits to be notified, based on its place in line. pub struct Waiter { inner: oneshot::Receiver<()>, } impl Future for Waiter { type Error = (); type Item = (); fn poll(&mut self) -> Poll<Self::Item, Self::Error> { self.inner.poll().map_err(|_| ()) } } /// An ordered queue of waiting participants. /// /// Every turn of the turnstyle, the next participant in queue is notified and removed from the /// queue. If the queue is empty, `turn` is a noop. /// /// Turnstyles can be cloned and are safe to share across threads. #[derive(Clone)] pub struct Turnstyle { waiters: Arc<Mutex<VecDeque<oneshot::Sender<()>>>>, } impl Turnstyle { /// Creates a new, empty turnstyle. pub fn new() -> Turnstyle { Turnstyle { waiters: Arc::new(Mutex::new(VecDeque::new())), } } /// Joins the waiting queue. /// /// Returns a `Waiter` to the caller, which will complete when the turnstyle turns and reaches /// the caller's position in the queue. pub fn join(&self) -> Waiter { let (tx, rx) = oneshot::channel(); { let mut waiters = self.waiters.lock().expect("turnstyle unable to join line"); waiters.push_back(tx); } Waiter { inner: rx } } /// Turns once, letting a single waiter through. /// /// The `Waiter` is notified by the future completing. pub fn turn(&self) { let waiter = { let mut waiters = self.waiters.lock().unwrap(); waiters.pop_front() }; if let Some(w) = waiter { w.send(()).expect("turnstyle failed to signal next in line"); } } } #[cfg(test)] mod tests { use super::Turnstyle; use futures::{future, Future}; #[test] fn single_waiter() { future::lazy(|| { let ts = Turnstyle::new(); let mut w = ts.join(); assert!(!w.poll().unwrap().is_ready()); ts.turn(); assert!(w.poll().unwrap().is_ready()); future::ok::<_, ()>(()) }).wait() .unwrap(); } #[test] fn multiple_waiters() { future::lazy(|| { let ts = Turnstyle::new(); let mut w1 = ts.join(); let mut w2 = ts.join(); let mut w3 = ts.join(); assert!(!w1.poll().unwrap().is_ready()); assert!(!w2.poll().unwrap().is_ready()); assert!(!w2.poll().unwrap().is_ready()); ts.turn(); assert!(w1.poll().unwrap().is_ready()); assert!(!w2.poll().unwrap().is_ready()); assert!(!w3.poll().unwrap().is_ready()); ts.turn(); assert!(w2.poll().unwrap().is_ready()); assert!(!w3.poll().unwrap().is_ready()); ts.turn(); assert!(w3.poll().unwrap().is_ready()); future::ok::<_, ()>(()) }).wait() .unwrap(); } }