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use worker::Worker; use futures::Poll; use std::error::Error; use std::fmt; /// Error raised by `blocking`. pub struct BlockingError { _p: (), } /// Enter a blocking section of code. /// /// The `blocking` function annotates a section of code that performs a blocking /// operation, either by issuing a blocking syscall or by performing a long /// running CPU-bound computation. /// /// When the `blocking` function enters, it hands off the responsibility of /// processing the current work queue to another thread. Then, it calls the /// supplied closure. The closure is permitted to block indefinitely. /// /// If the maximum number of concurrent `blocking` calls has been reached, then /// `NotReady` is returned and the task is notified once existing `blocking` /// calls complete. The maximum value is specified when creating a thread pool /// using [`Builder::max_blocking`][build] /// /// [build]: struct.Builder.html#method.max_blocking /// /// # Return /// /// When the blocking closure is executed, `Ok(T)` is returned, where `T` is the /// closure's return value. /// /// If the thread pool has shutdown, `Err` is returned. /// /// If the number of concurrent `blocking` calls has reached the maximum, /// `Ok(NotReady)` is returned and the current task is notified when a call to /// `blocking` will succeed. /// /// If `blocking` is called from outside the context of a Tokio thread pool, /// `Err` is returned. /// /// # Background /// /// By default, the Tokio thread pool expects that tasks will only run for short /// periods at a time before yielding back to the thread pool. This is the basic /// premise of cooperative multitasking. /// /// However, it is common to want to perform a blocking operation while /// processing an asynchronous computation. Examples of blocking operation /// include: /// /// * Performing synchronous file operations (reading and writing). /// * Blocking on acquiring a mutex. /// * Performing a CPU bound computation, like cryptographic encryption or /// decryption. /// /// One option for dealing with blocking operations in an asynchronous context /// is to use a thread pool dedicated to performing these operations. This not /// ideal as it requires bidirectional message passing as well as a channel to /// communicate which adds a level of buffering. /// /// Instead, `blocking` hands off the responsibility of processing the work queue /// to another thread. This hand off is light compared to a channel and does not /// require buffering. /// /// # Examples /// /// Block on receiving a message from a `std` channel. This example is a little /// silly as using the non-blocking channel from the `futures` crate would make /// more sense. The blocking receive can be replaced with any blocking operation /// that needs to be performed. /// /// ```rust /// # extern crate futures; /// # extern crate tokio_threadpool; /// /// use tokio_threadpool::{ThreadPool, blocking}; /// /// use futures::Future; /// use futures::future::{lazy, poll_fn}; /// /// use std::sync::mpsc; /// use std::thread; /// use std::time::Duration; /// /// pub fn main() { /// // This is a *blocking* channel /// let (tx, rx) = mpsc::channel(); /// /// // Spawn a thread to send a message /// thread::spawn(move || { /// thread::sleep(Duration::from_millis(500)); /// tx.send("hello").unwrap(); /// }); /// /// let pool = ThreadPool::new(); /// /// pool.spawn(lazy(move || { /// // Because `blocking` returns `Poll`, it is intended to be used /// // from the context of a `Future` implementation. Since we don't /// // have a complicated requirement, we can use `poll_fn` in this /// // case. /// poll_fn(move || { /// blocking(|| { /// let msg = rx.recv().unwrap(); /// println!("message = {}", msg); /// }).map_err(|_| panic!("the threadpool shut down")) /// }) /// })); /// /// // Wait for the task we just spawned to complete. /// pool.shutdown_on_idle().wait().unwrap(); /// } /// ``` pub fn blocking<F, T>(f: F) -> Poll<T, BlockingError> where F: FnOnce() -> T, { let res = Worker::with_current(|worker| { let worker = match worker { Some(worker) => worker, None => { return Err(BlockingError { _p: () }); } }; // Transition the worker state to blocking. This will exit the fn early // with `NotReady` if the pool does not have enough capacity to enter // blocking mode. worker.transition_to_blocking() }); // If the transition cannot happen, exit early try_ready!(res); // Currently in blocking mode, so call the inner closure let ret = f(); // Try to transition out of blocking mode. This is a fast path that takes // back ownership of the worker if the worker handoff didn't complete yet. Worker::with_current(|worker| { // Worker must be set since it was above. worker.unwrap() .transition_from_blocking(); }); // Return the result Ok(ret.into()) } impl fmt::Display for BlockingError { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { write!(fmt, "{}", self.description()) } } impl fmt::Debug for BlockingError { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.debug_struct("BlockingError") .field("reason", &self.description()) .finish() } } impl Error for BlockingError { fn description(&self) -> &str { "`blocking` annotation used from outside the context of a thread pool" } }