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use super::{BlockingError, BlockingImpl}; use futures::Poll; use std::cell::Cell; use std::fmt; use std::marker::PhantomData; use tokio_executor::Enter; thread_local! { static CURRENT: Cell<BlockingImpl> = Cell::new(super::default_blocking); } /// Ensures that the executor is removed from the thread-local context /// when leaving the scope. This handles cases that involve panicking. /// /// **NOTE:** This is intended specifically for use by `tokio` 0.2's /// backwards-compatibility layer. In general, user code should not override the /// blocking implementation. If you use this, make sure you know what you're /// doing. pub struct DefaultGuard<'a> { prior: BlockingImpl, _lifetime: PhantomData<&'a ()>, } /// Set the default blocking implementation, returning a guard that resets the /// blocking implementation when dropped. /// /// **NOTE:** This is intended specifically for use by `tokio` 0.2's /// backwards-compatibility layer. In general, user code should not override the /// blocking implementation. If you use this, make sure you know what you're /// doing. pub fn set_default<'a>(blocking: BlockingImpl) -> DefaultGuard<'a> { CURRENT.with(|cell| { let prior = cell.replace(blocking); DefaultGuard { prior, _lifetime: PhantomData, } }) } /// Set the default blocking implementation for the duration of the closure. /// /// **NOTE:** This is intended specifically for use by `tokio` 0.2's /// backwards-compatibility layer. In general, user code should not override the /// blocking implementation. If you use this, make sure you know what you're /// doing. pub fn with_default<F, R>(blocking: BlockingImpl, enter: &mut Enter, f: F) -> R where F: FnOnce(&mut Enter) -> R, { let _guard = set_default(blocking); f(enter) } /// 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] /// /// NB: The entire task that called `blocking` is blocked whenever the supplied /// closure blocks, even if you have used future combinators such as `select` - /// the other futures in this task will not make progress until the closure /// returns. /// If this is not desired, ensure that `blocking` runs in its own task (e.g. /// using `futures::sync::oneshot::spawn`). /// /// [build]: struct.Builder.html#method.max_blocking /// /// # Return /// /// When the blocking closure is executed, `Ok(Ready(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, { CURRENT.with(|cell| { let blocking = cell.get(); // Object-safety workaround: the `Blocking` trait must be object-safe, // since we use a trait object in the thread-local. However, a blocking // _operation_ will be generic over the return type of the blocking // function. Therefore, rather than passing a function with a return // type to `Blocking::run_blocking`, we pass a _new_ closure which // doesn't have a return value. That closure invokes the blocking // function and assigns its value to `ret`, which we then unpack when // the blocking call finishes. let mut f = Some(f); let mut ret = None; { let ret2 = &mut ret; let mut run = move || { let f = f .take() .expect("blocking closure invoked twice; this is a bug!"); *ret2 = Some((f)()); }; try_ready!((blocking)(&mut run)); } // Return the result let ret = ret.expect("blocking function finished, but return value was unset; this is a bug!"); Ok(ret.into()) }) } // === impl DefaultGuard === impl<'a> fmt::Debug for DefaultGuard<'a> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.pad("DefaultGuard { .. }") } } impl<'a> Drop for DefaultGuard<'a> { fn drop(&mut self) { // if the TLS value has already been torn down, there's nothing else we // can do. we're almost certainly panicking anyway. let _ = CURRENT.try_with(|cell| { cell.set(self.prior); }); } }