blocking.rs

use crate::task::JoinHandle;

cfg_rt_multi_thread! {
    /// Runs the provided blocking function on the current thread without
    /// blocking the executor.
    ///
    /// In general, issuing a blocking call or performing a lot of compute in a
    /// future without yielding is problematic, as it may prevent the executor
    /// from driving other tasks forward. Calling this function informs the
    /// executor that the currently executing task is about to block the thread,
    /// so the executor is able to hand off any other tasks it has to a new
    /// worker thread before that happens. See the [CPU-bound tasks and blocking
    /// code][blocking] section for more information.
    ///
    /// Be aware that although this function avoids starving other independently
    /// spawned tasks, any other code running concurrently in the same task will
    /// be suspended during the call to `block_in_place`. This can happen e.g.
    /// when using the [`join!`] macro. To avoid this issue, use
    /// [`spawn_blocking`] instead of `block_in_place`.
    ///
    /// Note that this function cannot be used within a [`current_thread`] runtime
    /// because in this case there are no other worker threads to hand off tasks
    /// to. On the other hand, calling the function outside a runtime is
    /// allowed. In this case, `block_in_place` just calls the provided closure
    /// normally.
    ///
    /// Code running behind `block_in_place` cannot be cancelled. When you shut
    /// down the executor, it will wait indefinitely for all blocking operations
    /// to finish. You can use [`shutdown_timeout`] to stop waiting for them
    /// after a certain timeout. Be aware that this will still not cancel the
    /// tasks — they are simply allowed to keep running after the method
    /// returns.
    ///
    /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
    /// [`spawn_blocking`]: fn@crate::task::spawn_blocking
    /// [`join!`]: macro@join
    /// [`thread::spawn`]: fn@std::thread::spawn
    /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::task;
    ///
    /// # async fn docs() {
    /// task::block_in_place(move || {
    ///     // do some compute-heavy work or call synchronous code
    /// });
    /// # }
    /// ```
    ///
    /// Code running inside `block_in_place` may use `block_on` to reenter the
    /// async context.
    ///
    /// ```
    /// use tokio::task;
    /// use tokio::runtime::Handle;
    ///
    /// # async fn docs() {
    /// task::block_in_place(move || {
    ///     Handle::current().block_on(async move {
    ///         // do something async
    ///     });
    /// });
    /// # }
    /// ```
    ///
    /// # Panics
    ///
    /// This function panics if called from a [`current_thread`] runtime.
    ///
    /// [`current_thread`]: fn@crate::runtime::Builder::new_current_thread
    pub fn block_in_place<F, R>(f: F) -> R
    where
        F: FnOnce() -> R,
    {
        crate::runtime::thread_pool::block_in_place(f)
    }
}

cfg_rt! {
    /// Runs the provided closure on a thread where blocking is acceptable.
    ///
    /// In general, issuing a blocking call or performing a lot of compute in a
    /// future without yielding is problematic, as it may prevent the executor from
    /// driving other futures forward. This function runs the provided closure on a
    /// thread dedicated to blocking operations. See the [CPU-bound tasks and
    /// blocking code][blocking] section for more information.
    ///
    /// Tokio will spawn more blocking threads when they are requested through this
    /// function until the upper limit configured on the [`Builder`] is reached.
    /// After reaching the upper limit, the tasks are put in a queue.
    /// The thread limit is very large by default, because `spawn_blocking` is often
    /// used for various kinds of IO operations that cannot be performed
    /// asynchronously.  When you run CPU-bound code using `spawn_blocking`, you
    /// should keep this large upper limit in mind. When running many CPU-bound
    /// computations, a semaphore or some other synchronization primitive should be
    /// used to limit the number of computation executed in parallel. Specialized
    /// CPU-bound executors, such as [rayon], may also be a good fit.
    ///
    /// This function is intended for non-async operations that eventually finish on
    /// their own. If you want to spawn an ordinary thread, you should use
    /// [`thread::spawn`] instead.
    ///
    /// Closures spawned using `spawn_blocking` cannot be cancelled abruptly; there
    /// is no standard low level API to cause a thread to stop running.  However,
    /// a useful pattern is to pass some form of "cancellation token" into
    /// the thread.  This could be an [`AtomicBool`], and inner loops in the
    /// thread check the boolean periodically.  Another approach is to
    /// have the thread primarily read from a channel, and to exit when
    /// the channel closes; assuming the sender side of the channel is dropped
    /// when cancellation occurs, this will cause the receiver thread to exit
    /// soon after as well.
    ///
    /// When you shut down the executor, it will wait indefinitely for all blocking operations to
    /// finish. You can use [`shutdown_timeout`] to stop waiting for them after a
    /// certain timeout. Be aware that this will still not cancel the tasks — they
    /// are simply allowed to keep running after the method returns.
    ///
    /// Note that if you are using the single threaded runtime, this function will
    /// still spawn additional threads for blocking operations. The basic
    /// scheduler's single thread is only used for asynchronous code.
    ///
    /// # Related APIs and patterns for bridging asynchronous and blocking code
    ///
    /// In simple cases, it is sufficient to have the closure accept input
    /// parameters at creation time and return a single value (or struct/tuple, etc.).
    ///
    /// For more complex situations in which it is desirable to stream data to or from
    /// the synchronous context, the [`mpsc channel`] has `blocking_send` and
    /// `blocking_recv` methods for use in non-async code such as the thread created
    /// by `spawn_blocking`.
    ///
    /// Another option is [`SyncIoBridge`] for cases where the synchronous context
    /// is operating on byte streams.  For example, you might use an asynchronous
    /// HTTP client such as [hyper] to fetch data, but perform complex parsing
    /// of the payload body using a library written for synchronous I/O.
    ///
    /// Finally, see also [Bridging with sync code][bridgesync] for discussions
    /// around the opposite case of using Tokio as part of a larger synchronous
    /// codebase.
    ///
    /// [`Builder`]: struct@crate::runtime::Builder
    /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
    /// [rayon]: https://docs.rs/rayon
    /// [`mpsc channel`]: crate::sync::mpsc
    /// [`SyncIoBridge`]: https://docs.rs/tokio-util/0.6/tokio_util/io/struct.SyncIoBridge.html
    /// [hyper]: https://docs.rs/hyper
    /// [`thread::spawn`]: fn@std::thread::spawn
    /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
    /// [bridgesync]: https://tokio.rs/tokio/topics/bridging
    /// [`AtomicBool`]: https://doc.rust-lang.org/core/sync/atomic/struct.AtomicBool.html
    ///
    /// # Examples
    ///
    /// Pass an input value and receive result of computation:
    ///
    /// ```
    /// use tokio::task;
    ///
    /// # async fn docs() -> Result<(), Box<dyn std::error::Error>>{
    /// // Initial input
    /// let mut v = "Hello, ".to_string();
    /// let res = task::spawn_blocking(move || {
    ///     // Stand-in for compute-heavy work or using synchronous APIs
    ///     v.push_str("world");
    ///     // Pass ownership of the value back to the asynchronous context
    ///     v
    /// }).await?;
    ///
    /// // `res` is the value returned from the thread
    /// assert_eq!(res.as_str(), "Hello, world");
    /// # Ok(())
    /// # }
    /// ```
    ///
    /// Use a channel:
    ///
    /// ```
    /// use tokio::task;
    /// use tokio::sync::mpsc;
    ///
    /// # async fn docs() {
    /// let (tx, mut rx) = mpsc::channel(2);
    /// let start = 5;
    /// let worker = task::spawn_blocking(move || {
    ///     for x in 0..10 {
    ///         // Stand in for complex computation
    ///         tx.blocking_send(start + x).unwrap();
    ///     }
    /// });
    ///
    /// let mut acc = 0;
    /// while let Some(v) = rx.recv().await {
    ///     acc += v;
    /// }
    /// assert_eq!(acc, 95);
    /// worker.await.unwrap();
    /// # }
    /// ```
    #[track_caller]
    pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R>
    where
        F: FnOnce() -> R + Send + 'static,
        R: Send + 'static,
    {
        crate::runtime::spawn_blocking(f)
    }
}