tokio 0.3.0

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 not okay, as it may prevent the executor from
    /// driving other futures forward.  This function runs the closure on the
    /// current thread by having the thread temporarily cease from being a core
    /// thread, and turns it into a blocking thread. See the [CPU-bound tasks
    /// and blocking code][blocking] section for more information.
    ///
    /// 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.
    ///
    /// Note that this function can only be used when using the `multi_thread` runtime.
    ///
    /// 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
    /// });
    /// # }
    /// ```
    pub fn block_in_place<F, R>(f: F) -> R
    where
        F: FnOnce() -> R,
    {
        crate::runtime::thread_pool::block_in_place(f)
    }
}

/// 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.
/// This 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. 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.
///
/// [`Builder`]: struct@crate::runtime::Builder
/// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
/// [rayon]: https://docs.rs/rayon
/// [`thread::spawn`]: fn@std::thread::spawn
/// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
///
/// # Examples
///
/// ```
/// use tokio::task;
///
/// # async fn docs() -> Result<(), Box<dyn std::error::Error>>{
/// let res = task::spawn_blocking(move || {
///     // do some compute-heavy work or call synchronous code
///     "done computing"
/// }).await?;
///
/// assert_eq!(res, "done computing");
/// # Ok(())
/// # }
/// ```
pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R>
where
    F: FnOnce() -> R + Send + 'static,
    R: Send + 'static,
{
    #[cfg(feature = "tracing")]
    let f = {
        let span = tracing::trace_span!(
            target: "tokio::task",
            "task",
            kind = %"blocking",
            function = %std::any::type_name::<F>(),
        );
        move || {
            let _g = span.enter();
            f()
        }
    };
    crate::runtime::spawn_blocking(f)
}