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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.
/// 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(())
/// # }
/// ```
#[cfg_attr(tokio_track_caller, 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)
}
}
|
