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Diffstat (limited to 'third_party/rust/tokio/src/task/blocking.rs')
| -rw-r--r-- | third_party/rust/tokio/src/task/blocking.rs | 199 |
1 files changed, 199 insertions, 0 deletions
diff --git a/third_party/rust/tokio/src/task/blocking.rs b/third_party/rust/tokio/src/task/blocking.rs new file mode 100644 index 0000000000..5fe358f3e5 --- /dev/null +++ b/third_party/rust/tokio/src/task/blocking.rs @@ -0,0 +1,199 @@ +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. 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 + /// + /// # 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) + } +} |