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Diffstat (limited to 'third_party/rust/tokio/src/task/mod.rs')
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diff --git a/third_party/rust/tokio/src/task/mod.rs b/third_party/rust/tokio/src/task/mod.rs new file mode 100644 index 0000000000..d532155a1f --- /dev/null +++ b/third_party/rust/tokio/src/task/mod.rs @@ -0,0 +1,317 @@ +//! Asynchronous green-threads. +//! +//! ## What are Tasks? +//! +//! A _task_ is a light weight, non-blocking unit of execution. A task is similar +//! to an OS thread, but rather than being managed by the OS scheduler, they are +//! managed by the [Tokio runtime][rt]. Another name for this general pattern is +//! [green threads]. If you are familiar with [Go's goroutines], [Kotlin's +//! coroutines], or [Erlang's processes], you can think of Tokio's tasks as +//! something similar. +//! +//! Key points about tasks include: +//! +//! * Tasks are **light weight**. Because tasks are scheduled by the Tokio +//! runtime rather than the operating system, creating new tasks or switching +//! between tasks does not require a context switch and has fairly low +//! overhead. Creating, running, and destroying large numbers of tasks is +//! quite cheap, especially compared to OS threads. +//! +//! * Tasks are scheduled **cooperatively**. Most operating systems implement +//! _preemptive multitasking_. This is a scheduling technique where the +//! operating system allows each thread to run for a period of time, and then +//! _preempts_ it, temporarily pausing that thread and switching to another. +//! Tasks, on the other hand, implement _cooperative multitasking_. In +//! cooperative multitasking, a task is allowed to run until it _yields_, +//! indicating to the Tokio runtime's scheduler that it cannot currently +//! continue executing. When a task yields, the Tokio runtime switches to +//! executing the next task. +//! +//! * Tasks are **non-blocking**. Typically, when an OS thread performs I/O or +//! must synchronize with another thread, it _blocks_, allowing the OS to +//! schedule another thread. When a task cannot continue executing, it must +//! yield instead, allowing the Tokio runtime to schedule another task. Tasks +//! should generally not perform system calls or other operations that could +//! block a thread, as this would prevent other tasks running on the same +//! thread from executing as well. Instead, this module provides APIs for +//! running blocking operations in an asynchronous context. +//! +//! [rt]: crate::runtime +//! [green threads]: https://en.wikipedia.org/wiki/Green_threads +//! [Go's goroutines]: https://tour.golang.org/concurrency/1 +//! [Kotlin's coroutines]: https://kotlinlang.org/docs/reference/coroutines-overview.html +//! [Erlang's processes]: http://erlang.org/doc/getting_started/conc_prog.html#processes +//! +//! ## Working with Tasks +//! +//! This module provides the following APIs for working with tasks: +//! +//! ### Spawning +//! +//! Perhaps the most important function in this module is [`task::spawn`]. This +//! function can be thought of as an async equivalent to the standard library's +//! [`thread::spawn`][`std::thread::spawn`]. It takes an `async` block or other +//! [future], and creates a new task to run that work concurrently: +//! +//! ``` +//! use tokio::task; +//! +//! # async fn doc() { +//! task::spawn(async { +//! // perform some work here... +//! }); +//! # } +//! ``` +//! +//! Like [`std::thread::spawn`], `task::spawn` returns a [`JoinHandle`] struct. +//! A `JoinHandle` is itself a future which may be used to await the output of +//! the spawned task. For example: +//! +//! ``` +//! use tokio::task; +//! +//! # #[tokio::main] async fn main() -> Result<(), Box<dyn std::error::Error>> { +//! let join = task::spawn(async { +//! // ... +//! "hello world!" +//! }); +//! +//! // ... +//! +//! // Await the result of the spawned task. +//! let result = join.await?; +//! assert_eq!(result, "hello world!"); +//! # Ok(()) +//! # } +//! ``` +//! +//! Again, like `std::thread`'s [`JoinHandle` type][thread_join], if the spawned +//! task panics, awaiting its `JoinHandle` will return a [`JoinError`]. For +//! example: +//! +//! ``` +//! use tokio::task; +//! +//! # #[tokio::main] async fn main() { +//! let join = task::spawn(async { +//! panic!("something bad happened!") +//! }); +//! +//! // The returned result indicates that the task failed. +//! assert!(join.await.is_err()); +//! # } +//! ``` +//! +//! `spawn`, `JoinHandle`, and `JoinError` are present when the "rt" +//! feature flag is enabled. +//! +//! [`task::spawn`]: crate::task::spawn() +//! [future]: std::future::Future +//! [`std::thread::spawn`]: std::thread::spawn +//! [`JoinHandle`]: crate::task::JoinHandle +//! [thread_join]: std::thread::JoinHandle +//! [`JoinError`]: crate::task::JoinError +//! +//! ### Blocking and Yielding +//! +//! As we discussed above, code running in asynchronous tasks should not perform +//! operations that can block. A blocking operation performed in a task running +//! on a thread that is also running other tasks would block the entire thread, +//! preventing other tasks from running. +//! +//! Instead, Tokio provides two APIs for running blocking operations in an +//! asynchronous context: [`task::spawn_blocking`] and [`task::block_in_place`]. +//! +//! Be aware that if you call a non-async method from async code, that non-async +//! method is still inside the asynchronous context, so you should also avoid +//! blocking operations there. This includes destructors of objects destroyed in +//! async code. +//! +//! #### spawn_blocking +//! +//! The `task::spawn_blocking` function is similar to the `task::spawn` function +//! discussed in the previous section, but rather than spawning an +//! _non-blocking_ future on the Tokio runtime, it instead spawns a +//! _blocking_ function on a dedicated thread pool for blocking tasks. For +//! example: +//! +//! ``` +//! use tokio::task; +//! +//! # async fn docs() { +//! task::spawn_blocking(|| { +//! // do some compute-heavy work or call synchronous code +//! }); +//! # } +//! ``` +//! +//! Just like `task::spawn`, `task::spawn_blocking` returns a `JoinHandle` +//! which we can use to await the result of the blocking operation: +//! +//! ```rust +//! # use tokio::task; +//! # async fn docs() -> Result<(), Box<dyn std::error::Error>>{ +//! let join = task::spawn_blocking(|| { +//! // do some compute-heavy work or call synchronous code +//! "blocking completed" +//! }); +//! +//! let result = join.await?; +//! assert_eq!(result, "blocking completed"); +//! # Ok(()) +//! # } +//! ``` +//! +//! #### block_in_place +//! +//! When using the [multi-threaded runtime][rt-multi-thread], the [`task::block_in_place`] +//! function is also available. Like `task::spawn_blocking`, this function +//! allows running a blocking operation from an asynchronous context. Unlike +//! `spawn_blocking`, however, `block_in_place` works by transitioning the +//! _current_ worker thread to a blocking thread, moving other tasks running on +//! that thread to another worker thread. This can improve performance by avoiding +//! context switches. +//! +//! For example: +//! +//! ``` +//! use tokio::task; +//! +//! # async fn docs() { +//! let result = task::block_in_place(|| { +//! // do some compute-heavy work or call synchronous code +//! "blocking completed" +//! }); +//! +//! assert_eq!(result, "blocking completed"); +//! # } +//! ``` +//! +//! #### yield_now +//! +//! In addition, this module provides a [`task::yield_now`] async function +//! that is analogous to the standard library's [`thread::yield_now`]. Calling +//! and `await`ing this function will cause the current task to yield to the +//! Tokio runtime's scheduler, allowing other tasks to be +//! scheduled. Eventually, the yielding task will be polled again, allowing it +//! to execute. For example: +//! +//! ```rust +//! use tokio::task; +//! +//! # #[tokio::main] async fn main() { +//! async { +//! task::spawn(async { +//! // ... +//! println!("spawned task done!") +//! }); +//! +//! // Yield, allowing the newly-spawned task to execute first. +//! task::yield_now().await; +//! println!("main task done!"); +//! } +//! # .await; +//! # } +//! ``` +//! +//! ### Cooperative scheduling +//! +//! A single call to [`poll`] on a top-level task may potentially do a lot of +//! work before it returns `Poll::Pending`. If a task runs for a long period of +//! time without yielding back to the executor, it can starve other tasks +//! waiting on that executor to execute them, or drive underlying resources. +//! Since Rust does not have a runtime, it is difficult to forcibly preempt a +//! long-running task. Instead, this module provides an opt-in mechanism for +//! futures to collaborate with the executor to avoid starvation. +//! +//! Consider a future like this one: +//! +//! ``` +//! # use tokio_stream::{Stream, StreamExt}; +//! async fn drop_all<I: Stream + Unpin>(mut input: I) { +//! while let Some(_) = input.next().await {} +//! } +//! ``` +//! +//! It may look harmless, but consider what happens under heavy load if the +//! input stream is _always_ ready. If we spawn `drop_all`, the task will never +//! yield, and will starve other tasks and resources on the same executor. +//! +//! To account for this, Tokio has explicit yield points in a number of library +//! functions, which force tasks to return to the executor periodically. +//! +//! +//! #### unconstrained +//! +//! If necessary, [`task::unconstrained`] lets you opt out a future of Tokio's cooperative +//! scheduling. When a future is wrapped with `unconstrained`, it will never be forced to yield to +//! Tokio. For example: +//! +//! ``` +//! # #[tokio::main] +//! # async fn main() { +//! use tokio::{task, sync::mpsc}; +//! +//! let fut = async { +//! let (tx, mut rx) = mpsc::unbounded_channel(); +//! +//! for i in 0..1000 { +//! let _ = tx.send(()); +//! // This will always be ready. If coop was in effect, this code would be forced to yield +//! // periodically. However, if left unconstrained, then this code will never yield. +//! rx.recv().await; +//! } +//! }; +//! +//! task::unconstrained(fut).await; +//! # } +//! ``` +//! +//! [`task::spawn_blocking`]: crate::task::spawn_blocking +//! [`task::block_in_place`]: crate::task::block_in_place +//! [rt-multi-thread]: ../runtime/index.html#threaded-scheduler +//! [`task::yield_now`]: crate::task::yield_now() +//! [`thread::yield_now`]: std::thread::yield_now +//! [`task::unconstrained`]: crate::task::unconstrained() +//! [`poll`]: method@std::future::Future::poll + +cfg_rt! { + pub use crate::runtime::task::{JoinError, JoinHandle}; + + mod blocking; + pub use blocking::spawn_blocking; + + mod spawn; + pub use spawn::spawn; + + cfg_rt_multi_thread! { + pub use blocking::block_in_place; + } + + mod yield_now; + pub use yield_now::yield_now; + + mod local; + pub use local::{spawn_local, LocalSet}; + + mod task_local; + pub use task_local::LocalKey; + + mod unconstrained; + pub use unconstrained::{unconstrained, Unconstrained}; + + cfg_unstable! { + mod join_set; + pub use join_set::JoinSet; + } + + cfg_trace! { + mod builder; + pub use builder::Builder; + } + + /// Task-related futures. + pub mod futures { + pub use super::task_local::TaskLocalFuture; + } +} |