| #![cfg_attr(loom, allow(dead_code, unreachable_pub, unused_imports))] |
| |
| //! Synchronization primitives for use in asynchronous contexts. |
| //! |
| //! Tokio programs tend to be organized as a set of [tasks] where each task |
| //! operates independently and may be executed on separate physical threads. The |
| //! synchronization primitives provided in this module permit these independent |
| //! tasks to communicate together. |
| //! |
| //! [tasks]: crate::task |
| //! |
| //! # Message passing |
| //! |
| //! The most common form of synchronization in a Tokio program is message |
| //! passing. Two tasks operate independently and send messages to each other to |
| //! synchronize. Doing so has the advantage of avoiding shared state. |
| //! |
| //! Message passing is implemented using channels. A channel supports sending a |
| //! message from one producer task to one or more consumer tasks. There are a |
| //! few flavors of channels provided by Tokio. Each channel flavor supports |
| //! different message passing patterns. When a channel supports multiple |
| //! producers, many separate tasks may **send** messages. When a channel |
| //! supports multiple consumers, many different separate tasks may **receive** |
| //! messages. |
| //! |
| //! Tokio provides many different channel flavors as different message passing |
| //! patterns are best handled with different implementations. |
| //! |
| //! ## `oneshot` channel |
| //! |
| //! The [`oneshot` channel][oneshot] supports sending a **single** value from a |
| //! single producer to a single consumer. This channel is usually used to send |
| //! the result of a computation to a waiter. |
| //! |
| //! **Example:** using a [`oneshot` channel][oneshot] to receive the result of a |
| //! computation. |
| //! |
| //! ``` |
| //! use tokio::sync::oneshot; |
| //! |
| //! async fn some_computation() -> String { |
| //! "represents the result of the computation".to_string() |
| //! } |
| //! |
| //! #[tokio::main] |
| //! async fn main() { |
| //! let (tx, rx) = oneshot::channel(); |
| //! |
| //! tokio::spawn(async move { |
| //! let res = some_computation().await; |
| //! tx.send(res).unwrap(); |
| //! }); |
| //! |
| //! // Do other work while the computation is happening in the background |
| //! |
| //! // Wait for the computation result |
| //! let res = rx.await.unwrap(); |
| //! } |
| //! ``` |
| //! |
| //! Note, if the task produces a computation result as its final |
| //! action before terminating, the [`JoinHandle`] can be used to |
| //! receive that value instead of allocating resources for the |
| //! `oneshot` channel. Awaiting on [`JoinHandle`] returns `Result`. If |
| //! the task panics, the `Joinhandle` yields `Err` with the panic |
| //! cause. |
| //! |
| //! **Example:** |
| //! |
| //! ``` |
| //! async fn some_computation() -> String { |
| //! "the result of the computation".to_string() |
| //! } |
| //! |
| //! #[tokio::main] |
| //! async fn main() { |
| //! let join_handle = tokio::spawn(async move { |
| //! some_computation().await |
| //! }); |
| //! |
| //! // Do other work while the computation is happening in the background |
| //! |
| //! // Wait for the computation result |
| //! let res = join_handle.await.unwrap(); |
| //! } |
| //! ``` |
| //! |
| //! [oneshot]: oneshot |
| //! [`JoinHandle`]: crate::task::JoinHandle |
| //! |
| //! ## `mpsc` channel |
| //! |
| //! The [`mpsc` channel][mpsc] supports sending **many** values from **many** |
| //! producers to a single consumer. This channel is often used to send work to a |
| //! task or to receive the result of many computations. |
| //! |
| //! This is also the channel you should use if you want to send many messages |
| //! from a single producer to a single consumer. There is no dedicated spsc |
| //! channel. |
| //! |
| //! **Example:** using an mpsc to incrementally stream the results of a series |
| //! of computations. |
| //! |
| //! ``` |
| //! use tokio::sync::mpsc; |
| //! |
| //! async fn some_computation(input: u32) -> String { |
| //! format!("the result of computation {}", input) |
| //! } |
| //! |
| //! #[tokio::main] |
| //! async fn main() { |
| //! let (tx, mut rx) = mpsc::channel(100); |
| //! |
| //! tokio::spawn(async move { |
| //! for i in 0..10 { |
| //! let res = some_computation(i).await; |
| //! tx.send(res).await.unwrap(); |
| //! } |
| //! }); |
| //! |
| //! while let Some(res) = rx.recv().await { |
| //! println!("got = {}", res); |
| //! } |
| //! } |
| //! ``` |
| //! |
| //! The argument to `mpsc::channel` is the channel capacity. This is the maximum |
| //! number of values that can be stored in the channel pending receipt at any |
| //! given time. Properly setting this value is key in implementing robust |
| //! programs as the channel capacity plays a critical part in handling back |
| //! pressure. |
| //! |
| //! A common concurrency pattern for resource management is to spawn a task |
| //! dedicated to managing that resource and using message passing between other |
| //! tasks to interact with the resource. The resource may be anything that may |
| //! not be concurrently used. Some examples include a socket and program state. |
| //! For example, if multiple tasks need to send data over a single socket, spawn |
| //! a task to manage the socket and use a channel to synchronize. |
| //! |
| //! **Example:** sending data from many tasks over a single socket using message |
| //! passing. |
| //! |
| //! ```no_run |
| //! use tokio::io::{self, AsyncWriteExt}; |
| //! use tokio::net::TcpStream; |
| //! use tokio::sync::mpsc; |
| //! |
| //! #[tokio::main] |
| //! async fn main() -> io::Result<()> { |
| //! let mut socket = TcpStream::connect("www.example.com:1234").await?; |
| //! let (tx, mut rx) = mpsc::channel(100); |
| //! |
| //! for _ in 0..10 { |
| //! // Each task needs its own `tx` handle. This is done by cloning the |
| //! // original handle. |
| //! let tx = tx.clone(); |
| //! |
| //! tokio::spawn(async move { |
| //! tx.send(&b"data to write"[..]).await.unwrap(); |
| //! }); |
| //! } |
| //! |
| //! // The `rx` half of the channel returns `None` once **all** `tx` clones |
| //! // drop. To ensure `None` is returned, drop the handle owned by the |
| //! // current task. If this `tx` handle is not dropped, there will always |
| //! // be a single outstanding `tx` handle. |
| //! drop(tx); |
| //! |
| //! while let Some(res) = rx.recv().await { |
| //! socket.write_all(res).await?; |
| //! } |
| //! |
| //! Ok(()) |
| //! } |
| //! ``` |
| //! |
| //! The [`mpsc`][mpsc] and [`oneshot`][oneshot] channels can be combined to |
| //! provide a request / response type synchronization pattern with a shared |
| //! resource. A task is spawned to synchronize a resource and waits on commands |
| //! received on a [`mpsc`][mpsc] channel. Each command includes a |
| //! [`oneshot`][oneshot] `Sender` on which the result of the command is sent. |
| //! |
| //! **Example:** use a task to synchronize a `u64` counter. Each task sends an |
| //! "fetch and increment" command. The counter value **before** the increment is |
| //! sent over the provided `oneshot` channel. |
| //! |
| //! ``` |
| //! use tokio::sync::{oneshot, mpsc}; |
| //! use Command::Increment; |
| //! |
| //! enum Command { |
| //! Increment, |
| //! // Other commands can be added here |
| //! } |
| //! |
| //! #[tokio::main] |
| //! async fn main() { |
| //! let (cmd_tx, mut cmd_rx) = mpsc::channel::<(Command, oneshot::Sender<u64>)>(100); |
| //! |
| //! // Spawn a task to manage the counter |
| //! tokio::spawn(async move { |
| //! let mut counter: u64 = 0; |
| //! |
| //! while let Some((cmd, response)) = cmd_rx.recv().await { |
| //! match cmd { |
| //! Increment => { |
| //! let prev = counter; |
| //! counter += 1; |
| //! response.send(prev).unwrap(); |
| //! } |
| //! } |
| //! } |
| //! }); |
| //! |
| //! let mut join_handles = vec![]; |
| //! |
| //! // Spawn tasks that will send the increment command. |
| //! for _ in 0..10 { |
| //! let cmd_tx = cmd_tx.clone(); |
| //! |
| //! join_handles.push(tokio::spawn(async move { |
| //! let (resp_tx, resp_rx) = oneshot::channel(); |
| //! |
| //! cmd_tx.send((Increment, resp_tx)).await.ok().unwrap(); |
| //! let res = resp_rx.await.unwrap(); |
| //! |
| //! println!("previous value = {}", res); |
| //! })); |
| //! } |
| //! |
| //! // Wait for all tasks to complete |
| //! for join_handle in join_handles.drain(..) { |
| //! join_handle.await.unwrap(); |
| //! } |
| //! } |
| //! ``` |
| //! |
| //! [mpsc]: mpsc |
| //! |
| //! ## `broadcast` channel |
| //! |
| //! The [`broadcast` channel] supports sending **many** values from |
| //! **many** producers to **many** consumers. Each consumer will receive |
| //! **each** value. This channel can be used to implement "fan out" style |
| //! patterns common with pub / sub or "chat" systems. |
| //! |
| //! This channel tends to be used less often than `oneshot` and `mpsc` but still |
| //! has its use cases. |
| //! |
| //! This is also the channel you should use if you want to broadcast values from |
| //! a single producer to many consumers. There is no dedicated spmc broadcast |
| //! channel. |
| //! |
| //! Basic usage |
| //! |
| //! ``` |
| //! use tokio::sync::broadcast; |
| //! |
| //! #[tokio::main] |
| //! async fn main() { |
| //! let (tx, mut rx1) = broadcast::channel(16); |
| //! let mut rx2 = tx.subscribe(); |
| //! |
| //! tokio::spawn(async move { |
| //! assert_eq!(rx1.recv().await.unwrap(), 10); |
| //! assert_eq!(rx1.recv().await.unwrap(), 20); |
| //! }); |
| //! |
| //! tokio::spawn(async move { |
| //! assert_eq!(rx2.recv().await.unwrap(), 10); |
| //! assert_eq!(rx2.recv().await.unwrap(), 20); |
| //! }); |
| //! |
| //! tx.send(10).unwrap(); |
| //! tx.send(20).unwrap(); |
| //! } |
| //! ``` |
| //! |
| //! [`broadcast` channel]: crate::sync::broadcast |
| //! |
| //! ## `watch` channel |
| //! |
| //! The [`watch` channel] supports sending **many** values from a **single** |
| //! producer to **many** consumers. However, only the **most recent** value is |
| //! stored in the channel. Consumers are notified when a new value is sent, but |
| //! there is no guarantee that consumers will see **all** values. |
| //! |
| //! The [`watch` channel] is similar to a [`broadcast` channel] with capacity 1. |
| //! |
| //! Use cases for the [`watch` channel] include broadcasting configuration |
| //! changes or signalling program state changes, such as transitioning to |
| //! shutdown. |
| //! |
| //! **Example:** use a [`watch` channel] to notify tasks of configuration |
| //! changes. In this example, a configuration file is checked periodically. When |
| //! the file changes, the configuration changes are signalled to consumers. |
| //! |
| //! ``` |
| //! use tokio::sync::watch; |
| //! use tokio::time::{self, Duration, Instant}; |
| //! |
| //! use std::io; |
| //! |
| //! #[derive(Debug, Clone, Eq, PartialEq)] |
| //! struct Config { |
| //! timeout: Duration, |
| //! } |
| //! |
| //! impl Config { |
| //! async fn load_from_file() -> io::Result<Config> { |
| //! // file loading and deserialization logic here |
| //! # Ok(Config { timeout: Duration::from_secs(1) }) |
| //! } |
| //! } |
| //! |
| //! async fn my_async_operation() { |
| //! // Do something here |
| //! } |
| //! |
| //! #[tokio::main] |
| //! async fn main() { |
| //! // Load initial configuration value |
| //! let mut config = Config::load_from_file().await.unwrap(); |
| //! |
| //! // Create the watch channel, initialized with the loaded configuration |
| //! let (tx, rx) = watch::channel(config.clone()); |
| //! |
| //! // Spawn a task to monitor the file. |
| //! tokio::spawn(async move { |
| //! loop { |
| //! // Wait 10 seconds between checks |
| //! time::sleep(Duration::from_secs(10)).await; |
| //! |
| //! // Load the configuration file |
| //! let new_config = Config::load_from_file().await.unwrap(); |
| //! |
| //! // If the configuration changed, send the new config value |
| //! // on the watch channel. |
| //! if new_config != config { |
| //! tx.send(new_config.clone()).unwrap(); |
| //! config = new_config; |
| //! } |
| //! } |
| //! }); |
| //! |
| //! let mut handles = vec![]; |
| //! |
| //! // Spawn tasks that runs the async operation for at most `timeout`. If |
| //! // the timeout elapses, restart the operation. |
| //! // |
| //! // The task simultaneously watches the `Config` for changes. When the |
| //! // timeout duration changes, the timeout is updated without restarting |
| //! // the in-flight operation. |
| //! for _ in 0..5 { |
| //! // Clone a config watch handle for use in this task |
| //! let mut rx = rx.clone(); |
| //! |
| //! let handle = tokio::spawn(async move { |
| //! // Start the initial operation and pin the future to the stack. |
| //! // Pinning to the stack is required to resume the operation |
| //! // across multiple calls to `select!` |
| //! let op = my_async_operation(); |
| //! tokio::pin!(op); |
| //! |
| //! // Get the initial config value |
| //! let mut conf = rx.borrow().clone(); |
| //! |
| //! let mut op_start = Instant::now(); |
| //! let sleep = time::sleep_until(op_start + conf.timeout); |
| //! tokio::pin!(sleep); |
| //! |
| //! loop { |
| //! tokio::select! { |
| //! _ = &mut sleep => { |
| //! // The operation elapsed. Restart it |
| //! op.set(my_async_operation()); |
| //! |
| //! // Track the new start time |
| //! op_start = Instant::now(); |
| //! |
| //! // Restart the timeout |
| //! sleep.set(time::sleep_until(op_start + conf.timeout)); |
| //! } |
| //! _ = rx.changed() => { |
| //! conf = rx.borrow().clone(); |
| //! |
| //! // The configuration has been updated. Update the |
| //! // `sleep` using the new `timeout` value. |
| //! sleep.as_mut().reset(op_start + conf.timeout); |
| //! } |
| //! _ = &mut op => { |
| //! // The operation completed! |
| //! return |
| //! } |
| //! } |
| //! } |
| //! }); |
| //! |
| //! handles.push(handle); |
| //! } |
| //! |
| //! for handle in handles.drain(..) { |
| //! handle.await.unwrap(); |
| //! } |
| //! } |
| //! ``` |
| //! |
| //! [`watch` channel]: mod@crate::sync::watch |
| //! [`broadcast` channel]: mod@crate::sync::broadcast |
| //! |
| //! # State synchronization |
| //! |
| //! The remaining synchronization primitives focus on synchronizing state. |
| //! These are asynchronous equivalents to versions provided by `std`. They |
| //! operate in a similar way as their `std` counterparts but will wait |
| //! asynchronously instead of blocking the thread. |
| //! |
| //! * [`Barrier`](Barrier) Ensures multiple tasks will wait for each other to |
| //! reach a point in the program, before continuing execution all together. |
| //! |
| //! * [`Mutex`](Mutex) Mutual Exclusion mechanism, which ensures that at most |
| //! one thread at a time is able to access some data. |
| //! |
| //! * [`Notify`](Notify) Basic task notification. `Notify` supports notifying a |
| //! receiving task without sending data. In this case, the task wakes up and |
| //! resumes processing. |
| //! |
| //! * [`RwLock`](RwLock) Provides a mutual exclusion mechanism which allows |
| //! multiple readers at the same time, while allowing only one writer at a |
| //! time. In some cases, this can be more efficient than a mutex. |
| //! |
| //! * [`Semaphore`](Semaphore) Limits the amount of concurrency. A semaphore |
| //! holds a number of permits, which tasks may request in order to enter a |
| //! critical section. Semaphores are useful for implementing limiting or |
| //! bounding of any kind. |
| |
| cfg_sync! { |
| /// Named future types. |
| pub mod futures { |
| pub use super::notify::Notified; |
| } |
| |
| mod barrier; |
| pub use barrier::{Barrier, BarrierWaitResult}; |
| |
| pub mod broadcast; |
| |
| pub mod mpsc; |
| |
| mod mutex; |
| pub use mutex::{Mutex, MutexGuard, TryLockError, OwnedMutexGuard, MappedMutexGuard, OwnedMappedMutexGuard}; |
| |
| pub(crate) mod notify; |
| pub use notify::Notify; |
| |
| pub mod oneshot; |
| |
| pub(crate) mod batch_semaphore; |
| pub use batch_semaphore::{AcquireError, TryAcquireError}; |
| |
| mod semaphore; |
| pub use semaphore::{Semaphore, SemaphorePermit, OwnedSemaphorePermit}; |
| |
| mod rwlock; |
| pub use rwlock::RwLock; |
| pub use rwlock::owned_read_guard::OwnedRwLockReadGuard; |
| pub use rwlock::owned_write_guard::OwnedRwLockWriteGuard; |
| pub use rwlock::owned_write_guard_mapped::OwnedRwLockMappedWriteGuard; |
| pub use rwlock::read_guard::RwLockReadGuard; |
| pub use rwlock::write_guard::RwLockWriteGuard; |
| pub use rwlock::write_guard_mapped::RwLockMappedWriteGuard; |
| |
| mod task; |
| pub(crate) use task::AtomicWaker; |
| |
| mod once_cell; |
| pub use self::once_cell::{OnceCell, SetError}; |
| |
| pub mod watch; |
| } |
| |
| cfg_not_sync! { |
| cfg_fs! { |
| pub(crate) mod batch_semaphore; |
| mod mutex; |
| pub(crate) use mutex::Mutex; |
| } |
| |
| #[cfg(any(feature = "rt", feature = "signal", all(unix, feature = "process")))] |
| pub(crate) mod notify; |
| |
| #[cfg(any(feature = "rt", all(windows, feature = "process")))] |
| pub(crate) mod oneshot; |
| |
| cfg_atomic_waker_impl! { |
| mod task; |
| pub(crate) use task::AtomicWaker; |
| } |
| |
| #[cfg(any(feature = "signal", all(unix, feature = "process")))] |
| pub(crate) mod watch; |
| } |
| |
| /// Unit tests |
| #[cfg(test)] |
| mod tests; |