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#![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();
//! }
//! ```
//!
//! [`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`] and [`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`] channel. Each command includes a [`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();
//! }
//! }
//! ```
//!
//! ## `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 **many**
//! 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_and_update().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`] Ensures multiple tasks will wait for each other to reach a
//! point in the program, before continuing execution all together.
//!
//! * [`Mutex`] Mutual Exclusion mechanism, which ensures that at most one
//! thread at a time is able to access some data.
//!
//! * [`Notify`] Basic task notification. `Notify` supports notifying a
//! receiving task without sending data. In this case, the task wakes up and
//! resumes processing.
//!
//! * [`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`] 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;