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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();
    //! }
    //! ```
    //!
    //! [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.
    //!
    //! **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.
    //!
    //! 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! {
        mod barrier;
        pub use barrier::{Barrier, BarrierWaitResult};

        pub mod broadcast;

        pub mod mpsc;

        mod mutex;
        pub use mutex::{Mutex, MutexGuard, TryLockError, OwnedMutexGuard};

        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, RwLockReadGuard, RwLockWriteGuard};

        mod task;
        pub(crate) use task::AtomicWaker;

        pub mod watch;
    }

    cfg_not_sync! {
        #[cfg(any(feature = "fs", feature = "signal", all(unix, feature = "process")))]
        pub(crate) mod batch_semaphore;

        cfg_fs! {
            mod mutex;
            pub(crate) use mutex::Mutex;
        }

        #[cfg(any(feature = "rt", feature = "signal", all(unix, feature = "process")))]
        pub(crate) mod notify;

        cfg_atomic_waker_impl! {
            mod task;
            pub(crate) use task::AtomicWaker;
        }

        #[cfg(any(
                feature = "rt",
                feature = "process",
                feature = "signal"))]
        pub(crate) mod oneshot;

        cfg_signal_internal! {
            pub(crate) mod mpsc;
        }
    }

    /// Unit tests
    #[cfg(test)]
    mod tests;