crates/tokio-util/src/task/task_tracker.rs - platform/external/rust/android-crates-io - Git at Google

 //! Types related to the [`TaskTracker`] collection.
 //!
 //! See the documentation of [`TaskTracker`] for more information.

 use pin_project_lite::pin_project;
 use std::fmt;
 use std::future::Future;
 use std::pin::Pin;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;
 use std::task::{Context, Poll};
 use tokio::sync::{futures::Notified, Notify};

 #[cfg(feature = "rt")]
 use tokio::{
     runtime::Handle,
     task::{JoinHandle, LocalSet},
 };

 /// A task tracker used for waiting until tasks exit.
 ///
 /// This is usually used together with [`CancellationToken`] to implement [graceful shutdown]. The
 /// `CancellationToken` is used to signal to tasks that they should shut down, and the
 /// `TaskTracker` is used to wait for them to finish shutting down.
 ///
 /// The `TaskTracker` will also keep track of a `closed` boolean. This is used to handle the case
 /// where the `TaskTracker` is empty, but we don't want to shut down yet. This means that the
 /// [`wait`] method will wait until *both* of the following happen at the same time:
 ///
 ///  * The `TaskTracker` must be closed using the [`close`] method.
 ///  * The `TaskTracker` must be empty, that is, all tasks that it is tracking must have exited.
 ///
 /// When a call to [`wait`] returns, it is guaranteed that all tracked tasks have exited and that
 /// the destructor of the future has finished running. However, there might be a short amount of
 /// time where [`JoinHandle::is_finished`] returns false.
 ///
 /// # Comparison to `JoinSet`
 ///
 /// The main Tokio crate has a similar collection known as [`JoinSet`]. The `JoinSet` type has a
 /// lot more features than `TaskTracker`, so `TaskTracker` should only be used when one of its
 /// unique features is required:
 ///
 ///  1. When tasks exit, a `TaskTracker` will allow the task to immediately free its memory.
 ///  2. By not closing the `TaskTracker`, [`wait`] will be prevented from returning even if
 ///     the `TaskTracker` is empty.
 ///  3. A `TaskTracker` does not require mutable access to insert tasks.
 ///  4. A `TaskTracker` can be cloned to share it with many tasks.
 ///
 /// The first point is the most important one. A [`JoinSet`] keeps track of the return value of
 /// every inserted task. This means that if the caller keeps inserting tasks and never calls
 /// [`join_next`], then their return values will keep building up and consuming memory, _even if_
 /// most of the tasks have already exited. This can cause the process to run out of memory. With a
 /// `TaskTracker`, this does not happen. Once tasks exit, they are immediately removed from the
 /// `TaskTracker`.
 ///
 /// # Examples
 ///
 /// For more examples, please see the topic page on [graceful shutdown].
 ///
 /// ## Spawn tasks and wait for them to exit
 ///
 /// This is a simple example. For this case, [`JoinSet`] should probably be used instead.
 ///
 /// ```
 /// use tokio_util::task::TaskTracker;
 ///
 /// #[tokio::main]
 /// async fn main() {
 ///     let tracker = TaskTracker::new();
 ///
 ///     for i in 0..10 {
 ///         tracker.spawn(async move {
 ///             println!("Task {} is running!", i);
 ///         });
 ///     }
 ///     // Once we spawned everything, we close the tracker.
 ///     tracker.close();
 ///
 ///     // Wait for everything to finish.
 ///     tracker.wait().await;
 ///
 ///     println!("This is printed after all of the tasks.");
 /// }
 /// ```
 ///
 /// ## Wait for tasks to exit
 ///
 /// This example shows the intended use-case of `TaskTracker`. It is used together with
 /// [`CancellationToken`] to implement graceful shutdown.
 /// ```
 /// use tokio_util::sync::CancellationToken;
 /// use tokio_util::task::TaskTracker;
 /// use tokio::time::{self, Duration};
 ///
 /// async fn background_task(num: u64) {
 ///     for i in 0..10 {
 ///         time::sleep(Duration::from_millis(100*num)).await;
 ///         println!("Background task {} in iteration {}.", num, i);
 ///     }
 /// }
 ///
 /// #[tokio::main]
 /// # async fn _hidden() {}
 /// # #[tokio::main(flavor = "current_thread", start_paused = true)]
 /// async fn main() {
 ///     let tracker = TaskTracker::new();
 ///     let token = CancellationToken::new();
 ///
 ///     for i in 0..10 {
 ///         let token = token.clone();
 ///         tracker.spawn(async move {
 ///             // Use a `tokio::select!` to kill the background task if the token is
 ///             // cancelled.
 ///             tokio::select! {
 ///                 () = background_task(i) => {
 ///                     println!("Task {} exiting normally.", i);
 ///                 },
 ///                 () = token.cancelled() => {
 ///                     // Do some cleanup before we really exit.
 ///                     time::sleep(Duration::from_millis(50)).await;
 ///                     println!("Task {} finished cleanup.", i);
 ///                 },
 ///             }
 ///         });
 ///     }
 ///
 ///     // Spawn a background task that will send the shutdown signal.
 ///     {
 ///         let tracker = tracker.clone();
 ///         tokio::spawn(async move {
 ///             // Normally you would use something like ctrl-c instead of
 ///             // sleeping.
 ///             time::sleep(Duration::from_secs(2)).await;
 ///             tracker.close();
 ///             token.cancel();
 ///         });
 ///     }
 ///
 ///     // Wait for all tasks to exit.
 ///     tracker.wait().await;
 ///
 ///     println!("All tasks have exited now.");
 /// }
 /// ```
 ///
 /// [`CancellationToken`]: crate::sync::CancellationToken
 /// [`JoinHandle::is_finished`]: tokio::task::JoinHandle::is_finished
 /// [`JoinSet`]: tokio::task::JoinSet
 /// [`close`]: Self::close
 /// [`join_next`]: tokio::task::JoinSet::join_next
 /// [`wait`]: Self::wait
 /// [graceful shutdown]: https://tokio.rs/tokio/topics/shutdown
 pub struct TaskTracker {
     inner: Arc<TaskTrackerInner>,
 }

 /// Represents a task tracked by a [`TaskTracker`].
 #[must_use]
 #[derive(Debug)]
 pub struct TaskTrackerToken {
     task_tracker: TaskTracker,
 }

 struct TaskTrackerInner {
     /// Keeps track of the state.
     ///
     /// The lowest bit is whether the task tracker is closed.
     ///
     /// The rest of the bits count the number of tracked tasks.
     state: AtomicUsize,
     /// Used to notify when the last task exits.
     on_last_exit: Notify,
 }

 pin_project! {
     /// A future that is tracked as a task by a [`TaskTracker`].
     ///
     /// The associated [`TaskTracker`] cannot complete until this future is dropped.
     ///
     /// This future is returned by [`TaskTracker::track_future`].
     #[must_use = "futures do nothing unless polled"]
     pub struct TrackedFuture<F> {
         #[pin]
         future: F,
         token: TaskTrackerToken,
     }
 }

 pin_project! {
     /// A future that completes when the [`TaskTracker`] is empty and closed.
     ///
     /// This future is returned by [`TaskTracker::wait`].
     #[must_use = "futures do nothing unless polled"]
     pub struct TaskTrackerWaitFuture<'a> {
         #[pin]
         future: Notified<'a>,
         inner: Option<&'a TaskTrackerInner>,
     }
 }

 impl TaskTrackerInner {
     #[inline]
     fn new() -> Self {
         Self {
             state: AtomicUsize::new(0),
             on_last_exit: Notify::new(),
         }
     }

     #[inline]
     fn is_closed_and_empty(&self) -> bool {
         // If empty and closed bit set, then we are done.
         //
         // The acquire load will synchronize with the release store of any previous call to
         // `set_closed` and `drop_task`.
         self.state.load(Ordering::Acquire) == 1
     }

     #[inline]
     fn set_closed(&self) -> bool {
         // The AcqRel ordering makes the closed bit behave like a `Mutex<bool>` for synchronization
         // purposes. We do this because it makes the return value of `TaskTracker::{close,reopen}`
         // more meaningful for the user. Without these orderings, this assert could fail:
         // ```
         // // thread 1
         // some_other_atomic.store(true, Relaxed);
         // tracker.close();
         //
         // // thread 2
         // if tracker.reopen() {
         //     assert!(some_other_atomic.load(Relaxed));
         // }
         // ```
         // However, with the AcqRel ordering, we establish a happens-before relationship from the
         // call to `close` and the later call to `reopen` that returned true.
         let state = self.state.fetch_or(1, Ordering::AcqRel);

         // If there are no tasks, and if it was not already closed:
         if state == 0 {
             self.notify_now();
         }

         (state & 1) == 0
     }

     #[inline]
     fn set_open(&self) -> bool {
         // See `set_closed` regarding the AcqRel ordering.
         let state = self.state.fetch_and(!1, Ordering::AcqRel);
         (state & 1) == 1
     }

     #[inline]
     fn add_task(&self) {
         self.state.fetch_add(2, Ordering::Relaxed);
     }

     #[inline]
     fn drop_task(&self) {
         let state = self.state.fetch_sub(2, Ordering::Release);

         // If this was the last task and we are closed:
         if state == 3 {
             self.notify_now();
         }
     }

     #[cold]
     fn notify_now(&self) {
         // Insert an acquire fence. This matters for `drop_task` but doesn't matter for
         // `set_closed` since it already uses AcqRel.
         //
         // This synchronizes with the release store of any other call to `drop_task`, and with the
         // release store in the call to `set_closed`. That ensures that everything that happened
         // before those other calls to `drop_task` or `set_closed` will be visible after this load,
         // and those things will also be visible to anything woken by the call to `notify_waiters`.
         self.state.load(Ordering::Acquire);

         self.on_last_exit.notify_waiters();
     }
 }

 impl TaskTracker {
     /// Creates a new `TaskTracker`.
     ///
     /// The `TaskTracker` will start out as open.
     #[must_use]
     pub fn new() -> Self {
         Self {
             inner: Arc::new(TaskTrackerInner::new()),
         }
     }

     /// Waits until this `TaskTracker` is both closed and empty.
     ///
     /// If the `TaskTracker` is already closed and empty when this method is called, then it
     /// returns immediately.
     ///
     /// The `wait` future is resistant against [ABA problems][aba]. That is, if the `TaskTracker`
     /// becomes both closed and empty for a short amount of time, then it is guarantee that all
     /// `wait` futures that were created before the short time interval will trigger, even if they
     /// are not polled during that short time interval.
     ///
     /// # Cancel safety
     ///
     /// This method is cancel safe.
     ///
     /// However, the resistance against [ABA problems][aba] is lost when using `wait` as the
     /// condition in a `tokio::select!` loop.
     ///
     /// [aba]: https://en.wikipedia.org/wiki/ABA_problem
     #[inline]
     pub fn wait(&self) -> TaskTrackerWaitFuture<'_> {
         TaskTrackerWaitFuture {
             future: self.inner.on_last_exit.notified(),
             inner: if self.inner.is_closed_and_empty() {
                 None
             } else {
                 Some(&self.inner)
             },
         }
     }

     /// Close this `TaskTracker`.
     ///
     /// This allows [`wait`] futures to complete. It does not prevent you from spawning new tasks.
     ///
     /// Returns `true` if this closed the `TaskTracker`, or `false` if it was already closed.
     ///
     /// [`wait`]: Self::wait
     #[inline]
     pub fn close(&self) -> bool {
         self.inner.set_closed()
     }

     /// Reopen this `TaskTracker`.
     ///
     /// This prevents [`wait`] futures from completing even if the `TaskTracker` is empty.
     ///
     /// Returns `true` if this reopened the `TaskTracker`, or `false` if it was already open.
     ///
     /// [`wait`]: Self::wait
     #[inline]
     pub fn reopen(&self) -> bool {
         self.inner.set_open()
     }

     /// Returns `true` if this `TaskTracker` is [closed](Self::close).
     #[inline]
     #[must_use]
     pub fn is_closed(&self) -> bool {
         (self.inner.state.load(Ordering::Acquire) & 1) != 0
     }

     /// Returns the number of tasks tracked by this `TaskTracker`.
     #[inline]
     #[must_use]
     pub fn len(&self) -> usize {
         self.inner.state.load(Ordering::Acquire) >> 1
     }

     /// Returns `true` if there are no tasks in this `TaskTracker`.
     #[inline]
     #[must_use]
     pub fn is_empty(&self) -> bool {
         self.inner.state.load(Ordering::Acquire) <= 1
     }

     /// Spawn the provided future on the current Tokio runtime, and track it in this `TaskTracker`.
     ///
     /// This is equivalent to `tokio::spawn(tracker.track_future(task))`.
     #[inline]
     #[track_caller]
     #[cfg(feature = "rt")]
     #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
     pub fn spawn<F>(&self, task: F) -> JoinHandle<F::Output>
     where
         F: Future + Send + 'static,
         F::Output: Send + 'static,
     {
         tokio::task::spawn(self.track_future(task))
     }

     /// Spawn the provided future on the provided Tokio runtime, and track it in this `TaskTracker`.
     ///
     /// This is equivalent to `handle.spawn(tracker.track_future(task))`.
     #[inline]
     #[track_caller]
     #[cfg(feature = "rt")]
     #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
     pub fn spawn_on<F>(&self, task: F, handle: &Handle) -> JoinHandle<F::Output>
     where
         F: Future + Send + 'static,
         F::Output: Send + 'static,
     {
         handle.spawn(self.track_future(task))
     }

     /// Spawn the provided future on the current [`LocalSet`], and track it in this `TaskTracker`.
     ///
     /// This is equivalent to `tokio::task::spawn_local(tracker.track_future(task))`.
     ///
     /// [`LocalSet`]: tokio::task::LocalSet
     #[inline]
     #[track_caller]
     #[cfg(feature = "rt")]
     #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
     pub fn spawn_local<F>(&self, task: F) -> JoinHandle<F::Output>
     where
         F: Future + 'static,
         F::Output: 'static,
     {
         tokio::task::spawn_local(self.track_future(task))
     }

     /// Spawn the provided future on the provided [`LocalSet`], and track it in this `TaskTracker`.
     ///
     /// This is equivalent to `local_set.spawn_local(tracker.track_future(task))`.
     ///
     /// [`LocalSet`]: tokio::task::LocalSet
     #[inline]
     #[track_caller]
     #[cfg(feature = "rt")]
     #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
     pub fn spawn_local_on<F>(&self, task: F, local_set: &LocalSet) -> JoinHandle<F::Output>
     where
         F: Future + 'static,
         F::Output: 'static,
     {
         local_set.spawn_local(self.track_future(task))
     }

     /// Spawn the provided blocking task on the current Tokio runtime, and track it in this `TaskTracker`.
     ///
     /// This is equivalent to `tokio::task::spawn_blocking(tracker.track_future(task))`.
     #[inline]
     #[track_caller]
     #[cfg(feature = "rt")]
     #[cfg(not(target_family = "wasm"))]
     #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
     pub fn spawn_blocking<F, T>(&self, task: F) -> JoinHandle<T>
     where
         F: FnOnce() -> T,
         F: Send + 'static,
         T: Send + 'static,
     {
         let token = self.token();
         tokio::task::spawn_blocking(move || {
             let res = task();
             drop(token);
             res
         })
     }

     /// Spawn the provided blocking task on the provided Tokio runtime, and track it in this `TaskTracker`.
     ///
     /// This is equivalent to `handle.spawn_blocking(tracker.track_future(task))`.
     #[inline]
     #[track_caller]
     #[cfg(feature = "rt")]
     #[cfg(not(target_family = "wasm"))]
     #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
     pub fn spawn_blocking_on<F, T>(&self, task: F, handle: &Handle) -> JoinHandle<T>
     where
         F: FnOnce() -> T,
         F: Send + 'static,
         T: Send + 'static,
     {
         let token = self.token();
         handle.spawn_blocking(move || {
             let res = task();
             drop(token);
             res
         })
     }

     /// Track the provided future.
     ///
     /// The returned [`TrackedFuture`] will count as a task tracked by this collection, and will
     /// prevent calls to [`wait`] from returning until the task is dropped.
     ///
     /// The task is removed from the collection when it is dropped, not when [`poll`] returns
     /// [`Poll::Ready`].
     ///
     /// # Examples
     ///
     /// Track a future spawned with [`tokio::spawn`].
     ///
     /// ```
     /// # async fn my_async_fn() {}
     /// use tokio_util::task::TaskTracker;
     ///
     /// # #[tokio::main(flavor = "current_thread")]
     /// # async fn main() {
     /// let tracker = TaskTracker::new();
     ///
     /// tokio::spawn(tracker.track_future(my_async_fn()));
     /// # }
     /// ```
     ///
     /// Track a future spawned on a [`JoinSet`].
     /// ```
     /// # async fn my_async_fn() {}
     /// use tokio::task::JoinSet;
     /// use tokio_util::task::TaskTracker;
     ///
     /// # #[tokio::main(flavor = "current_thread")]
     /// # async fn main() {
     /// let tracker = TaskTracker::new();
     /// let mut join_set = JoinSet::new();
     ///
     /// join_set.spawn(tracker.track_future(my_async_fn()));
     /// # }
     /// ```
     ///
     /// [`JoinSet`]: tokio::task::JoinSet
     /// [`Poll::Pending`]: std::task::Poll::Pending
     /// [`poll`]: std::future::Future::poll
     /// [`wait`]: Self::wait
     #[inline]
     pub fn track_future<F: Future>(&self, future: F) -> TrackedFuture<F> {
         TrackedFuture {
             future,
             token: self.token(),
         }
     }

     /// Creates a [`TaskTrackerToken`] representing a task tracked by this `TaskTracker`.
     ///
     /// This token is a lower-level utility than the spawn methods. Each token is considered to
     /// correspond to a task. As long as the token exists, the `TaskTracker` cannot complete.
     /// Furthermore, the count returned by the [`len`] method will include the tokens in the count.
     ///
     /// Dropping the token indicates to the `TaskTracker` that the task has exited.
     ///
     /// [`len`]: TaskTracker::len
     #[inline]
     pub fn token(&self) -> TaskTrackerToken {
         self.inner.add_task();
         TaskTrackerToken {
             task_tracker: self.clone(),
         }
     }

     /// Returns `true` if both task trackers correspond to the same set of tasks.
     ///
     /// # Examples
     ///
     /// ```
     /// use tokio_util::task::TaskTracker;
     ///
     /// let tracker_1 = TaskTracker::new();
     /// let tracker_2 = TaskTracker::new();
     /// let tracker_1_clone = tracker_1.clone();
     ///
     /// assert!(TaskTracker::ptr_eq(&tracker_1, &tracker_1_clone));
     /// assert!(!TaskTracker::ptr_eq(&tracker_1, &tracker_2));
     /// ```
     #[inline]
     #[must_use]
     pub fn ptr_eq(left: &TaskTracker, right: &TaskTracker) -> bool {
         Arc::ptr_eq(&left.inner, &right.inner)
     }
 }

 impl Default for TaskTracker {
     /// Creates a new `TaskTracker`.
     ///
     /// The `TaskTracker` will start out as open.
     #[inline]
     fn default() -> TaskTracker {
         TaskTracker::new()
     }
 }

 impl Clone for TaskTracker {
     /// Returns a new `TaskTracker` that tracks the same set of tasks.
     ///
     /// Since the new `TaskTracker` shares the same set of tasks, changes to one set are visible in
     /// all other clones.
     ///
     /// # Examples
     ///
     /// ```
     /// use tokio_util::task::TaskTracker;
     ///
     /// #[tokio::main]
     /// # async fn _hidden() {}
     /// # #[tokio::main(flavor = "current_thread")]
     /// async fn main() {
     ///     let tracker = TaskTracker::new();
     ///     let cloned = tracker.clone();
     ///
     ///     // Spawns on `tracker` are visible in `cloned`.
     ///     tracker.spawn(std::future::pending::<()>());
     ///     assert_eq!(cloned.len(), 1);
     ///
     ///     // Spawns on `cloned` are visible in `tracker`.
     ///     cloned.spawn(std::future::pending::<()>());
     ///     assert_eq!(tracker.len(), 2);
     ///
     ///     // Calling `close` is visible to `cloned`.
     ///     tracker.close();
     ///     assert!(cloned.is_closed());
     ///
     ///     // Calling `reopen` is visible to `tracker`.
     ///     cloned.reopen();
     ///     assert!(!tracker.is_closed());
     /// }
     /// ```
     #[inline]
     fn clone(&self) -> TaskTracker {
         Self {
             inner: self.inner.clone(),
         }
     }
 }

 fn debug_inner(inner: &TaskTrackerInner, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     let state = inner.state.load(Ordering::Acquire);
     let is_closed = (state & 1) != 0;
     let len = state >> 1;

     f.debug_struct("TaskTracker")
         .field("len", &len)
         .field("is_closed", &is_closed)
         .field("inner", &(inner as *const TaskTrackerInner))
         .finish()
 }

 impl fmt::Debug for TaskTracker {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         debug_inner(&self.inner, f)
     }
 }

 impl TaskTrackerToken {
     /// Returns the [`TaskTracker`] that this token is associated with.
     #[inline]
     #[must_use]
     pub fn task_tracker(&self) -> &TaskTracker {
         &self.task_tracker
     }
 }

 impl Clone for TaskTrackerToken {
     /// Returns a new `TaskTrackerToken` associated with the same [`TaskTracker`].
     ///
     /// This is equivalent to `token.task_tracker().token()`.
     #[inline]
     fn clone(&self) -> TaskTrackerToken {
         self.task_tracker.token()
     }
 }

 impl Drop for TaskTrackerToken {
     /// Dropping the token indicates to the [`TaskTracker`] that the task has exited.
     #[inline]
     fn drop(&mut self) {
         self.task_tracker.inner.drop_task();
     }
 }

 impl<F: Future> Future for TrackedFuture<F> {
     type Output = F::Output;

     #[inline]
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<F::Output> {
         self.project().future.poll(cx)
     }
 }

 impl<F: fmt::Debug> fmt::Debug for TrackedFuture<F> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("TrackedFuture")
             .field("future", &self.future)
             .field("task_tracker", self.token.task_tracker())
             .finish()
     }
 }

 impl<'a> Future for TaskTrackerWaitFuture<'a> {
     type Output = ();

     #[inline]
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
         let me = self.project();

         let inner = match me.inner.as_ref() {
             None => return Poll::Ready(()),
             Some(inner) => inner,
         };

         let ready = inner.is_closed_and_empty() || me.future.poll(cx).is_ready();
         if ready {
             *me.inner = None;
             Poll::Ready(())
         } else {
             Poll::Pending
         }
     }
 }

 impl<'a> fmt::Debug for TaskTrackerWaitFuture<'a> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         struct Helper<'a>(&'a TaskTrackerInner);

         impl fmt::Debug for Helper<'_> {
             fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                 debug_inner(self.0, f)
             }
         }

         f.debug_struct("TaskTrackerWaitFuture")
             .field("future", &self.future)
             .field("task_tracker", &self.inner.map(Helper))
             .finish()
     }
 }
	//! Types related to the [`TaskTracker`] collection.
	//!
	//! See the documentation of [`TaskTracker`] for more information.

	use pin_project_lite::pin_project;
	use std::fmt;
	use std::future::Future;
	use std::pin::Pin;
	use std::sync::atomic::{AtomicUsize, Ordering};
	use std::sync::Arc;
	use std::task::{Context, Poll};
	use tokio::sync::{futures::Notified, Notify};

	#[cfg(feature = "rt")]
	use tokio::{
	runtime::Handle,
	task::{JoinHandle, LocalSet},
	};

	/// A task tracker used for waiting until tasks exit.
	///
	/// This is usually used together with [`CancellationToken`] to implement [graceful shutdown]. The
	/// `CancellationToken` is used to signal to tasks that they should shut down, and the
	/// `TaskTracker` is used to wait for them to finish shutting down.
	///
	/// The `TaskTracker` will also keep track of a `closed` boolean. This is used to handle the case
	/// where the `TaskTracker` is empty, but we don't want to shut down yet. This means that the
	/// [`wait`] method will wait until both of the following happen at the same time:
	///
	/// * The `TaskTracker` must be closed using the [`close`] method.
	/// * The `TaskTracker` must be empty, that is, all tasks that it is tracking must have exited.
	///
	/// When a call to [`wait`] returns, it is guaranteed that all tracked tasks have exited and that
	/// the destructor of the future has finished running. However, there might be a short amount of
	/// time where [`JoinHandle::is_finished`] returns false.
	///
	/// # Comparison to `JoinSet`
	///
	/// The main Tokio crate has a similar collection known as [`JoinSet`]. The `JoinSet` type has a
	/// lot more features than `TaskTracker`, so `TaskTracker` should only be used when one of its
	/// unique features is required:
	///
	/// 1. When tasks exit, a `TaskTracker` will allow the task to immediately free its memory.
	/// 2. By not closing the `TaskTracker`, [`wait`] will be prevented from returning even if
	/// the `TaskTracker` is empty.
	/// 3. A `TaskTracker` does not require mutable access to insert tasks.
	/// 4. A `TaskTracker` can be cloned to share it with many tasks.
	///
	/// The first point is the most important one. A [`JoinSet`] keeps track of the return value of
	/// every inserted task. This means that if the caller keeps inserting tasks and never calls
	/// [`join_next`], then their return values will keep building up and consuming memory, _even if_
	/// most of the tasks have already exited. This can cause the process to run out of memory. With a
	/// `TaskTracker`, this does not happen. Once tasks exit, they are immediately removed from the
	/// `TaskTracker`.
	///
	/// # Examples
	///
	/// For more examples, please see the topic page on [graceful shutdown].
	///
	/// ## Spawn tasks and wait for them to exit
	///
	/// This is a simple example. For this case, [`JoinSet`] should probably be used instead.
	///
	/// ```
	/// use tokio_util::task::TaskTracker;
	///
	/// #[tokio::main]
	/// async fn main() {
	/// let tracker = TaskTracker::new();
	///
	/// for i in 0..10 {
	/// tracker.spawn(async move {
	/// println!("Task {} is running!", i);
	/// });
	/// }
	/// // Once we spawned everything, we close the tracker.
	/// tracker.close();
	///
	/// // Wait for everything to finish.
	/// tracker.wait().await;
	///
	/// println!("This is printed after all of the tasks.");
	/// }
	/// ```
	///
	/// ## Wait for tasks to exit
	///
	/// This example shows the intended use-case of `TaskTracker`. It is used together with
	/// [`CancellationToken`] to implement graceful shutdown.
	/// ```
	/// use tokio_util::sync::CancellationToken;
	/// use tokio_util::task::TaskTracker;
	/// use tokio::time::{self, Duration};
	///
	/// async fn background_task(num: u64) {
	/// for i in 0..10 {
	/// time::sleep(Duration::from_millis(100*num)).await;
	/// println!("Background task {} in iteration {}.", num, i);
	/// }
	/// }
	///
	/// #[tokio::main]
	/// # async fn _hidden() {}
	/// # #[tokio::main(flavor = "current_thread", start_paused = true)]
	/// async fn main() {
	/// let tracker = TaskTracker::new();
	/// let token = CancellationToken::new();
	///
	/// for i in 0..10 {
	/// let token = token.clone();
	/// tracker.spawn(async move {
	/// // Use a `tokio::select!` to kill the background task if the token is
	/// // cancelled.
	/// tokio::select! {
	/// () = background_task(i) => {
	/// println!("Task {} exiting normally.", i);
	/// },
	/// () = token.cancelled() => {
	/// // Do some cleanup before we really exit.
	/// time::sleep(Duration::from_millis(50)).await;
	/// println!("Task {} finished cleanup.", i);
	/// },
	/// }
	/// });
	/// }
	///
	/// // Spawn a background task that will send the shutdown signal.
	/// {
	/// let tracker = tracker.clone();
	/// tokio::spawn(async move {
	/// // Normally you would use something like ctrl-c instead of
	/// // sleeping.
	/// time::sleep(Duration::from_secs(2)).await;
	/// tracker.close();
	/// token.cancel();
	/// });
	/// }
	///
	/// // Wait for all tasks to exit.
	/// tracker.wait().await;
	///
	/// println!("All tasks have exited now.");
	/// }
	/// ```
	///
	/// [`CancellationToken`]: crate::sync::CancellationToken
	/// [`JoinHandle::is_finished`]: tokio::task::JoinHandle::is_finished
	/// [`JoinSet`]: tokio::task::JoinSet
	/// [`close`]: Self::close
	/// [`join_next`]: tokio::task::JoinSet::join_next
	/// [`wait`]: Self::wait
	/// [graceful shutdown]: https://tokio.rs/tokio/topics/shutdown
	pub struct TaskTracker {
	inner: Arc<TaskTrackerInner>,
	}

	/// Represents a task tracked by a [`TaskTracker`].
	#[must_use]
	#[derive(Debug)]
	pub struct TaskTrackerToken {
	task_tracker: TaskTracker,
	}

	struct TaskTrackerInner {
	/// Keeps track of the state.
	///
	/// The lowest bit is whether the task tracker is closed.
	///
	/// The rest of the bits count the number of tracked tasks.
	state: AtomicUsize,
	/// Used to notify when the last task exits.
	on_last_exit: Notify,
	}

	pin_project! {
	/// A future that is tracked as a task by a [`TaskTracker`].
	///
	/// The associated [`TaskTracker`] cannot complete until this future is dropped.
	///
	/// This future is returned by [`TaskTracker::track_future`].
	#[must_use = "futures do nothing unless polled"]
	pub struct TrackedFuture<F> {
	#[pin]
	future: F,
	token: TaskTrackerToken,
	}
	}

	pin_project! {
	/// A future that completes when the [`TaskTracker`] is empty and closed.
	///
	/// This future is returned by [`TaskTracker::wait`].
	#[must_use = "futures do nothing unless polled"]
	pub struct TaskTrackerWaitFuture<'a> {
	#[pin]
	future: Notified<'a>,
	inner: Option<&'a TaskTrackerInner>,
	}
	}

	impl TaskTrackerInner {
	#[inline]
	fn new() -> Self {
	Self {
	state: AtomicUsize::new(0),
	on_last_exit: Notify::new(),
	}
	}

	#[inline]
	fn is_closed_and_empty(&self) -> bool {
	// If empty and closed bit set, then we are done.
	//
	// The acquire load will synchronize with the release store of any previous call to
	// `set_closed` and `drop_task`.
	self.state.load(Ordering::Acquire) == 1
	}

	#[inline]
	fn set_closed(&self) -> bool {
	// The AcqRel ordering makes the closed bit behave like a `Mutex<bool>` for synchronization
	// purposes. We do this because it makes the return value of `TaskTracker::{close,reopen}`
	// more meaningful for the user. Without these orderings, this assert could fail:
	// ```
	// // thread 1
	// some_other_atomic.store(true, Relaxed);
	// tracker.close();
	//
	// // thread 2
	// if tracker.reopen() {
	// assert!(some_other_atomic.load(Relaxed));
	// }
	// ```
	// However, with the AcqRel ordering, we establish a happens-before relationship from the
	// call to `close` and the later call to `reopen` that returned true.
	let state = self.state.fetch_or(1, Ordering::AcqRel);

	// If there are no tasks, and if it was not already closed:
	if state == 0 {
	self.notify_now();
	}

	(state & 1) == 0
	}

	#[inline]
	fn set_open(&self) -> bool {
	// See `set_closed` regarding the AcqRel ordering.
	let state = self.state.fetch_and(!1, Ordering::AcqRel);
	(state & 1) == 1
	}

	#[inline]
	fn add_task(&self) {
	self.state.fetch_add(2, Ordering::Relaxed);
	}

	#[inline]
	fn drop_task(&self) {
	let state = self.state.fetch_sub(2, Ordering::Release);

	// If this was the last task and we are closed:
	if state == 3 {
	self.notify_now();
	}
	}

	#[cold]
	fn notify_now(&self) {
	// Insert an acquire fence. This matters for `drop_task` but doesn't matter for
	// `set_closed` since it already uses AcqRel.
	//
	// This synchronizes with the release store of any other call to `drop_task`, and with the
	// release store in the call to `set_closed`. That ensures that everything that happened
	// before those other calls to `drop_task` or `set_closed` will be visible after this load,
	// and those things will also be visible to anything woken by the call to `notify_waiters`.
	self.state.load(Ordering::Acquire);

	self.on_last_exit.notify_waiters();
	}
	}

	impl TaskTracker {
	/// Creates a new `TaskTracker`.
	///
	/// The `TaskTracker` will start out as open.
	#[must_use]
	pub fn new() -> Self {
	Self {
	inner: Arc::new(TaskTrackerInner::new()),
	}
	}

	/// Waits until this `TaskTracker` is both closed and empty.
	///
	/// If the `TaskTracker` is already closed and empty when this method is called, then it
	/// returns immediately.
	///
	/// The `wait` future is resistant against [ABA problems][aba]. That is, if the `TaskTracker`
	/// becomes both closed and empty for a short amount of time, then it is guarantee that all
	/// `wait` futures that were created before the short time interval will trigger, even if they
	/// are not polled during that short time interval.
	///
	/// # Cancel safety
	///
	/// This method is cancel safe.
	///
	/// However, the resistance against [ABA problems][aba] is lost when using `wait` as the
	/// condition in a `tokio::select!` loop.
	///
	/// [aba]: https://en.wikipedia.org/wiki/ABA_problem
	#[inline]
	pub fn wait(&self) -> TaskTrackerWaitFuture<'_> {
	TaskTrackerWaitFuture {
	future: self.inner.on_last_exit.notified(),
	inner: if self.inner.is_closed_and_empty() {
	None
	} else {
	Some(&self.inner)
	},
	}
	}

	/// Close this `TaskTracker`.
	///
	/// This allows [`wait`] futures to complete. It does not prevent you from spawning new tasks.
	///
	/// Returns `true` if this closed the `TaskTracker`, or `false` if it was already closed.
	///
	/// [`wait`]: Self::wait
	#[inline]
	pub fn close(&self) -> bool {
	self.inner.set_closed()
	}

	/// Reopen this `TaskTracker`.
	///
	/// This prevents [`wait`] futures from completing even if the `TaskTracker` is empty.
	///
	/// Returns `true` if this reopened the `TaskTracker`, or `false` if it was already open.
	///
	/// [`wait`]: Self::wait
	#[inline]
	pub fn reopen(&self) -> bool {
	self.inner.set_open()
	}

	/// Returns `true` if this `TaskTracker` is [closed](Self::close).
	#[inline]
	#[must_use]
	pub fn is_closed(&self) -> bool {
	(self.inner.state.load(Ordering::Acquire) & 1) != 0
	}

	/// Returns the number of tasks tracked by this `TaskTracker`.
	#[inline]
	#[must_use]
	pub fn len(&self) -> usize {
	self.inner.state.load(Ordering::Acquire) >> 1
	}

	/// Returns `true` if there are no tasks in this `TaskTracker`.
	#[inline]
	#[must_use]
	pub fn is_empty(&self) -> bool {
	self.inner.state.load(Ordering::Acquire) <= 1
	}

	/// Spawn the provided future on the current Tokio runtime, and track it in this `TaskTracker`.
	///
	/// This is equivalent to `tokio::spawn(tracker.track_future(task))`.
	#[inline]
	#[track_caller]
	#[cfg(feature = "rt")]
	#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
	pub fn spawn<F>(&self, task: F) -> JoinHandle<F::Output>
	where
	F: Future + Send + 'static,
	F::Output: Send + 'static,
	{
	tokio::task::spawn(self.track_future(task))
	}

	/// Spawn the provided future on the provided Tokio runtime, and track it in this `TaskTracker`.
	///
	/// This is equivalent to `handle.spawn(tracker.track_future(task))`.
	#[inline]
	#[track_caller]
	#[cfg(feature = "rt")]
	#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
	pub fn spawn_on<F>(&self, task: F, handle: &Handle) -> JoinHandle<F::Output>
	where
	F: Future + Send + 'static,
	F::Output: Send + 'static,
	{
	handle.spawn(self.track_future(task))
	}

	/// Spawn the provided future on the current [`LocalSet`], and track it in this `TaskTracker`.
	///
	/// This is equivalent to `tokio::task::spawn_local(tracker.track_future(task))`.
	///
	/// [`LocalSet`]: tokio::task::LocalSet
	#[inline]
	#[track_caller]
	#[cfg(feature = "rt")]
	#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
	pub fn spawn_local<F>(&self, task: F) -> JoinHandle<F::Output>
	where
	F: Future + 'static,
	F::Output: 'static,
	{
	tokio::task::spawn_local(self.track_future(task))
	}

	/// Spawn the provided future on the provided [`LocalSet`], and track it in this `TaskTracker`.
	///
	/// This is equivalent to `local_set.spawn_local(tracker.track_future(task))`.
	///
	/// [`LocalSet`]: tokio::task::LocalSet
	#[inline]
	#[track_caller]
	#[cfg(feature = "rt")]
	#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
	pub fn spawn_local_on<F>(&self, task: F, local_set: &LocalSet) -> JoinHandle<F::Output>
	where
	F: Future + 'static,
	F::Output: 'static,
	{
	local_set.spawn_local(self.track_future(task))
	}

	/// Spawn the provided blocking task on the current Tokio runtime, and track it in this `TaskTracker`.
	///
	/// This is equivalent to `tokio::task::spawn_blocking(tracker.track_future(task))`.
	#[inline]
	#[track_caller]
	#[cfg(feature = "rt")]
	#[cfg(not(target_family = "wasm"))]
	#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
	pub fn spawn_blocking<F, T>(&self, task: F) -> JoinHandle<T>
	where
	F: FnOnce() -> T,
	F: Send + 'static,
	T: Send + 'static,
	{
	let token = self.token();
	tokio::task::spawn_blocking(move \|\| {
	let res = task();
	drop(token);
	res
	})
	}

	/// Spawn the provided blocking task on the provided Tokio runtime, and track it in this `TaskTracker`.
	///
	/// This is equivalent to `handle.spawn_blocking(tracker.track_future(task))`.
	#[inline]
	#[track_caller]
	#[cfg(feature = "rt")]
	#[cfg(not(target_family = "wasm"))]
	#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
	pub fn spawn_blocking_on<F, T>(&self, task: F, handle: &Handle) -> JoinHandle<T>
	where
	F: FnOnce() -> T,
	F: Send + 'static,
	T: Send + 'static,
	{
	let token = self.token();
	handle.spawn_blocking(move \|\| {
	let res = task();
	drop(token);
	res
	})
	}

	/// Track the provided future.
	///
	/// The returned [`TrackedFuture`] will count as a task tracked by this collection, and will
	/// prevent calls to [`wait`] from returning until the task is dropped.
	///
	/// The task is removed from the collection when it is dropped, not when [`poll`] returns
	/// [`Poll::Ready`].
	///
	/// # Examples
	///
	/// Track a future spawned with [`tokio::spawn`].
	///
	/// ```
	/// # async fn my_async_fn() {}
	/// use tokio_util::task::TaskTracker;
	///
	/// # #[tokio::main(flavor = "current_thread")]
	/// # async fn main() {
	/// let tracker = TaskTracker::new();
	///
	/// tokio::spawn(tracker.track_future(my_async_fn()));
	/// # }
	/// ```
	///
	/// Track a future spawned on a [`JoinSet`].
	/// ```
	/// # async fn my_async_fn() {}
	/// use tokio::task::JoinSet;
	/// use tokio_util::task::TaskTracker;
	///
	/// # #[tokio::main(flavor = "current_thread")]
	/// # async fn main() {
	/// let tracker = TaskTracker::new();
	/// let mut join_set = JoinSet::new();
	///
	/// join_set.spawn(tracker.track_future(my_async_fn()));
	/// # }
	/// ```
	///
	/// [`JoinSet`]: tokio::task::JoinSet
	/// [`Poll::Pending`]: std::task::Poll::Pending
	/// [`poll`]: std::future::Future::poll
	/// [`wait`]: Self::wait
	#[inline]
	pub fn track_future<F: Future>(&self, future: F) -> TrackedFuture<F> {
	TrackedFuture {
	future,
	token: self.token(),
	}
	}

	/// Creates a [`TaskTrackerToken`] representing a task tracked by this `TaskTracker`.
	///
	/// This token is a lower-level utility than the spawn methods. Each token is considered to
	/// correspond to a task. As long as the token exists, the `TaskTracker` cannot complete.
	/// Furthermore, the count returned by the [`len`] method will include the tokens in the count.
	///
	/// Dropping the token indicates to the `TaskTracker` that the task has exited.
	///
	/// [`len`]: TaskTracker::len
	#[inline]
	pub fn token(&self) -> TaskTrackerToken {
	self.inner.add_task();
	TaskTrackerToken {
	task_tracker: self.clone(),
	}
	}

	/// Returns `true` if both task trackers correspond to the same set of tasks.
	///
	/// # Examples
	///
	/// ```
	/// use tokio_util::task::TaskTracker;
	///
	/// let tracker_1 = TaskTracker::new();
	/// let tracker_2 = TaskTracker::new();
	/// let tracker_1_clone = tracker_1.clone();
	///
	/// assert!(TaskTracker::ptr_eq(&tracker_1, &tracker_1_clone));
	/// assert!(!TaskTracker::ptr_eq(&tracker_1, &tracker_2));
	/// ```
	#[inline]
	#[must_use]
	pub fn ptr_eq(left: &TaskTracker, right: &TaskTracker) -> bool {
	Arc::ptr_eq(&left.inner, &right.inner)
	}
	}

	impl Default for TaskTracker {
	/// Creates a new `TaskTracker`.
	///
	/// The `TaskTracker` will start out as open.
	#[inline]
	fn default() -> TaskTracker {
	TaskTracker::new()
	}
	}

	impl Clone for TaskTracker {
	/// Returns a new `TaskTracker` that tracks the same set of tasks.
	///
	/// Since the new `TaskTracker` shares the same set of tasks, changes to one set are visible in
	/// all other clones.
	///
	/// # Examples
	///
	/// ```
	/// use tokio_util::task::TaskTracker;
	///
	/// #[tokio::main]
	/// # async fn _hidden() {}
	/// # #[tokio::main(flavor = "current_thread")]
	/// async fn main() {
	/// let tracker = TaskTracker::new();
	/// let cloned = tracker.clone();
	///
	/// // Spawns on `tracker` are visible in `cloned`.
	/// tracker.spawn(std::future::pending::<()>());
	/// assert_eq!(cloned.len(), 1);
	///
	/// // Spawns on `cloned` are visible in `tracker`.
	/// cloned.spawn(std::future::pending::<()>());
	/// assert_eq!(tracker.len(), 2);
	///
	/// // Calling `close` is visible to `cloned`.
	/// tracker.close();
	/// assert!(cloned.is_closed());
	///
	/// // Calling `reopen` is visible to `tracker`.
	/// cloned.reopen();
	/// assert!(!tracker.is_closed());
	/// }
	/// ```
	#[inline]
	fn clone(&self) -> TaskTracker {
	Self {
	inner: self.inner.clone(),
	}
	}
	}

	fn debug_inner(inner: &TaskTrackerInner, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	let state = inner.state.load(Ordering::Acquire);
	let is_closed = (state & 1) != 0;
	let len = state >> 1;

	f.debug_struct("TaskTracker")
	.field("len", &len)
	.field("is_closed", &is_closed)
	.field("inner", &(inner as *const TaskTrackerInner))
	.finish()
	}

	impl fmt::Debug for TaskTracker {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	debug_inner(&self.inner, f)
	}
	}

	impl TaskTrackerToken {
	/// Returns the [`TaskTracker`] that this token is associated with.
	#[inline]
	#[must_use]
	pub fn task_tracker(&self) -> &TaskTracker {
	&self.task_tracker
	}
	}

	impl Clone for TaskTrackerToken {
	/// Returns a new `TaskTrackerToken` associated with the same [`TaskTracker`].
	///
	/// This is equivalent to `token.task_tracker().token()`.
	#[inline]
	fn clone(&self) -> TaskTrackerToken {
	self.task_tracker.token()
	}
	}

	impl Drop for TaskTrackerToken {
	/// Dropping the token indicates to the [`TaskTracker`] that the task has exited.
	#[inline]
	fn drop(&mut self) {
	self.task_tracker.inner.drop_task();
	}
	}

	impl<F: Future> Future for TrackedFuture<F> {
	type Output = F::Output;

	#[inline]
	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<F::Output> {
	self.project().future.poll(cx)
	}
	}

	impl<F: fmt::Debug> fmt::Debug for TrackedFuture<F> {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	f.debug_struct("TrackedFuture")
	.field("future", &self.future)
	.field("task_tracker", self.token.task_tracker())
	.finish()
	}
	}

	impl<'a> Future for TaskTrackerWaitFuture<'a> {
	type Output = ();

	#[inline]
	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
	let me = self.project();

	let inner = match me.inner.as_ref() {
	None => return Poll::Ready(()),
	Some(inner) => inner,
	};

	let ready = inner.is_closed_and_empty() \|\| me.future.poll(cx).is_ready();
	if ready {
	*me.inner = None;
	Poll::Ready(())
	} else {
	Poll::Pending
	}
	}
	}

	impl<'a> fmt::Debug for TaskTrackerWaitFuture<'a> {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	struct Helper<'a>(&'a TaskTrackerInner);

	impl fmt::Debug for Helper<'_> {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	debug_inner(self.0, f)
	}
	}

	f.debug_struct("TaskTrackerWaitFuture")
	.field("future", &self.future)
	.field("task_tracker", &self.inner.map(Helper))
	.finish()
	}
	}