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use std::fmt;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use crate::runtime::Handle;
use crate::task::{AbortHandle, Id, JoinError, JoinHandle, LocalSet};
use crate::util::IdleNotifiedSet;
/// A collection of tasks spawned on a Tokio runtime.
///
/// A `JoinSet` can be used to await the completion of some or all of the tasks
/// in the set. The set is not ordered, and the tasks will be returned in the
/// order they complete.
///
/// All of the tasks must have the same return type `T`.
///
/// When the `JoinSet` is dropped, all tasks in the `JoinSet` are immediately aborted.
///
/// **Note**: This is an [unstable API][unstable]. The public API of this type
/// may break in 1.x releases. See [the documentation on unstable
/// features][unstable] for details.
///
/// # Examples
///
/// Spawn multiple tasks and wait for them.
///
/// ```
/// use tokio::task::JoinSet;
///
/// #[tokio::main]
/// async fn main() {
/// let mut set = JoinSet::new();
///
/// for i in 0..10 {
/// set.spawn(async move { i });
/// }
///
/// let mut seen = [false; 10];
/// while let Some(res) = set.join_one().await.unwrap() {
/// seen[res] = true;
/// }
///
/// for i in 0..10 {
/// assert!(seen[i]);
/// }
/// }
/// ```
///
/// [unstable]: crate#unstable-features
#[cfg_attr(docsrs, doc(cfg(all(feature = "rt", tokio_unstable))))]
pub struct JoinSet<T> {
inner: IdleNotifiedSet<JoinHandle<T>>,
}
impl<T> JoinSet<T> {
/// Create a new `JoinSet`.
pub fn new() -> Self {
Self {
inner: IdleNotifiedSet::new(),
}
}
/// Returns the number of tasks currently in the `JoinSet`.
pub fn len(&self) -> usize {
self.inner.len()
}
/// Returns whether the `JoinSet` is empty.
pub fn is_empty(&self) -> bool {
self.inner.is_empty()
}
}
impl<T: 'static> JoinSet<T> {
/// Spawn the provided task on the `JoinSet`, returning an [`AbortHandle`]
/// that can be used to remotely cancel the task.
///
/// # Panics
///
/// This method panics if called outside of a Tokio runtime.
///
/// [`AbortHandle`]: crate::task::AbortHandle
pub fn spawn<F>(&mut self, task: F) -> AbortHandle
where
F: Future<Output = T>,
F: Send + 'static,
T: Send,
{
self.insert(crate::spawn(task))
}
/// Spawn the provided task on the provided runtime and store it in this
/// `JoinSet` returning an [`AbortHandle`] that can be used to remotely
/// cancel the task.
///
/// [`AbortHandle`]: crate::task::AbortHandle
pub fn spawn_on<F>(&mut self, task: F, handle: &Handle) -> AbortHandle
where
F: Future<Output = T>,
F: Send + 'static,
T: Send,
{
self.insert(handle.spawn(task))
}
/// Spawn the provided task on the current [`LocalSet`] and store it in this
/// `JoinSet`, returning an [`AbortHandle`] that can be used to remotely
/// cancel the task.
///
/// # Panics
///
/// This method panics if it is called outside of a `LocalSet`.
///
/// [`LocalSet`]: crate::task::LocalSet
/// [`AbortHandle`]: crate::task::AbortHandle
pub fn spawn_local<F>(&mut self, task: F) -> AbortHandle
where
F: Future<Output = T>,
F: 'static,
{
self.insert(crate::task::spawn_local(task))
}
/// Spawn the provided task on the provided [`LocalSet`] and store it in
/// this `JoinSet`, returning an [`AbortHandle`] that can be used to
/// remotely cancel the task.
///
/// [`LocalSet`]: crate::task::LocalSet
/// [`AbortHandle`]: crate::task::AbortHandle
pub fn spawn_local_on<F>(&mut self, task: F, local_set: &LocalSet) -> AbortHandle
where
F: Future<Output = T>,
F: 'static,
{
self.insert(local_set.spawn_local(task))
}
fn insert(&mut self, jh: JoinHandle<T>) -> AbortHandle {
let abort = jh.abort_handle();
let mut entry = self.inner.insert_idle(jh);
// Set the waker that is notified when the task completes.
entry.with_value_and_context(|jh, ctx| jh.set_join_waker(ctx.waker()));
abort
}
/// Waits until one of the tasks in the set completes and returns its output.
///
/// Returns `None` if the set is empty.
///
/// # Cancel Safety
///
/// This method is cancel safe. If `join_one` is used as the event in a `tokio::select!`
/// statement and some other branch completes first, it is guaranteed that no tasks were
/// removed from this `JoinSet`.
pub async fn join_one(&mut self) -> Result<Option<T>, JoinError> {
crate::future::poll_fn(|cx| self.poll_join_one(cx))
.await
.map(|opt| opt.map(|(_, res)| res))
}
/// Waits until one of the tasks in the set completes and returns its
/// output, along with the [task ID] of the completed task.
///
/// Returns `None` if the set is empty.
///
/// # Cancel Safety
///
/// This method is cancel safe. If `join_one_with_id` is used as the event in a `tokio::select!`
/// statement and some other branch completes first, it is guaranteed that no tasks were
/// removed from this `JoinSet`.
///
/// [task ID]: crate::task::Id
pub async fn join_one_with_id(&mut self) -> Result<Option<(Id, T)>, JoinError> {
crate::future::poll_fn(|cx| self.poll_join_one(cx)).await
}
/// Aborts all tasks and waits for them to finish shutting down.
///
/// Calling this method is equivalent to calling [`abort_all`] and then calling [`join_one`] in
/// a loop until it returns `Ok(None)`.
///
/// This method ignores any panics in the tasks shutting down. When this call returns, the
/// `JoinSet` will be empty.
///
/// [`abort_all`]: fn@Self::abort_all
/// [`join_one`]: fn@Self::join_one
pub async fn shutdown(&mut self) {
self.abort_all();
while self.join_one().await.transpose().is_some() {}
}
/// Aborts all tasks on this `JoinSet`.
///
/// This does not remove the tasks from the `JoinSet`. To wait for the tasks to complete
/// cancellation, you should call `join_one` in a loop until the `JoinSet` is empty.
pub fn abort_all(&mut self) {
self.inner.for_each(|jh| jh.abort());
}
/// Removes all tasks from this `JoinSet` without aborting them.
///
/// The tasks removed by this call will continue to run in the background even if the `JoinSet`
/// is dropped.
pub fn detach_all(&mut self) {
self.inner.drain(drop);
}
/// Polls for one of the tasks in the set to complete.
///
/// If this returns `Poll::Ready(Some((_, Ok(_))))` or `Poll::Ready(Some((_,
/// Err(_)))`, then the task that completed is removed from the set.
///
/// When the method returns `Poll::Pending`, the `Waker` in the provided `Context` is scheduled
/// to receive a wakeup when a task in the `JoinSet` completes. Note that on multiple calls to
/// `poll_join_one`, only the `Waker` from the `Context` passed to the most recent call is
/// scheduled to receive a wakeup.
///
/// # Returns
///
/// This function returns:
///
/// * `Poll::Pending` if the `JoinSet` is not empty but there is no task whose output is
/// available right now.
/// * `Poll::Ready(Ok(Some((id, value)))` if one of the tasks in this `JoinSet` has completed. The
/// `value` is the return value of one of the tasks that completed, while
/// `id` is the [task ID] of that task.
/// * `Poll::Ready(Err(err))` if one of the tasks in this `JoinSet` has panicked or been
/// aborted. The `err` is the `JoinError` from the panicked/aborted task.
/// * `Poll::Ready(Ok(None))` if the `JoinSet` is empty.
///
/// Note that this method may return `Poll::Pending` even if one of the tasks has completed.
/// This can happen if the [coop budget] is reached.
///
/// [coop budget]: crate::task#cooperative-scheduling
/// [task ID]: crate::task::Id
fn poll_join_one(&mut self, cx: &mut Context<'_>) -> Poll<Result<Option<(Id, T)>, JoinError>> {
// The call to `pop_notified` moves the entry to the `idle` list. It is moved back to
// the `notified` list if the waker is notified in the `poll` call below.
let mut entry = match self.inner.pop_notified(cx.waker()) {
Some(entry) => entry,
None => {
if self.is_empty() {
return Poll::Ready(Ok(None));
} else {
// The waker was set by `pop_notified`.
return Poll::Pending;
}
}
};
let res = entry.with_value_and_context(|jh, ctx| Pin::new(jh).poll(ctx));
if let Poll::Ready(res) = res {
let entry = entry.remove();
// If the task succeeded, add the task ID to the output. Otherwise, the
// `JoinError` will already have the task's ID.
let res = res.map(|output| (entry.id(), output));
Poll::Ready(Some(res).transpose())
} else {
// A JoinHandle generally won't emit a wakeup without being ready unless
// the coop limit has been reached. We yield to the executor in this
// case.
cx.waker().wake_by_ref();
Poll::Pending
}
}
}
impl<T> Drop for JoinSet<T> {
fn drop(&mut self) {
self.inner.drain(|join_handle| join_handle.abort());
}
}
impl<T> fmt::Debug for JoinSet<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("JoinSet").field("len", &self.len()).finish()
}
}
impl<T> Default for JoinSet<T> {
fn default() -> Self {
Self::new()
}
}