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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
use std::{
collections::btree_map::BTreeMap,
fmt::Debug,
future::Future,
mem,
num::NonZeroUsize,
pin::Pin,
sync::{Arc, Mutex},
task::{Context, Poll, Waker},
};
use futures::future::{self, Either};
/// An object that signals when a task, or a group of tasks,
/// should be aborted.
///
/// An [`AbortController`] works like the DOM API of the same name.
/// Each controller has one or more associated [`AbortSignal`]s, which are
/// futures that observe the controller. When the controller tells its signals
/// to abort, any tasks that are waiting on those signals can resume executing.
///
/// [`AbortController`] is similar to [`future::AbortHandle`], but can wake up
/// more than one task that's waiting on an associated [`AbortSignal`].
pub struct AbortController {
state: Arc<Mutex<AbortState>>,
}
impl AbortController {
/// Creates a new controller.
pub fn new() -> Self {
Self {
state: Arc::new(Mutex::new(AbortState::Armed {
next_id: NonZeroUsize::MIN,
wakers: BTreeMap::new(),
})),
}
}
/// Creates an associated signal which can be waited on.
pub fn signal(&self) -> AbortSignal {
AbortSignal {
id: AbortSignalId::Unreserved,
state: self.state.clone(),
}
}
/// Tells all associated signals to abort.
pub fn abort(&self) {
let wakers = match mem::replace(&mut *self.state.lock().unwrap(), AbortState::Aborted) {
AbortState::Aborted => return,
AbortState::Armed { wakers, .. } => wakers,
};
for waker in wakers.into_values() {
waker.wake()
}
}
}
impl Default for AbortController {
fn default() -> Self {
Self::new()
}
}
impl Debug for AbortController {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.write_str("AbortController { .. }")
}
}
/// A future that completes when its associated [`AbortController`]
/// is aborted.
///
/// [`AbortSignal`] is similar to [`future::AbortRegistration`] and
/// [`future::Abortable`], but can be cloned and shared between
/// multiple tasks.
pub struct AbortSignal {
id: AbortSignalId,
state: Arc<Mutex<AbortState>>,
}
impl AbortSignal {
/// Consumes this signal and waits for either `f` to complete, or the
/// signal to fire; whichever happens first.
pub async fn aborting<T, E, F>(self, f: F) -> Result<T, E>
where
F: Future<Output = Result<T, E>>,
E: From<AbortError>,
{
futures::pin_mut!(f);
match future::select(f, self).await {
Either::Left((result, _)) => result,
Either::Right((_, _)) => Err(AbortError)?,
}
}
}
impl Clone for AbortSignal {
fn clone(&self) -> Self {
AbortSignal {
id: AbortSignalId::Unreserved,
state: Arc::clone(&self.state),
}
}
}
impl Future for AbortSignal {
type Output = ();
fn poll(self: Pin<&mut Self>, context: &mut Context<'_>) -> Poll<Self::Output> {
let this = self.get_mut();
let mut guard = this.state.lock().unwrap();
let AbortState::Armed { next_id, wakers } = &mut *guard else {
// Our controller has been aborted. Forget our ID now,
// so that we can skip re-locking and cleaning up in
// `drop` later.
this.id = AbortSignalId::Unreserved;
return Poll::Ready(());
};
let id = match this.id {
AbortSignalId::Reserved(id) => id,
AbortSignalId::Unreserved => {
// A task is waiting on us for the first time.
// Reserve our ID.
let this_id = *next_id;
*next_id = this_id.checked_add(1).unwrap();
this.id = AbortSignalId::Reserved(this_id);
this_id
}
};
wakers
.entry(id)
.and_modify(|waker| {
// The `Waker` docs recommend using `clone_from` to
// avoid cloning a waker that would wake the same task.
waker.clone_from(context.waker());
})
.or_insert_with(|| context.waker().clone());
Poll::Pending
}
}
impl Drop for AbortSignal {
fn drop(&mut self) {
if let AbortSignalId::Reserved(id) = self.id {
// We only need to lock and update the shared state if
// we have at least one task waiting on us.
if let AbortState::Armed { wakers, .. } = &mut *self.state.lock().unwrap() {
wakers.remove(&id);
}
}
}
}
impl Debug for AbortSignal {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "AbortSignal {{ id: {:?}, .. }}", self.id)
}
}
/// Uniquely identifies an [`AbortSignal`].
///
/// ### Notes
///
/// [`AbortSignalId`] takes advantage of Rust's [discriminant elision][1] for
/// option-like enums, so it has the same size and alignment as `usize`.
///
#[derive(Clone, Copy, Debug)]
enum AbortSignalId {
/// Indicates that an ID has not been reserved for this [`AbortSignal`],
/// because no tasks are currently waiting on it.
Unreserved,
/// The ID for an [`AbortSignal`] with one or more tasks waiting on it.
Reserved(NonZeroUsize),
}
/// State shared between an [`AbortController`] and its [`AbortSignal`]s.
///
/// ### Notes
///
/// Controllers and signals share state using an `Arc<Mutex<AbortState>>`.
/// There are more efficient implementations than a single shared lock,
/// but we're prioritizing readability and correctness at-a-glance over
/// raw performance.
enum AbortState {
/// The controller has not yet been aborted.
Armed {
/// The ID to reserve for the next [`AbortSignalId`].
next_id: NonZeroUsize,
/// A map of reserved signal IDs to waiting tasks
/// that should be woken up on abort.
wakers: BTreeMap<NonZeroUsize, Waker>,
},
/// The controller has been aborted.
Aborted,
}
#[derive(thiserror::Error, Debug)]
#[error("operation aborted")]
pub struct AbortError;