async_task/task.rs
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use core::fmt;
use core::future::Future;
use core::marker::PhantomData;
use core::mem;
use core::pin::Pin;
use core::ptr::NonNull;
use core::sync::atomic::Ordering;
use core::task::{Context, Poll};
use crate::header::Header;
use crate::raw::Panic;
use crate::runnable::ScheduleInfo;
use crate::state::*;
/// A spawned task.
///
/// A [`Task`] can be awaited to retrieve the output of its future.
///
/// Dropping a [`Task`] cancels it, which means its future won't be polled again. To drop the
/// [`Task`] handle without canceling it, use [`detach()`][`Task::detach()`] instead. To cancel a
/// task gracefully and wait until it is fully destroyed, use the [`cancel()`][Task::cancel()]
/// method.
///
/// Note that canceling a task actually wakes it and reschedules one last time. Then, the executor
/// can destroy the task by simply dropping its [`Runnable`][`super::Runnable`] or by invoking
/// [`run()`][`super::Runnable::run()`].
///
/// # Examples
///
/// ```
/// use smol::{future, Executor};
/// use std::thread;
///
/// let ex = Executor::new();
///
/// // Spawn a future onto the executor.
/// let task = ex.spawn(async {
/// println!("Hello from a task!");
/// 1 + 2
/// });
///
/// // Run an executor thread.
/// thread::spawn(move || future::block_on(ex.run(future::pending::<()>())));
///
/// // Wait for the task's output.
/// assert_eq!(future::block_on(task), 3);
/// ```
#[must_use = "tasks get canceled when dropped, use `.detach()` to run them in the background"]
pub struct Task<T, M = ()> {
/// A raw task pointer.
pub(crate) ptr: NonNull<()>,
/// A marker capturing generic types `T` and `M`.
pub(crate) _marker: PhantomData<(T, M)>,
}
unsafe impl<T: Send, M: Send + Sync> Send for Task<T, M> {}
unsafe impl<T, M: Send + Sync> Sync for Task<T, M> {}
impl<T, M> Unpin for Task<T, M> {}
#[cfg(feature = "std")]
impl<T, M> std::panic::UnwindSafe for Task<T, M> {}
#[cfg(feature = "std")]
impl<T, M> std::panic::RefUnwindSafe for Task<T, M> {}
impl<T, M> Task<T, M> {
/// Detaches the task to let it keep running in the background.
///
/// # Examples
///
/// ```
/// use smol::{Executor, Timer};
/// use std::time::Duration;
///
/// let ex = Executor::new();
///
/// // Spawn a deamon future.
/// ex.spawn(async {
/// loop {
/// println!("I'm a daemon task looping forever.");
/// Timer::after(Duration::from_secs(1)).await;
/// }
/// })
/// .detach();
/// ```
pub fn detach(self) {
let mut this = self;
let _out = this.set_detached();
mem::forget(this);
}
/// Cancels the task and waits for it to stop running.
///
/// Returns the task's output if it was completed just before it got canceled, or [`None`] if
/// it didn't complete.
///
/// While it's possible to simply drop the [`Task`] to cancel it, this is a cleaner way of
/// canceling because it also waits for the task to stop running.
///
/// # Examples
///
/// ```
/// # if cfg!(miri) { return; } // Miri does not support epoll
/// use smol::{future, Executor, Timer};
/// use std::thread;
/// use std::time::Duration;
///
/// let ex = Executor::new();
///
/// // Spawn a deamon future.
/// let task = ex.spawn(async {
/// loop {
/// println!("Even though I'm in an infinite loop, you can still cancel me!");
/// Timer::after(Duration::from_secs(1)).await;
/// }
/// });
///
/// // Run an executor thread.
/// thread::spawn(move || future::block_on(ex.run(future::pending::<()>())));
///
/// future::block_on(async {
/// Timer::after(Duration::from_secs(3)).await;
/// task.cancel().await;
/// });
/// ```
pub async fn cancel(self) -> Option<T> {
let mut this = self;
this.set_canceled();
this.fallible().await
}
/// Converts this task into a [`FallibleTask`].
///
/// Like [`Task`], a fallible task will poll the task's output until it is
/// completed or cancelled due to its [`Runnable`][`super::Runnable`] being
/// dropped without being run. Resolves to the task's output when completed,
/// or [`None`] if it didn't complete.
///
/// # Examples
///
/// ```
/// use smol::{future, Executor};
/// use std::thread;
///
/// let ex = Executor::new();
///
/// // Spawn a future onto the executor.
/// let task = ex.spawn(async {
/// println!("Hello from a task!");
/// 1 + 2
/// })
/// .fallible();
///
/// // Run an executor thread.
/// thread::spawn(move || future::block_on(ex.run(future::pending::<()>())));
///
/// // Wait for the task's output.
/// assert_eq!(future::block_on(task), Some(3));
/// ```
///
/// ```
/// use smol::future;
///
/// // Schedule function which drops the runnable without running it.
/// let schedule = move |runnable| drop(runnable);
///
/// // Create a task with the future and the schedule function.
/// let (runnable, task) = async_task::spawn(async {
/// println!("Hello from a task!");
/// 1 + 2
/// }, schedule);
/// runnable.schedule();
///
/// // Wait for the task's output.
/// assert_eq!(future::block_on(task.fallible()), None);
/// ```
pub fn fallible(self) -> FallibleTask<T, M> {
FallibleTask { task: self }
}
/// Puts the task in canceled state.
fn set_canceled(&mut self) {
let ptr = self.ptr.as_ptr();
let header = ptr as *const Header<M>;
unsafe {
let mut state = (*header).state.load(Ordering::Acquire);
loop {
// If the task has been completed or closed, it can't be canceled.
if state & (COMPLETED | CLOSED) != 0 {
break;
}
// If the task is not scheduled nor running, we'll need to schedule it.
let new = if state & (SCHEDULED | RUNNING) == 0 {
(state | SCHEDULED | CLOSED) + REFERENCE
} else {
state | CLOSED
};
// Mark the task as closed.
match (*header).state.compare_exchange_weak(
state,
new,
Ordering::AcqRel,
Ordering::Acquire,
) {
Ok(_) => {
// If the task is not scheduled nor running, schedule it one more time so
// that its future gets dropped by the executor.
if state & (SCHEDULED | RUNNING) == 0 {
((*header).vtable.schedule)(ptr, ScheduleInfo::new(false));
}
// Notify the awaiter that the task has been closed.
if state & AWAITER != 0 {
(*header).notify(None);
}
break;
}
Err(s) => state = s,
}
}
}
}
/// Puts the task in detached state.
fn set_detached(&mut self) -> Option<Result<T, Panic>> {
let ptr = self.ptr.as_ptr();
let header = ptr as *const Header<M>;
unsafe {
// A place where the output will be stored in case it needs to be dropped.
let mut output = None;
// Optimistically assume the `Task` is being detached just after creating the task.
// This is a common case so if the `Task` is datached, the overhead of it is only one
// compare-exchange operation.
if let Err(mut state) = (*header).state.compare_exchange_weak(
SCHEDULED | TASK | REFERENCE,
SCHEDULED | REFERENCE,
Ordering::AcqRel,
Ordering::Acquire,
) {
loop {
// If the task has been completed but not yet closed, that means its output
// must be dropped.
if state & COMPLETED != 0 && state & CLOSED == 0 {
// Mark the task as closed in order to grab its output.
match (*header).state.compare_exchange_weak(
state,
state | CLOSED,
Ordering::AcqRel,
Ordering::Acquire,
) {
Ok(_) => {
// Read the output.
output = Some(
(((*header).vtable.get_output)(ptr) as *mut Result<T, Panic>)
.read(),
);
// Update the state variable because we're continuing the loop.
state |= CLOSED;
}
Err(s) => state = s,
}
} else {
// If this is the last reference to the task and it's not closed, then
// close it and schedule one more time so that its future gets dropped by
// the executor.
let new = if state & (!(REFERENCE - 1) | CLOSED) == 0 {
SCHEDULED | CLOSED | REFERENCE
} else {
state & !TASK
};
// Unset the `TASK` flag.
match (*header).state.compare_exchange_weak(
state,
new,
Ordering::AcqRel,
Ordering::Acquire,
) {
Ok(_) => {
// If this is the last reference to the task, we need to either
// schedule dropping its future or destroy it.
if state & !(REFERENCE - 1) == 0 {
if state & CLOSED == 0 {
((*header).vtable.schedule)(ptr, ScheduleInfo::new(false));
} else {
((*header).vtable.destroy)(ptr);
}
}
break;
}
Err(s) => state = s,
}
}
}
}
output
}
}
/// Polls the task to retrieve its output.
///
/// Returns `Some` if the task has completed or `None` if it was closed.
///
/// A task becomes closed in the following cases:
///
/// 1. It gets canceled by `Runnable::drop()`, `Task::drop()`, or `Task::cancel()`.
/// 2. Its output gets awaited by the `Task`.
/// 3. It panics while polling the future.
/// 4. It is completed and the `Task` gets dropped.
fn poll_task(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
let ptr = self.ptr.as_ptr();
let header = ptr as *const Header<M>;
unsafe {
let mut state = (*header).state.load(Ordering::Acquire);
loop {
// If the task has been closed, notify the awaiter and return `None`.
if state & CLOSED != 0 {
// If the task is scheduled or running, we need to wait until its future is
// dropped.
if state & (SCHEDULED | RUNNING) != 0 {
// Replace the waker with one associated with the current task.
(*header).register(cx.waker());
// Reload the state after registering. It is possible changes occurred just
// before registration so we need to check for that.
state = (*header).state.load(Ordering::Acquire);
// If the task is still scheduled or running, we need to wait because its
// future is not dropped yet.
if state & (SCHEDULED | RUNNING) != 0 {
return Poll::Pending;
}
}
// Even though the awaiter is most likely the current task, it could also be
// another task.
(*header).notify(Some(cx.waker()));
return Poll::Ready(None);
}
// If the task is not completed, register the current task.
if state & COMPLETED == 0 {
// Replace the waker with one associated with the current task.
(*header).register(cx.waker());
// Reload the state after registering. It is possible that the task became
// completed or closed just before registration so we need to check for that.
state = (*header).state.load(Ordering::Acquire);
// If the task has been closed, restart.
if state & CLOSED != 0 {
continue;
}
// If the task is still not completed, we're blocked on it.
if state & COMPLETED == 0 {
return Poll::Pending;
}
}
// Since the task is now completed, mark it as closed in order to grab its output.
match (*header).state.compare_exchange(
state,
state | CLOSED,
Ordering::AcqRel,
Ordering::Acquire,
) {
Ok(_) => {
// Notify the awaiter. Even though the awaiter is most likely the current
// task, it could also be another task.
if state & AWAITER != 0 {
(*header).notify(Some(cx.waker()));
}
// Take the output from the task.
let output = ((*header).vtable.get_output)(ptr) as *mut Result<T, Panic>;
let output = output.read();
// Propagate the panic if the task panicked.
let output = match output {
Ok(output) => output,
Err(panic) => {
#[cfg(feature = "std")]
std::panic::resume_unwind(panic);
#[cfg(not(feature = "std"))]
match panic {}
}
};
return Poll::Ready(Some(output));
}
Err(s) => state = s,
}
}
}
}
fn header(&self) -> &Header<M> {
let ptr = self.ptr.as_ptr();
let header = ptr as *const Header<M>;
unsafe { &*header }
}
/// Returns `true` if the current task is finished.
///
/// Note that in a multithreaded environment, this task can change finish immediately after calling this function.
pub fn is_finished(&self) -> bool {
let ptr = self.ptr.as_ptr();
let header = ptr as *const Header<M>;
unsafe {
let state = (*header).state.load(Ordering::Acquire);
state & (CLOSED | COMPLETED) != 0
}
}
/// Get the metadata associated with this task.
///
/// Tasks can be created with a metadata object associated with them; by default, this
/// is a `()` value. See the [`Builder::metadata()`] method for more information.
pub fn metadata(&self) -> &M {
&self.header().metadata
}
}
impl<T, M> Drop for Task<T, M> {
fn drop(&mut self) {
self.set_canceled();
self.set_detached();
}
}
impl<T, M> Future for Task<T, M> {
type Output = T;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
match self.poll_task(cx) {
Poll::Ready(t) => Poll::Ready(t.expect("Task polled after completion")),
Poll::Pending => Poll::Pending,
}
}
}
impl<T, M: fmt::Debug> fmt::Debug for Task<T, M> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Task")
.field("header", self.header())
.finish()
}
}
/// A spawned task with a fallible response.
///
/// This type behaves like [`Task`], however it produces an `Option<T>` when
/// polled and will return `None` if the executor dropped its
/// [`Runnable`][`super::Runnable`] without being run.
///
/// This can be useful to avoid the panic produced when polling the `Task`
/// future if the executor dropped its `Runnable`.
#[must_use = "tasks get canceled when dropped, use `.detach()` to run them in the background"]
pub struct FallibleTask<T, M = ()> {
task: Task<T, M>,
}
impl<T, M> FallibleTask<T, M> {
/// Detaches the task to let it keep running in the background.
///
/// # Examples
///
/// ```
/// use smol::{Executor, Timer};
/// use std::time::Duration;
///
/// let ex = Executor::new();
///
/// // Spawn a deamon future.
/// ex.spawn(async {
/// loop {
/// println!("I'm a daemon task looping forever.");
/// Timer::after(Duration::from_secs(1)).await;
/// }
/// })
/// .fallible()
/// .detach();
/// ```
pub fn detach(self) {
self.task.detach()
}
/// Cancels the task and waits for it to stop running.
///
/// Returns the task's output if it was completed just before it got canceled, or [`None`] if
/// it didn't complete.
///
/// While it's possible to simply drop the [`Task`] to cancel it, this is a cleaner way of
/// canceling because it also waits for the task to stop running.
///
/// # Examples
///
/// ```
/// # if cfg!(miri) { return; } // Miri does not support epoll
/// use smol::{future, Executor, Timer};
/// use std::thread;
/// use std::time::Duration;
///
/// let ex = Executor::new();
///
/// // Spawn a deamon future.
/// let task = ex.spawn(async {
/// loop {
/// println!("Even though I'm in an infinite loop, you can still cancel me!");
/// Timer::after(Duration::from_secs(1)).await;
/// }
/// })
/// .fallible();
///
/// // Run an executor thread.
/// thread::spawn(move || future::block_on(ex.run(future::pending::<()>())));
///
/// future::block_on(async {
/// Timer::after(Duration::from_secs(3)).await;
/// task.cancel().await;
/// });
/// ```
pub async fn cancel(self) -> Option<T> {
self.task.cancel().await
}
/// Returns `true` if the current task is finished.
///
/// Note that in a multithreaded environment, this task can change finish immediately after calling this function.
pub fn is_finished(&self) -> bool {
self.task.is_finished()
}
}
impl<T, M> Future for FallibleTask<T, M> {
type Output = Option<T>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
self.task.poll_task(cx)
}
}
impl<T, M: fmt::Debug> fmt::Debug for FallibleTask<T, M> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("FallibleTask")
.field("header", self.task.header())
.finish()
}
}