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use crate::runtime::task::RawTask;
use std::fmt;
use std::future::Future;
use std::marker::PhantomData;
use std::pin::Pin;
use std::task::{Context, Poll};
doc_rt_core! {
/// An owned permission to join on a task (await its termination).
///
/// This can be thought of as the equivalent of [`std::thread::JoinHandle`] for
/// a task rather than a thread.
///
/// A `JoinHandle` *detaches* the associated task when it is dropped, which
/// means that there is no longer any handle to the task, and no way to `join`
/// on it.
///
/// This `struct` is created by the [`task::spawn`] and [`task::spawn_blocking`]
/// functions.
///
/// # Examples
///
/// Creation from [`task::spawn`]:
///
/// ```
/// use tokio::task;
///
/// # async fn doc() {
/// let join_handle: task::JoinHandle<_> = task::spawn(async {
/// // some work here
/// });
/// # }
/// ```
///
/// Creation from [`task::spawn_blocking`]:
///
/// ```
/// use tokio::task;
///
/// # async fn doc() {
/// let join_handle: task::JoinHandle<_> = task::spawn_blocking(|| {
/// // some blocking work here
/// });
/// # }
/// ```
///
/// Child being detached and outliving its parent:
///
/// ```no_run
/// use tokio::task;
/// use tokio::time;
/// use std::time::Duration;
///
/// # #[tokio::main] async fn main() {
/// let original_task = task::spawn(async {
/// let _detached_task = task::spawn(async {
/// // Here we sleep to make sure that the first task returns before.
/// time::delay_for(Duration::from_millis(10)).await;
/// // This will be called, even though the JoinHandle is dropped.
/// println!("♫ Still alive ♫");
/// });
/// });
///
/// original_task.await.expect("The task being joined has panicked");
/// println!("Original task is joined.");
///
/// // We make sure that the new task has time to run, before the main
/// // task returns.
///
/// time::delay_for(Duration::from_millis(1000)).await;
/// # }
/// ```
///
/// [`task::spawn`]: crate::task::spawn()
/// [`task::spawn_blocking`]: crate::task::spawn_blocking
/// [`std::thread::JoinHandle`]: std::thread::JoinHandle
pub struct JoinHandle<T> {
raw: Option<RawTask>,
_p: PhantomData<T>,
}
}
unsafe impl<T: Send> Send for JoinHandle<T> {}
unsafe impl<T: Send> Sync for JoinHandle<T> {}
impl<T> JoinHandle<T> {
pub(super) fn new(raw: RawTask) -> JoinHandle<T> {
JoinHandle {
raw: Some(raw),
_p: PhantomData,
}
}
}
impl<T> Unpin for JoinHandle<T> {}
impl<T> Future for JoinHandle<T> {
type Output = super::Result<T>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut ret = Poll::Pending;
// Keep track of task budget
ready!(crate::coop::poll_proceed(cx));
// Raw should always be set. If it is not, this is due to polling after
// completion
let raw = self
.raw
.as_ref()
.expect("polling after `JoinHandle` already completed");
// Try to read the task output. If the task is not yet complete, the
// waker is stored and is notified once the task does complete.
//
// The function must go via the vtable, which requires erasing generic
// types. To do this, the function "return" is placed on the stack
// **before** calling the function and is passed into the function using
// `*mut ()`.
//
// Safety:
//
// The type of `T` must match the task's output type.
unsafe {
raw.try_read_output(&mut ret as *mut _ as *mut (), cx.waker());
}
ret
}
}
impl<T> Drop for JoinHandle<T> {
fn drop(&mut self) {
if let Some(raw) = self.raw.take() {
if raw.header().state.drop_join_handle_fast().is_ok() {
return;
}
raw.drop_join_handle_slow();
}
}
}
impl<T> fmt::Debug for JoinHandle<T>
where
T: fmt::Debug,
{
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("JoinHandle").finish()
}
}