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task: use pin-project for TaskLocalFuture #5758

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merged 4 commits into from
Jun 10, 2023
Merged

task: use pin-project for TaskLocalFuture #5758

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c833947
task: use pin-project for `TaskLocalFuture`
BugenZhao Jun 3, 2023
714b63b
keep original field name
BugenZhao Jun 3, 2023
d2e6428
fix clippy
BugenZhao Jun 3, 2023
134cabd
bump minimal version
BugenZhao Jun 3, 2023
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97 changes: 50 additions & 47 deletions tokio/src/task/task_local.rs
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
use pin_project_lite::pin_project;
use std::cell::RefCell;
use std::error::Error;
use std::future::Future;
Expand Down Expand Up @@ -299,47 +300,62 @@ impl<T: 'static> fmt::Debug for LocalKey<T> {
}
}

/// A future that sets a value `T` of a task local for the future `F` during
/// its execution.
///
/// The value of the task-local must be `'static` and will be dropped on the
/// completion of the future.
///
/// Created by the function [`LocalKey::scope`](self::LocalKey::scope).
///
/// ### Examples
///
/// ```
/// # async fn dox() {
/// tokio::task_local! {
/// static NUMBER: u32;
/// }
///
/// NUMBER.scope(1, async move {
/// println!("task local value: {}", NUMBER.get());
/// }).await;
/// # }
/// ```
// Doesn't use pin_project due to custom Drop.
pub struct TaskLocalFuture<T, F>
where
T: 'static,
{
local: &'static LocalKey<T>,
slot: Option<T>,
future: Option<F>,
_pinned: PhantomPinned,
pin_project! {
/// A future that sets a value `T` of a task local for the future `F` during
/// its execution.
///
/// The value of the task-local must be `'static` and will be dropped on the
/// completion of the future.
///
/// Created by the function [`LocalKey::scope`](self::LocalKey::scope).
///
/// ### Examples
///
/// ```
/// # async fn dox() {
/// tokio::task_local! {
/// static NUMBER: u32;
/// }
///
/// NUMBER.scope(1, async move {
/// println!("task local value: {}", NUMBER.get());
/// }).await;
/// # }
/// ```
pub struct TaskLocalFuture<T, F>
where
T: 'static,
{
local: &'static LocalKey<T>,
slot: Option<T>,
#[pin]
future: Option<F>,
#[pin]
_pinned: PhantomPinned,
Comment on lines +333 to +334
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@BugenZhao BugenZhao Jun 3, 2023 •
edited

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I'm not really clear about whether we need this Future to be !Unpin, as making it Unpin seems to make life easier in some situations.

For example, if the feature in #5743 is added, it'll be common that we poll this with &mut fut to avoid the future being consumed, then we're able to take the value back after getting Ready. If the future is always !Unpin, we are not able to take back the ownership of the future. :(

Currently, I leave it untouched according to #3943 (comment). IIUC, this is considered to be a breaking change in the compatibility?

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Removing the PhantomPinned field would not be breaking, but we could never undo it because that would be breaking.

As for into_value, you make a good point. That method would not be able to take ownership of the future.

}

impl<T: 'static, F> PinnedDrop for TaskLocalFuture<T, F> {
fn drop(this: Pin<&mut Self>) {
let mut this = this.project();
if mem::needs_drop::<F>() && this.future.is_some() {
// Drop the future while the task-local is set, if possible. Otherwise
// the future is dropped normally when the `Option<F>` field drops.
let mut future = this.future;
let _ = this.local.scope_inner(&mut this.slot, || {
future.set(None);
});
}
}
}
}

impl<T: 'static, F: Future> Future for TaskLocalFuture<T, F> {
type Output = F::Output;

#[track_caller]
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
// safety: The TaskLocalFuture struct is `!Unpin` so there is no way to
// move `self.future` from now on.
let this = unsafe { Pin::into_inner_unchecked(self) };
let mut future_opt = unsafe { Pin::new_unchecked(&mut this.future) };
let mut this = self.project();
let mut future_opt = this.future;

let res =
this.local
Expand All @@ -362,19 +378,6 @@ impl<T: 'static, F: Future> Future for TaskLocalFuture<T, F> {
}
}

impl<T: 'static, F> Drop for TaskLocalFuture<T, F> {
fn drop(&mut self) {
if mem::needs_drop::<F>() && self.future.is_some() {
// Drop the future while the task-local is set, if possible. Otherwise
// the future is dropped normally when the `Option<F>` field drops.
let future = &mut self.future;
let _ = self.local.scope_inner(&mut self.slot, || {
*future = None;
});
}
}
}

impl<T: 'static, F> fmt::Debug for TaskLocalFuture<T, F>
where
T: fmt::Debug,
Expand Down