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Merge pull request #344 from glommer/unsafe-scoped
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mark ScopedTask as unsafe.
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Glauber Costa authored May 4, 2021
2 parents 75e0e96 + eaa13a6 commit bf8e442
Showing 1 changed file with 97 additions and 32 deletions.
129 changes: 97 additions & 32 deletions glommio/src/executor/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1217,7 +1217,8 @@ impl Default for LocalExecutor {
/// `'static`. Usually the pattern to make sure something is static is to use
/// `Rc<RefCell<T>>` or `Rc<Cell<T>>` and clone it, but that has a cost. If that
/// cost is deemed unacceptable, and you are able to have a well-defined
/// lifetime, you can use a [`ScopedTask`]
/// lifetime, and understand its safety considerations, you can use a
/// [`ScopedTask`]
///
/// Tasks are also futures themselves and yield the output of the spawned
/// future.
Expand Down Expand Up @@ -1723,6 +1724,21 @@ impl<T> Future for Task<T> {
/// Tasks that panic get immediately canceled. Awaiting a canceled task also
/// causes a panic.
///
/// # Safety
///
/// `ScopedTask` is safe to use so long as it is guaranteed to be either awaited
/// or dropped. Rust does not guarantee that destructors will be called, and if
/// they are not, `ScopedTask`s can be kept alive after the scope is terminated.
///
/// Typically, the only situations in which `drop` is not executed are:
///
/// * If you manually choose not to, with [`std::mem::forget`] or
/// [`ManuallyDrop`].
/// * If cyclic reference counts prevents the task from being destroyed.
///
/// If you believe any of the above situations are present (the first one is,
/// of course, considerably easier to spot), avoid using the `ScopedTask`.
///
/// # Examples
///
/// ```
Expand All @@ -1732,10 +1748,12 @@ impl<T> Future for Task<T> {
///
/// ex.run(async {
/// let a = 2;
/// let task = ScopedTask::local(async {
/// println!("Hello from a task!");
/// 1 + a // this is a reference, and it works just fine
/// });
/// let task = unsafe {
/// ScopedTask::local(async {
/// println!("Hello from a task!");
/// 1 + a // this is a reference, and it works just fine
/// })
/// };
///
/// assert_eq!(task.await, 3);
/// });
Expand All @@ -1749,9 +1767,11 @@ impl<T> Future for Task<T> {
/// # let ex = LocalExecutor::default();
/// # ex.run(async {
/// let mut a = 2;
/// let task = ScopedTask::local(async {
/// a = 3;
/// });
/// let task = unsafe {
/// ScopedTask::local(async {
/// a = 3;
/// })
/// };
/// task.await;
/// assert_eq!(a, 3);
/// # });
Expand All @@ -1766,9 +1786,11 @@ impl<T> Future for Task<T> {
/// # let ex = LocalExecutor::default();
/// # ex.run(async {
/// let mut a = 2;
/// let task = ScopedTask::local(async {
/// a = 3;
/// });
/// let task = unsafe {
/// ScopedTask::local(async {
/// a = 3;
/// })
/// };
/// assert_eq!(a, 3); // task hasn't completed yet!
/// task.await;
/// # });
Expand All @@ -1786,9 +1808,11 @@ impl<T> Future for Task<T> {
/// # let ex = LocalExecutor::default();
/// # ex.run(async {
/// let a = Cell::new(2);
/// let task = ScopedTask::local(async {
/// a.set(3);
/// });
/// let task = unsafe {
/// ScopedTask::local(async {
/// a.set(3);
/// })
/// };
///
/// assert!(a.get() == 3 || a.get() == 2); // impossible to know if it will be 2 or 3
/// task.await;
Expand All @@ -1797,9 +1821,32 @@ impl<T> Future for Task<T> {
/// # });
/// ```
///
/// The following code, however, will access invalid memory as drop is never
/// executed
///
/// ```no_run
/// # use glommio::{LocalExecutor, ScopedTask};
/// # use std::cell::Cell;
/// #
/// # let ex = LocalExecutor::default();
/// # ex.run(async {
/// {
/// let a = &mut "mayhem";
/// let task = unsafe {
/// ScopedTask::local(async {
/// *a = "doom";
/// })
/// };
/// std::mem::forget(task);
/// }
/// # });
/// ```

/// [`Task`]: crate::Task
/// [`Cell`]: std::cell::Cell
/// [`RefCell`]: std::cell::RefCell
/// [`std::mem::forget`]: std::mem::forget
/// [`ManuallyDrop`]: std::mem::ManuallyDrop
#[must_use = "scoped tasks get canceled when dropped, use a standard Task and `.detach()` to run \
them in the background"]
#[derive(Debug)]
Expand All @@ -1812,6 +1859,11 @@ impl<'a, T> ScopedTask<'a, T> {
///
/// Otherwise, this method panics.
///
/// # Safety
///
/// `ScopedTask` depends on `drop` running or `.await` being called for
/// safety. See the struct [`ScopedTask`] for details.
///
/// # Examples
///
/// ```
Expand All @@ -1821,11 +1873,11 @@ impl<'a, T> ScopedTask<'a, T> {
///
/// local_ex.run(async {
/// let non_static = 2;
/// let task = ScopedTask::local(async { 1 + non_static });
/// let task = unsafe { ScopedTask::local(async { 1 + non_static }) };
/// assert_eq!(task.await, 3);
/// });
/// ```
pub fn local(future: impl Future<Output = T> + 'a) -> Self {
pub unsafe fn local(future: impl Future<Output = T> + 'a) -> Self {
LOCAL_EX.with(|local_ex| Self(local_ex.spawn(future), PhantomData))
}

Expand All @@ -1836,6 +1888,11 @@ impl<'a, T> ScopedTask<'a, T> {
///
/// Otherwise, this method panics.
///
/// # Safety
///
/// `ScopedTask` depends on `drop` running or `.await` being called for
/// safety. See the struct [`ScopedTask`] for details.
///
/// # Examples
///
/// ```
Expand All @@ -1849,12 +1906,14 @@ impl<'a, T> ScopedTask<'a, T> {
/// "test_queue",
/// );
/// let non_static = 2;
/// let task = ScopedTask::<usize>::local_into(async { 1 + non_static }, handle)
/// .expect("failed to spawn task");
/// let task = unsafe {
/// ScopedTask::<usize>::local_into(async { 1 + non_static }, handle)
/// .expect("failed to spawn task")
/// };
/// assert_eq!(task.await, 3);
/// })
/// ```
pub fn local_into(
pub unsafe fn local_into(
future: impl Future<Output = T> + 'a,
handle: TaskQueueHandle,
) -> Result<Self> {
Expand Down Expand Up @@ -1883,12 +1942,14 @@ impl<'a, T> ScopedTask<'a, T> {
/// let ex = LocalExecutor::default();
///
/// ex.run(async {
/// let task = ScopedTask::local(async {
/// loop {
/// println!("Even though I'm in an infinite loop, you can still cancel me!");
/// future::yield_now().await;
/// }
/// });
/// let task = unsafe {
/// ScopedTask::local(async {
/// loop {
/// println!("Even though I'm in an infinite loop, you can still cancel me!");
/// future::yield_now().await;
/// }
/// })
/// };
///
/// task.cancel().await;
/// });
Expand Down Expand Up @@ -2708,17 +2769,21 @@ mod test {
fn scoped_task() {
LocalExecutor::default().run(async {
let mut a = 1;
ScopedTask::local(async {
a = 2;
})
.await;
unsafe {
ScopedTask::local(async {
a = 2;
})
.await;
}
Local::later().await;
assert_eq!(a, 2);

let mut a = 1;
let do_later = ScopedTask::local(async {
a = 2;
});
let do_later = unsafe {
ScopedTask::local(async {
a = 2;
})
};

Local::later().await;
do_later.await;
Expand Down

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