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//! Runs `!Send` futures on the current thread.
use crate::loom::cell::UnsafeCell;
use crate::loom::sync::{Arc, Mutex};
#[cfg(tokio_unstable)]
use crate::runtime;
use crate::runtime::task::{self, JoinHandle, LocalOwnedTasks, Task};
use crate::runtime::{context, ThreadId, BOX_FUTURE_THRESHOLD};
use crate::sync::AtomicWaker;
use crate::util::RcCell;
use std::cell::Cell;
use std::collections::VecDeque;
use std::fmt;
use std::future::Future;
use std::marker::PhantomData;
use std::pin::Pin;
use std::rc::Rc;
use std::task::Poll;
use pin_project_lite::pin_project;
cfg_rt! {
/// A set of tasks which are executed on the same thread.
///
/// In some cases, it is necessary to run one or more futures that do not
/// implement [`Send`] and thus are unsafe to send between threads. In these
/// cases, a [local task set] may be used to schedule one or more `!Send`
/// futures to run together on the same thread.
///
/// For example, the following code will not compile:
///
/// ```rust,compile_fail
/// use std::rc::Rc;
///
/// #[tokio::main]
/// async fn main() {
/// // `Rc` does not implement `Send`, and thus may not be sent between
/// // threads safely.
/// let nonsend_data = Rc::new("my nonsend data...");
///
/// let nonsend_data = nonsend_data.clone();
/// // Because the `async` block here moves `nonsend_data`, the future is `!Send`.
/// // Since `tokio::spawn` requires the spawned future to implement `Send`, this
/// // will not compile.
/// tokio::spawn(async move {
/// println!("{}", nonsend_data);
/// // ...
/// }).await.unwrap();
/// }
/// ```
///
/// # Use with `run_until`
///
/// To spawn `!Send` futures, we can use a local task set to schedule them
/// on the thread calling [`Runtime::block_on`]. When running inside of the
/// local task set, we can use [`task::spawn_local`], which can spawn
/// `!Send` futures. For example:
///
/// ```rust
/// use std::rc::Rc;
/// use tokio::task;
///
/// #[tokio::main]
/// async fn main() {
/// let nonsend_data = Rc::new("my nonsend data...");
///
/// // Construct a local task set that can run `!Send` futures.
/// let local = task::LocalSet::new();
///
/// // Run the local task set.
/// local.run_until(async move {
/// let nonsend_data = nonsend_data.clone();
/// // `spawn_local` ensures that the future is spawned on the local
/// // task set.
/// task::spawn_local(async move {
/// println!("{}", nonsend_data);
/// // ...
/// }).await.unwrap();
/// }).await;
/// }
/// ```
/// **Note:** The `run_until` method can only be used in `#[tokio::main]`,
/// `#[tokio::test]` or directly inside a call to [`Runtime::block_on`]. It
/// cannot be used inside a task spawned with `tokio::spawn`.
///
/// ## Awaiting a `LocalSet`
///
/// Additionally, a `LocalSet` itself implements `Future`, completing when
/// *all* tasks spawned on the `LocalSet` complete. This can be used to run
/// several futures on a `LocalSet` and drive the whole set until they
/// complete. For example,
///
/// ```rust
/// use tokio::{task, time};
/// use std::rc::Rc;
///
/// #[tokio::main]
/// async fn main() {
/// let nonsend_data = Rc::new("world");
/// let local = task::LocalSet::new();
///
/// let nonsend_data2 = nonsend_data.clone();
/// local.spawn_local(async move {
/// // ...
/// println!("hello {}", nonsend_data2)
/// });
///
/// local.spawn_local(async move {
/// time::sleep(time::Duration::from_millis(100)).await;
/// println!("goodbye {}", nonsend_data)
/// });
///
/// // ...
///
/// local.await;
/// }
/// ```
/// **Note:** Awaiting a `LocalSet` can only be done inside
/// `#[tokio::main]`, `#[tokio::test]` or directly inside a call to
/// [`Runtime::block_on`]. It cannot be used inside a task spawned with
/// `tokio::spawn`.
///
/// ## Use inside `tokio::spawn`
///
/// The two methods mentioned above cannot be used inside `tokio::spawn`, so
/// to spawn `!Send` futures from inside `tokio::spawn`, we need to do
/// something else. The solution is to create the `LocalSet` somewhere else,
/// and communicate with it using an [`mpsc`] channel.
///
/// The following example puts the `LocalSet` inside a new thread.
/// ```
/// use tokio::runtime::Builder;
/// use tokio::sync::{mpsc, oneshot};
/// use tokio::task::LocalSet;
///
/// // This struct describes the task you want to spawn. Here we include
/// // some simple examples. The oneshot channel allows sending a response
/// // to the spawner.
/// #[derive(Debug)]
/// enum Task {
/// PrintNumber(u32),
/// AddOne(u32, oneshot::Sender<u32>),
/// }
///
/// #[derive(Clone)]
/// struct LocalSpawner {
/// send: mpsc::UnboundedSender<Task>,
/// }
///
/// impl LocalSpawner {
/// pub fn new() -> Self {
/// let (send, mut recv) = mpsc::unbounded_channel();
///
/// let rt = Builder::new_current_thread()
/// .enable_all()
/// .build()
/// .unwrap();
///
/// std::thread::spawn(move || {
/// let local = LocalSet::new();
///
/// local.spawn_local(async move {
/// while let Some(new_task) = recv.recv().await {
/// tokio::task::spawn_local(run_task(new_task));
/// }
/// // If the while loop returns, then all the LocalSpawner
/// // objects have been dropped.
/// });
///
/// // This will return once all senders are dropped and all
/// // spawned tasks have returned.
/// rt.block_on(local);
/// });
///
/// Self {
/// send,
/// }
/// }
///
/// pub fn spawn(&self, task: Task) {
/// self.send.send(task).expect("Thread with LocalSet has shut down.");
/// }
/// }
///
/// // This task may do !Send stuff. We use printing a number as an example,
/// // but it could be anything.
/// //
/// // The Task struct is an enum to support spawning many different kinds
/// // of operations.
/// async fn run_task(task: Task) {
/// match task {
/// Task::PrintNumber(n) => {
/// println!("{}", n);
/// },
/// Task::AddOne(n, response) => {
/// // We ignore failures to send the response.
/// let _ = response.send(n + 1);
/// },
/// }
/// }
///
/// #[tokio::main]
/// async fn main() {
/// let spawner = LocalSpawner::new();
///
/// let (send, response) = oneshot::channel();
/// spawner.spawn(Task::AddOne(10, send));
/// let eleven = response.await.unwrap();
/// assert_eq!(eleven, 11);
/// }
/// ```
///
/// [`Send`]: trait@std::marker::Send
/// [local task set]: struct@LocalSet
/// [`Runtime::block_on`]: method@crate::runtime::Runtime::block_on
/// [`task::spawn_local`]: fn@spawn_local
/// [`mpsc`]: mod@crate::sync::mpsc
pub struct LocalSet {
/// Current scheduler tick.
tick: Cell<u8>,
/// State available from thread-local.
context: Rc<Context>,
/// This type should not be Send.
_not_send: PhantomData<*const ()>,
}
}
/// State available from the thread-local.
struct Context {
/// State shared between threads.
shared: Arc<Shared>,
/// True if a task panicked without being handled and the local set is
/// configured to shutdown on unhandled panic.
unhandled_panic: Cell<bool>,
}
/// `LocalSet` state shared between threads.
struct Shared {
/// # Safety
///
/// This field must *only* be accessed from the thread that owns the
/// `LocalSet` (i.e., `Thread::current().id() == owner`).
local_state: LocalState,
/// Remote run queue sender.
queue: Mutex<Option<VecDeque<task::Notified<Arc<Shared>>>>>,
/// Wake the `LocalSet` task.
waker: AtomicWaker,
/// How to respond to unhandled task panics.
#[cfg(tokio_unstable)]
pub(crate) unhandled_panic: crate::runtime::UnhandledPanic,
}
/// Tracks the `LocalSet` state that must only be accessed from the thread that
/// created the `LocalSet`.
struct LocalState {
/// The `ThreadId` of the thread that owns the `LocalSet`.
owner: ThreadId,
/// Local run queue sender and receiver.
local_queue: UnsafeCell<VecDeque<task::Notified<Arc<Shared>>>>,
/// Collection of all active tasks spawned onto this executor.
owned: LocalOwnedTasks<Arc<Shared>>,
}
pin_project! {
#[derive(Debug)]
struct RunUntil<'a, F> {
local_set: &'a LocalSet,
#[pin]
future: F,
}
}
tokio_thread_local!(static CURRENT: LocalData = const { LocalData {
ctx: RcCell::new(),
wake_on_schedule: Cell::new(false),
} });
struct LocalData {
ctx: RcCell<Context>,
wake_on_schedule: Cell<bool>,
}
impl LocalData {
/// Should be called except when we call `LocalSet::enter`.
/// Especially when we poll a `LocalSet`.
#[must_use = "dropping this guard will reset the entered state"]
fn enter(&self, ctx: Rc<Context>) -> LocalDataEnterGuard<'_> {
let ctx = self.ctx.replace(Some(ctx));
let wake_on_schedule = self.wake_on_schedule.replace(false);
LocalDataEnterGuard {
local_data_ref: self,
ctx,
wake_on_schedule,
}
}
}
/// A guard for `LocalData::enter()`
struct LocalDataEnterGuard<'a> {
local_data_ref: &'a LocalData,
ctx: Option<Rc<Context>>,
wake_on_schedule: bool,
}
impl<'a> Drop for LocalDataEnterGuard<'a> {
fn drop(&mut self) {
self.local_data_ref.ctx.set(self.ctx.take());
self.local_data_ref
.wake_on_schedule
.set(self.wake_on_schedule)
}
}
cfg_rt! {
/// Spawns a `!Send` future on the current [`LocalSet`].
///
/// The spawned future will run on the same thread that called `spawn_local`.
///
/// The provided future will start running in the background immediately
/// when `spawn_local` is called, even if you don't await the returned
/// `JoinHandle`.
///
/// # Panics
///
/// This function panics if called outside of a [`LocalSet`].
///
/// Note that if [`tokio::spawn`] is used from within a `LocalSet`, the
/// resulting new task will _not_ be inside the `LocalSet`, so you must use
/// `spawn_local` if you want to stay within the `LocalSet`.
///
/// # Examples
///
/// ```rust
/// use std::rc::Rc;
/// use tokio::task;
///
/// #[tokio::main]
/// async fn main() {
/// let nonsend_data = Rc::new("my nonsend data...");
///
/// let local = task::LocalSet::new();
///
/// // Run the local task set.
/// local.run_until(async move {
/// let nonsend_data = nonsend_data.clone();
/// task::spawn_local(async move {
/// println!("{}", nonsend_data);
/// // ...
/// }).await.unwrap();
/// }).await;
/// }
/// ```
///
/// [`LocalSet`]: struct@crate::task::LocalSet
/// [`tokio::spawn`]: fn@crate::task::spawn
#[track_caller]
pub fn spawn_local<F>(future: F) -> JoinHandle<F::Output>
where
F: Future + 'static,
F::Output: 'static,
{
if cfg!(debug_assertions) && std::mem::size_of::<F>() > BOX_FUTURE_THRESHOLD {
spawn_local_inner(Box::pin(future), None)
} else {
spawn_local_inner(future, None)
}
}
#[track_caller]
pub(super) fn spawn_local_inner<F>(future: F, name: Option<&str>) -> JoinHandle<F::Output>
where F: Future + 'static,
F::Output: 'static
{
match CURRENT.with(|LocalData { ctx, .. }| ctx.get()) {
None => panic!("`spawn_local` called from outside of a `task::LocalSet`"),
Some(cx) => cx.spawn(future, name)
}
}
}
/// Initial queue capacity.
const INITIAL_CAPACITY: usize = 64;
/// Max number of tasks to poll per tick.
const MAX_TASKS_PER_TICK: usize = 61;
/// How often it check the remote queue first.
const REMOTE_FIRST_INTERVAL: u8 = 31;
/// Context guard for `LocalSet`
pub struct LocalEnterGuard {
ctx: Option<Rc<Context>>,
/// Distinguishes whether the context was entered or being polled.
/// When we enter it, the value `wake_on_schedule` is set. In this case
/// `spawn_local` refers the context, whereas it is not being polled now.
wake_on_schedule: bool,
}
impl Drop for LocalEnterGuard {
fn drop(&mut self) {
CURRENT.with(
|LocalData {
ctx,
wake_on_schedule,
}| {
ctx.set(self.ctx.take());
wake_on_schedule.set(self.wake_on_schedule);
},
);
}
}
impl fmt::Debug for LocalEnterGuard {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("LocalEnterGuard").finish()
}
}
impl LocalSet {
/// Returns a new local task set.
pub fn new() -> LocalSet {
let owner = context::thread_id().expect("cannot create LocalSet during thread shutdown");
LocalSet {
tick: Cell::new(0),
context: Rc::new(Context {
shared: Arc::new(Shared {
local_state: LocalState {
owner,
owned: LocalOwnedTasks::new(),
local_queue: UnsafeCell::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
},
queue: Mutex::new(Some(VecDeque::with_capacity(INITIAL_CAPACITY))),
waker: AtomicWaker::new(),
#[cfg(tokio_unstable)]
unhandled_panic: crate::runtime::UnhandledPanic::Ignore,
}),
unhandled_panic: Cell::new(false),
}),
_not_send: PhantomData,
}
}
/// Enters the context of this `LocalSet`.
///
/// The [`spawn_local`] method will spawn tasks on the `LocalSet` whose
/// context you are inside.
///
/// [`spawn_local`]: fn@crate::task::spawn_local
pub fn enter(&self) -> LocalEnterGuard {
CURRENT.with(
|LocalData {
ctx,
wake_on_schedule,
..
}| {
let ctx = ctx.replace(Some(self.context.clone()));
let wake_on_schedule = wake_on_schedule.replace(true);
LocalEnterGuard {
ctx,
wake_on_schedule,
}
},
)
}
/// Spawns a `!Send` task onto the local task set.
///
/// This task is guaranteed to be run on the current thread.
///
/// Unlike the free function [`spawn_local`], this method may be used to
/// spawn local tasks when the `LocalSet` is _not_ running. The provided
/// future will start running once the `LocalSet` is next started, even if
/// you don't await the returned `JoinHandle`.
///
/// # Examples
///
/// ```rust
/// use tokio::task;
///
/// #[tokio::main]
/// async fn main() {
/// let local = task::LocalSet::new();
///
/// // Spawn a future on the local set. This future will be run when
/// // we call `run_until` to drive the task set.
/// local.spawn_local(async {
/// // ...
/// });
///
/// // Run the local task set.
/// local.run_until(async move {
/// // ...
/// }).await;
///
/// // When `run` finishes, we can spawn _more_ futures, which will
/// // run in subsequent calls to `run_until`.
/// local.spawn_local(async {
/// // ...
/// });
///
/// local.run_until(async move {
/// // ...
/// }).await;
/// }
/// ```
/// [`spawn_local`]: fn@spawn_local
#[track_caller]
pub fn spawn_local<F>(&self, future: F) -> JoinHandle<F::Output>
where
F: Future + 'static,
F::Output: 'static,
{
self.spawn_named(future, None)
}
/// Runs a future to completion on the provided runtime, driving any local
/// futures spawned on this task set on the current thread.
///
/// This runs the given future on the runtime, blocking until it is
/// complete, and yielding its resolved result. Any tasks or timers which
/// the future spawns internally will be executed on the runtime. The future
/// may also call [`spawn_local`] to `spawn_local` additional local futures on the
/// current thread.
///
/// This method should not be called from an asynchronous context.
///
/// # Panics
///
/// This function panics if the executor is at capacity, if the provided
/// future panics, or if called within an asynchronous execution context.
///
/// # Notes
///
/// Since this function internally calls [`Runtime::block_on`], and drives
/// futures in the local task set inside that call to `block_on`, the local
/// futures may not use [in-place blocking]. If a blocking call needs to be
/// issued from a local task, the [`spawn_blocking`] API may be used instead.
///
/// For example, this will panic:
/// ```should_panic
/// use tokio::runtime::Runtime;
/// use tokio::task;
///
/// let rt = Runtime::new().unwrap();
/// let local = task::LocalSet::new();
/// local.block_on(&rt, async {
/// let join = task::spawn_local(async {
/// let blocking_result = task::block_in_place(|| {
/// // ...
/// });
/// // ...
/// });
/// join.await.unwrap();
/// })
/// ```
/// This, however, will not panic:
/// ```
/// use tokio::runtime::Runtime;
/// use tokio::task;
///
/// let rt = Runtime::new().unwrap();
/// let local = task::LocalSet::new();
/// local.block_on(&rt, async {
/// let join = task::spawn_local(async {
/// let blocking_result = task::spawn_blocking(|| {
/// // ...
/// }).await;
/// // ...
/// });
/// join.await.unwrap();
/// })
/// ```
///
/// [`spawn_local`]: fn@spawn_local
/// [`Runtime::block_on`]: method@crate::runtime::Runtime::block_on
/// [in-place blocking]: fn@crate::task::block_in_place
/// [`spawn_blocking`]: fn@crate::task::spawn_blocking
#[track_caller]
#[cfg(feature = "rt")]
#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
pub fn block_on<F>(&self, rt: &crate::runtime::Runtime, future: F) -> F::Output
where
F: Future,
{
rt.block_on(self.run_until(future))
}
/// Runs a future to completion on the local set, returning its output.
///
/// This returns a future that runs the given future with a local set,
/// allowing it to call [`spawn_local`] to spawn additional `!Send` futures.
/// Any local futures spawned on the local set will be driven in the
/// background until the future passed to `run_until` completes. When the future
/// passed to `run_until` finishes, any local futures which have not completed
/// will remain on the local set, and will be driven on subsequent calls to
/// `run_until` or when [awaiting the local set] itself.
///
/// # Cancel safety
///
/// This method is cancel safe when `future` is cancel safe.
///
/// # Examples
///
/// ```rust
/// use tokio::task;
///
/// #[tokio::main]
/// async fn main() {
/// task::LocalSet::new().run_until(async {
/// task::spawn_local(async move {
/// // ...
/// }).await.unwrap();
/// // ...
/// }).await;
/// }
/// ```
///
/// [`spawn_local`]: fn@spawn_local
/// [awaiting the local set]: #awaiting-a-localset
pub async fn run_until<F>(&self, future: F) -> F::Output
where
F: Future,
{
let run_until = RunUntil {
future,
local_set: self,
};
run_until.await
}
#[track_caller]
pub(in crate::task) fn spawn_named<F>(
&self,
future: F,
name: Option<&str>,
) -> JoinHandle<F::Output>
where
F: Future + 'static,
F::Output: 'static,
{
if cfg!(debug_assertions) && std::mem::size_of::<F>() > BOX_FUTURE_THRESHOLD {
self.spawn_named_inner(Box::pin(future), name)
} else {
self.spawn_named_inner(future, name)
}
}
#[track_caller]
fn spawn_named_inner<F>(&self, future: F, name: Option<&str>) -> JoinHandle<F::Output>
where
F: Future + 'static,
F::Output: 'static,
{
let handle = self.context.spawn(future, name);
// Because a task was spawned from *outside* the `LocalSet`, wake the
// `LocalSet` future to execute the new task, if it hasn't been woken.
//
// Spawning via the free fn `spawn` does not require this, as it can
// only be called from *within* a future executing on the `LocalSet` —
// in that case, the `LocalSet` must already be awake.
self.context.shared.waker.wake();
handle
}
/// Ticks the scheduler, returning whether the local future needs to be
/// notified again.
fn tick(&self) -> bool {
for _ in 0..MAX_TASKS_PER_TICK {
// Make sure we didn't hit an unhandled panic
assert!(!self.context.unhandled_panic.get(), "a spawned task panicked and the LocalSet is configured to shutdown on unhandled panic");
match self.next_task() {
// Run the task
//
// Safety: As spawned tasks are `!Send`, `run_unchecked` must be
// used. We are responsible for maintaining the invariant that
// `run_unchecked` is only called on threads that spawned the
// task initially. Because `LocalSet` itself is `!Send`, and
// `spawn_local` spawns into the `LocalSet` on the current
// thread, the invariant is maintained.
Some(task) => crate::runtime::coop::budget(|| task.run()),
// We have fully drained the queue of notified tasks, so the
// local future doesn't need to be notified again — it can wait
// until something else wakes a task in the local set.
None => return false,
}
}
true
}
fn next_task(&self) -> Option<task::LocalNotified<Arc<Shared>>> {
let tick = self.tick.get();
self.tick.set(tick.wrapping_add(1));
let task = if tick % REMOTE_FIRST_INTERVAL == 0 {
self.context
.shared
.queue
.lock()
.as_mut()
.and_then(|queue| queue.pop_front())
.or_else(|| self.pop_local())
} else {
self.pop_local().or_else(|| {
self.context
.shared
.queue
.lock()
.as_mut()
.and_then(VecDeque::pop_front)
})
};
task.map(|task| unsafe {
// Safety: because the `LocalSet` itself is `!Send`, we know we are
// on the same thread if we have access to the `LocalSet`, and can
// therefore access the local run queue.
self.context.shared.local_state.assert_owner(task)
})
}
fn pop_local(&self) -> Option<task::Notified<Arc<Shared>>> {
unsafe {
// Safety: because the `LocalSet` itself is `!Send`, we know we are
// on the same thread if we have access to the `LocalSet`, and can
// therefore access the local run queue.
self.context.shared.local_state.task_pop_front()
}
}
fn with<T>(&self, f: impl FnOnce() -> T) -> T {
CURRENT.with(|local_data| {
let _guard = local_data.enter(self.context.clone());
f()
})
}
/// This method is like `with`, but it just calls `f` without setting the thread-local if that
/// fails.
fn with_if_possible<T>(&self, f: impl FnOnce() -> T) -> T {
let mut f = Some(f);
let res = CURRENT.try_with(|local_data| {
let _guard = local_data.enter(self.context.clone());
(f.take().unwrap())()
});
match res {
Ok(res) => res,
Err(_access_error) => (f.take().unwrap())(),
}
}
}
cfg_unstable! {
impl LocalSet {
/// Configure how the `LocalSet` responds to an unhandled panic on a
/// spawned task.
///
/// By default, an unhandled panic (i.e. a panic not caught by
/// [`std::panic::catch_unwind`]) has no impact on the `LocalSet`'s
/// execution. The panic is error value is forwarded to the task's
/// [`JoinHandle`] and all other spawned tasks continue running.
///
/// The `unhandled_panic` option enables configuring this behavior.
///
/// * `UnhandledPanic::Ignore` is the default behavior. Panics on
/// spawned tasks have no impact on the `LocalSet`'s execution.
/// * `UnhandledPanic::ShutdownRuntime` will force the `LocalSet` to
/// shutdown immediately when a spawned task panics even if that
/// task's `JoinHandle` has not been dropped. All other spawned tasks
/// will immediately terminate and further calls to
/// [`LocalSet::block_on`] and [`LocalSet::run_until`] will panic.
///
/// # Panics
///
/// This method panics if called after the `LocalSet` has started
/// running.
///
/// # Unstable
///
/// This option is currently unstable and its implementation is
/// incomplete. The API may change or be removed in the future. See
/// tokio-rs/tokio#4516 for more details.
///
/// # Examples
///
/// The following demonstrates a `LocalSet` configured to shutdown on
/// panic. The first spawned task panics and results in the `LocalSet`
/// shutting down. The second spawned task never has a chance to
/// execute. The call to `run_until` will panic due to the runtime being
/// forcibly shutdown.
///
/// ```should_panic
/// use tokio::runtime::UnhandledPanic;
///
/// # #[tokio::main]
/// # async fn main() {
/// tokio::task::LocalSet::new()
/// .unhandled_panic(UnhandledPanic::ShutdownRuntime)
/// .run_until(async {
/// tokio::task::spawn_local(async { panic!("boom"); });
/// tokio::task::spawn_local(async {
/// // This task never completes
/// });
///
/// // Do some work, but `run_until` will panic before it completes
/// # loop { tokio::task::yield_now().await; }
/// })
/// .await;
/// # }
/// ```
///
/// [`JoinHandle`]: struct@crate::task::JoinHandle
pub fn unhandled_panic(&mut self, behavior: crate::runtime::UnhandledPanic) -> &mut Self {
// TODO: This should be set as a builder
Rc::get_mut(&mut self.context)
.and_then(|ctx| Arc::get_mut(&mut ctx.shared))
.expect("Unhandled Panic behavior modified after starting LocalSet")
.unhandled_panic = behavior;
self
}
/// Returns the [`Id`] of the current `LocalSet` runtime.
///
/// # Examples
///
/// ```rust
/// use tokio::task;
///
/// #[tokio::main]
/// async fn main() {
/// let local_set = task::LocalSet::new();
/// println!("Local set id: {}", local_set.id());
/// }
/// ```
///
/// **Note**: This is an [unstable API][unstable]. The public API of this type
/// may break in 1.x releases. See [the documentation on unstable
/// features][unstable] for details.
///
/// [unstable]: crate#unstable-features
/// [`Id`]: struct@crate::runtime::Id
pub fn id(&self) -> runtime::Id {
self.context.shared.local_state.owned.id.into()
}
}
}
impl fmt::Debug for LocalSet {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("LocalSet").finish()
}
}
impl Future for LocalSet {
type Output = ();
fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
// Register the waker before starting to work
self.context.shared.waker.register_by_ref(cx.waker());
if self.with(|| self.tick()) {
// If `tick` returns true, we need to notify the local future again:
// there are still tasks remaining in the run queue.
cx.waker().wake_by_ref();
Poll::Pending
// Safety: called from the thread that owns `LocalSet`. Because
// `LocalSet` is `!Send`, this is safe.
} else if unsafe { self.context.shared.local_state.owned_is_empty() } {
// If the scheduler has no remaining futures, we're done!
Poll::Ready(())
} else {
// There are still futures in the local set, but we've polled all the
// futures in the run queue. Therefore, we can just return Pending
// since the remaining futures will be woken from somewhere else.
Poll::Pending
}
}
}
impl Default for LocalSet {
fn default() -> LocalSet {
LocalSet::new()
}
}
impl Drop for LocalSet {
fn drop(&mut self) {
self.with_if_possible(|| {
// Shut down all tasks in the LocalOwnedTasks and close it to
// prevent new tasks from ever being added.
unsafe {
// Safety: called from the thread that owns `LocalSet`
self.context.shared.local_state.close_and_shutdown_all();
}
// We already called shutdown on all tasks above, so there is no
// need to call shutdown.
// Safety: note that this *intentionally* bypasses the unsafe
// `Shared::local_queue()` method. This is in order to avoid the
// debug assertion that we are on the thread that owns the
// `LocalSet`, because on some systems (e.g. at least some macOS
// versions), attempting to get the current thread ID can panic due
// to the thread's local data that stores the thread ID being
// dropped *before* the `LocalSet`.
//
// Despite avoiding the assertion here, it is safe for us to access
// the local queue in `Drop`, because the `LocalSet` itself is
// `!Send`, so we can reasonably guarantee that it will not be
// `Drop`ped from another thread.
let local_queue = unsafe {
// Safety: called from the thread that owns `LocalSet`
self.context.shared.local_state.take_local_queue()
};
for task in local_queue {
drop(task);
}
// Take the queue from the Shared object to prevent pushing
// notifications to it in the future.
let queue = self.context.shared.queue.lock().take().unwrap();
for task in queue {
drop(task);
}
// Safety: called from the thread that owns `LocalSet`
assert!(unsafe { self.context.shared.local_state.owned_is_empty() });
});
}
}
// === impl Context ===
impl Context {
#[track_caller]
fn spawn<F>(&self, future: F, name: Option<&str>) -> JoinHandle<F::Output>
where
F: Future + 'static,
F::Output: 'static,
{
let id = crate::runtime::task::Id::next();
let future = crate::util::trace::task(future, "local", name, id.as_u64());
// Safety: called from the thread that owns the `LocalSet`
let (handle, notified) = {
self.shared.local_state.assert_called_from_owner_thread();
self.shared
.local_state
.owned
.bind(future, self.shared.clone(), id)
};
if let Some(notified) = notified {
self.shared.schedule(notified);
}
handle
}
}
// === impl LocalFuture ===
impl<T: Future> Future for RunUntil<'_, T> {
type Output = T::Output;
fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
let me = self.project();
me.local_set.with(|| {
me.local_set
.context
.shared
.waker
.register_by_ref(cx.waker());
let _no_blocking = crate::runtime::context::disallow_block_in_place();
let f = me.future;
if let Poll::Ready(output) = f.poll(cx) {
return Poll::Ready(output);
}
if me.local_set.tick() {
// If `tick` returns `true`, we need to notify the local future again:
// there are still tasks remaining in the run queue.
cx.waker().wake_by_ref();
}
Poll::Pending
})
}
}
impl Shared {
/// Schedule the provided task on the scheduler.
fn schedule(&self, task: task::Notified<Arc<Self>>) {
CURRENT.with(|localdata| {
match localdata.ctx.get() {
// If the current `LocalSet` is being polled, we don't need to wake it.
// When we `enter` it, then the value `wake_on_schedule` is set to be true.
// In this case it is not being polled, so we need to wake it.
Some(cx) if cx.shared.ptr_eq(self) && !localdata.wake_on_schedule.get() => unsafe {
// Safety: if the current `LocalSet` context points to this
// `LocalSet`, then we are on the thread that owns it.
cx.shared.local_state.task_push_back(task);
},
// We are on the thread that owns the `LocalSet`, so we can
// wake to the local queue.
_ if context::thread_id().ok() == Some(self.local_state.owner) => {
unsafe {
// Safety: we just checked that the thread ID matches
// the localset's owner, so this is safe.
self.local_state.task_push_back(task);
}
// We still have to wake the `LocalSet`, because it isn't
// currently being polled.
self.waker.wake();
}
// We are *not* on the thread that owns the `LocalSet`, so we
// have to wake to the remote queue.
_ => {
// First, check whether the queue is still there (if not, the
// LocalSet is dropped). Then push to it if so, and if not,
// do nothing.
let mut lock = self.queue.lock();
if let Some(queue) = lock.as_mut() {
queue.push_back(task);
drop(lock);
self.waker.wake();
}
}
}
});
}
fn ptr_eq(&self, other: &Shared) -> bool {
std::ptr::eq(self, other)
}
}
// This is safe because (and only because) we *pinky pwomise* to never touch the
// local run queue except from the thread that owns the `LocalSet`.
unsafe impl Sync for Shared {}
impl task::Schedule for Arc<Shared> {
fn release(&self, task: &Task<Self>) -> Option<Task<Self>> {
// Safety, this is always called from the thread that owns `LocalSet`
unsafe { self.local_state.task_remove(task) }
}
fn schedule(&self, task: task::Notified<Self>) {
Shared::schedule(self, task);
}
cfg_unstable! {
fn unhandled_panic(&self) {
use crate::runtime::UnhandledPanic;
match self.unhandled_panic {
UnhandledPanic::Ignore => {
// Do nothing
}
UnhandledPanic::ShutdownRuntime => {
// This hook is only called from within the runtime, so
// `CURRENT` should match with `&self`, i.e. there is no
// opportunity for a nested scheduler to be called.
CURRENT.with(|LocalData { ctx, .. }| match ctx.get() {
Some(cx) if Arc::ptr_eq(self, &cx.shared) => {
cx.unhandled_panic.set(true);
// Safety: this is always called from the thread that owns `LocalSet`
unsafe { cx.shared.local_state.close_and_shutdown_all(); }
}
_ => unreachable!("runtime core not set in CURRENT thread-local"),
})
}
}
}
}
}
impl LocalState {
unsafe fn task_pop_front(&self) -> Option<task::Notified<Arc<Shared>>> {
// The caller ensures it is called from the same thread that owns
// the LocalSet.
self.assert_called_from_owner_thread();
self.local_queue.with_mut(|ptr| (*ptr).pop_front())
}
unsafe fn task_push_back(&self, task: task::Notified<Arc<Shared>>) {
// The caller ensures it is called from the same thread that owns
// the LocalSet.
self.assert_called_from_owner_thread();
self.local_queue.with_mut(|ptr| (*ptr).push_back(task));
}
unsafe fn take_local_queue(&self) -> VecDeque<task::Notified<Arc<Shared>>> {
// The caller ensures it is called from the same thread that owns
// the LocalSet.
self.assert_called_from_owner_thread();
self.local_queue.with_mut(|ptr| std::mem::take(&mut (*ptr)))
}
unsafe fn task_remove(&self, task: &Task<Arc<Shared>>) -> Option<Task<Arc<Shared>>> {
// The caller ensures it is called from the same thread that owns
// the LocalSet.
self.assert_called_from_owner_thread();
self.owned.remove(task)
}
/// Returns true if the `LocalSet` does not have any spawned tasks
unsafe fn owned_is_empty(&self) -> bool {
// The caller ensures it is called from the same thread that owns
// the LocalSet.
self.assert_called_from_owner_thread();
self.owned.is_empty()
}
unsafe fn assert_owner(
&self,
task: task::Notified<Arc<Shared>>,
) -> task::LocalNotified<Arc<Shared>> {
// The caller ensures it is called from the same thread that owns
// the LocalSet.
self.assert_called_from_owner_thread();
self.owned.assert_owner(task)
}
unsafe fn close_and_shutdown_all(&self) {
// The caller ensures it is called from the same thread that owns
// the LocalSet.
self.assert_called_from_owner_thread();
self.owned.close_and_shutdown_all();
}
#[track_caller]
fn assert_called_from_owner_thread(&self) {
// FreeBSD has some weirdness around thread-local destruction.
// TODO: remove this hack when thread id is cleaned up
#[cfg(not(any(target_os = "openbsd", target_os = "freebsd")))]
debug_assert!(
// if we couldn't get the thread ID because we're dropping the local
// data, skip the assertion --- the `Drop` impl is not going to be
// called from another thread, because `LocalSet` is `!Send`
context::thread_id()
.map(|id| id == self.owner)
.unwrap_or(true),
"`LocalSet`'s local run queue must not be accessed by another thread!"
);
}
}
// This is `Send` because it is stored in `Shared`. It is up to the caller to
// ensure they are on the same thread that owns the `LocalSet`.
unsafe impl Send for LocalState {}
#[cfg(all(test, not(loom)))]
mod tests {
use super::*;
// Does a `LocalSet` running on a current-thread runtime...basically work?
//
// This duplicates a test in `tests/task_local_set.rs`, but because this is
// a lib test, it will run under Miri, so this is necessary to catch stacked
// borrows violations in the `LocalSet` implementation.
#[test]
fn local_current_thread_scheduler() {
let f = async {
LocalSet::new()
.run_until(async {
spawn_local(async {}).await.unwrap();
})
.await;
};
crate::runtime::Builder::new_current_thread()
.build()
.expect("rt")
.block_on(f)
}
// Tests that when a task on a `LocalSet` is woken by an io driver on the
// same thread, the task is woken to the localset's local queue rather than
// its remote queue.
//
// This test has to be defined in the `local.rs` file as a lib test, rather
// than in `tests/`, because it makes assertions about the local set's
// internal state.
#[test]
fn wakes_to_local_queue() {
use super::*;
use crate::sync::Notify;
let rt = crate::runtime::Builder::new_current_thread()
.build()
.expect("rt");
rt.block_on(async {
let local = LocalSet::new();
let notify = Arc::new(Notify::new());
let task = local.spawn_local({
let notify = notify.clone();
async move {
notify.notified().await;
}
});
let mut run_until = Box::pin(local.run_until(async move {
task.await.unwrap();
}));
// poll the run until future once
crate::future::poll_fn(|cx| {
let _ = run_until.as_mut().poll(cx);
Poll::Ready(())
})
.await;
notify.notify_one();
let task = unsafe { local.context.shared.local_state.task_pop_front() };
// TODO(eliza): it would be nice to be able to assert that this is
// the local task.
assert!(
task.is_some(),
"task should have been notified to the LocalSet's local queue"
);
})
}
}