zed/crates/gpui/src/executor.rs

use crate::{App, GpuiRunnable, PlatformDispatcher, RunnableMeta, TaskTiming, profiler};
use async_task::Runnable;
use futures::channel::mpsc;
use parking_lot::{Condvar, Mutex};
use smol::prelude::*;
use std::{
    cell::Cell,
    fmt::Debug,
    marker::PhantomData,
    mem::{self, ManuallyDrop},
    num::NonZeroUsize,
    panic::Location,
    pin::Pin,
    rc::Rc,
    sync::{
        Arc,
        atomic::{AtomicUsize, Ordering},
    },
    task::{Context, Poll},
    thread::{self, ThreadId},
    time::{Duration, Instant},
};
use util::TryFutureExt as _;
use waker_fn::waker_fn;

#[cfg(any(test, feature = "test-support"))]
use rand::rngs::StdRng;

/// A pointer to the executor that is currently running,
/// for spawning background tasks.
#[derive(Clone)]
pub struct BackgroundExecutor {
    #[doc(hidden)]
    pub dispatcher: Arc<dyn PlatformDispatcher>,
}

/// A pointer to the executor that is currently running,
/// for spawning tasks on the main thread.
///
/// This is intentionally `!Send` via the `not_send` marker field. This is because
/// `ForegroundExecutor::spawn` does not require `Send` but checks at runtime that the future is
/// only polled from the same thread it was spawned from. These checks would fail when spawning
/// foreground tasks from background threads.
#[derive(Clone)]
pub struct ForegroundExecutor {
    #[doc(hidden)]
    pub dispatcher: Arc<dyn PlatformDispatcher>,
    liveness: std::sync::Weak<()>,
    not_send: PhantomData<Rc<()>>,
}

/// Realtime task priority
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
#[repr(u8)]
pub enum RealtimePriority {
    /// Audio task
    Audio,
    /// Other realtime task
    #[default]
    Other,
}

/// Task priority
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
#[repr(u8)]
pub enum Priority {
    /// Realtime priority
    ///
    /// Spawning a task with this priority will spin it off on a separate thread dedicated just to that task.
    Realtime(RealtimePriority),
    /// High priority
    ///
    /// Only use for tasks that are critical to the user experience / responsiveness of the editor.
    High,
    /// Medium priority, probably suits most of your use cases.
    #[default]
    Medium,
    /// Low priority
    ///
    /// Prioritize this for background work that can come in large quantities
    /// to not starve the executor of resources for high priority tasks
    Low,
}

thread_local! {
static CURRENT_TASKS_PRIORITY: Cell<Priority> = const { Cell::new(Priority::Medium) }; }

impl Priority {
    /// Sets the priority any spawn call from the runnable about
    /// to be run will use
    pub(crate) fn set_as_default_for_spawns(&self) {
        CURRENT_TASKS_PRIORITY.set(*self);
    }

    /// Returns the priority from the currently running task
    pub fn inherit() -> Self {
        CURRENT_TASKS_PRIORITY.get()
    }

    #[allow(dead_code)]
    pub(crate) const fn probability(&self) -> u32 {
        match self {
            // realtime priorities are not considered for probability scheduling
            Priority::Realtime(_) => 0,
            Priority::High => 60,
            Priority::Medium => 30,
            Priority::Low => 10,
        }
    }
}

/// Task is a primitive that allows work to happen in the background.
///
/// It implements [`Future`] so you can `.await` on it.
///
/// If you drop a task it will be cancelled immediately. Calling [`Task::detach`] allows
/// the task to continue running, but with no way to return a value.
#[must_use]
#[derive(Debug)]
pub struct Task<T>(TaskState<T>);

#[derive(Debug)]
enum TaskState<T> {
    /// A task that is ready to return a value
    Ready(Option<T>),

    /// A task that is currently running.
    Spawned(async_task::Task<T, RunnableMeta>),
}

impl<T> Task<T> {
    /// Creates a new task that will resolve with the value
    pub fn ready(val: T) -> Self {
        Task(TaskState::Ready(Some(val)))
    }

    /// Detaching a task runs it to completion in the background
    pub fn detach(self) {
        match self {
            Task(TaskState::Ready(_)) => {}
            Task(TaskState::Spawned(task)) => task.detach(),
        }
    }

    /// Converts this task into a fallible task that returns `Option<T>`.
    ///
    /// Unlike the standard `Task<T>`, a [`FallibleTask`] will return `None`
    /// if the app was dropped while the task is executing.
    ///
    /// # Example
    ///
    /// ```ignore
    /// // Background task that gracefully handles app shutdown:
    /// cx.background_spawn(async move {
    ///     let result = foreground_task.fallible().await;
    ///     if let Some(value) = result {
    ///         // Process the value
    ///     }
    ///     // If None, app was shut down - just exit gracefully
    /// }).detach();
    /// ```
    pub fn fallible(self) -> FallibleTask<T> {
        FallibleTask(match self.0 {
            TaskState::Ready(val) => FallibleTaskState::Ready(val),
            TaskState::Spawned(task) => FallibleTaskState::Spawned(task.fallible()),
        })
    }
}

impl<E, T> Task<Result<T, E>>
where
    T: 'static,
    E: 'static + Debug,
{
    /// Run the task to completion in the background and log any
    /// errors that occur.
    #[track_caller]
    pub fn detach_and_log_err(self, cx: &App) {
        let location = core::panic::Location::caller();
        cx.foreground_executor()
            .spawn(self.log_tracked_err(*location))
            .detach();
    }
}

impl<T> Future for Task<T> {
    type Output = T;

    fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
        match unsafe { self.get_unchecked_mut() } {
            Task(TaskState::Ready(val)) => Poll::Ready(val.take().unwrap()),
            Task(TaskState::Spawned(task)) => task.poll(cx),
        }
    }
}

/// A task that returns `Option<T>` instead of panicking when cancelled.
#[must_use]
pub struct FallibleTask<T>(FallibleTaskState<T>);

enum FallibleTaskState<T> {
    /// A task that is ready to return a value
    Ready(Option<T>),

    /// A task that is currently running (wraps async_task::FallibleTask).
    Spawned(async_task::FallibleTask<T, RunnableMeta>),
}

impl<T> FallibleTask<T> {
    /// Creates a new fallible task that will resolve with the value.
    pub fn ready(val: T) -> Self {
        FallibleTask(FallibleTaskState::Ready(Some(val)))
    }

    /// Detaching a task runs it to completion in the background.
    pub fn detach(self) {
        match self.0 {
            FallibleTaskState::Ready(_) => {}
            FallibleTaskState::Spawned(task) => task.detach(),
        }
    }
}

impl<T> Future for FallibleTask<T> {
    type Output = Option<T>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
        match unsafe { self.get_unchecked_mut() } {
            FallibleTask(FallibleTaskState::Ready(val)) => Poll::Ready(val.take()),
            FallibleTask(FallibleTaskState::Spawned(task)) => Pin::new(task).poll(cx),
        }
    }
}

impl<T> std::fmt::Debug for FallibleTask<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match &self.0 {
            FallibleTaskState::Ready(_) => f.debug_tuple("FallibleTask::Ready").finish(),
            FallibleTaskState::Spawned(task) => {
                f.debug_tuple("FallibleTask::Spawned").field(task).finish()
            }
        }
    }
}

/// A task label is an opaque identifier that you can use to
/// refer to a task in tests.
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub struct TaskLabel(NonZeroUsize);

impl Default for TaskLabel {
    fn default() -> Self {
        Self::new()
    }
}

impl TaskLabel {
    /// Construct a new task label.
    pub fn new() -> Self {
        static NEXT_TASK_LABEL: AtomicUsize = AtomicUsize::new(1);
        Self(
            NEXT_TASK_LABEL
                .fetch_add(1, Ordering::SeqCst)
                .try_into()
                .unwrap(),
        )
    }
}

type AnyLocalFuture<R> = Pin<Box<dyn 'static + Future<Output = R>>>;

type AnyFuture<R> = Pin<Box<dyn 'static + Send + Future<Output = R>>>;

/// BackgroundExecutor lets you run things on background threads.
/// In production this is a thread pool with no ordering guarantees.
/// In tests this is simulated by running tasks one by one in a deterministic
/// (but arbitrary) order controlled by the `SEED` environment variable.
impl BackgroundExecutor {
    #[doc(hidden)]
    pub fn new(dispatcher: Arc<dyn PlatformDispatcher>) -> Self {
        Self { dispatcher }
    }

    /// Enqueues the given future to be run to completion on a background thread.
    #[track_caller]
    pub fn spawn<R>(&self, future: impl Future<Output = R> + Send + 'static) -> Task<R>
    where
        R: Send + 'static,
    {
        self.spawn_with_priority(Priority::default(), future)
    }

    /// Enqueues the given future to be run to completion on a background thread.
    #[track_caller]
    pub fn spawn_with_priority<R>(
        &self,
        priority: Priority,
        future: impl Future<Output = R> + Send + 'static,
    ) -> Task<R>
    where
        R: Send + 'static,
    {
        self.spawn_internal::<R>(Box::pin(future), None, priority)
    }

    /// Enqueues the given future to be run to completion on a background thread and blocking the current task on it.
    ///
    /// This allows to spawn background work that borrows from its scope. Note that the supplied future will run to
    /// completion before the current task is resumed, even if the current task is slated for cancellation.
    pub async fn await_on_background<R>(&self, future: impl Future<Output = R> + Send) -> R
    where
        R: Send,
    {
        // We need to ensure that cancellation of the parent task does not drop the environment
        // before the our own task has completed or got cancelled.
        struct NotifyOnDrop<'a>(&'a (Condvar, Mutex<bool>));

        impl Drop for NotifyOnDrop<'_> {
            fn drop(&mut self) {
                *self.0.1.lock() = true;
                self.0.0.notify_all();
            }
        }

        struct WaitOnDrop<'a>(&'a (Condvar, Mutex<bool>));

        impl Drop for WaitOnDrop<'_> {
            fn drop(&mut self) {
                let mut done = self.0.1.lock();
                if !*done {
                    self.0.0.wait(&mut done);
                }
            }
        }

        let dispatcher = self.dispatcher.clone();
        let location = core::panic::Location::caller();

        let pair = &(Condvar::new(), Mutex::new(false));
        let _wait_guard = WaitOnDrop(pair);

        let (runnable, task) = unsafe {
            async_task::Builder::new()
                .metadata(RunnableMeta {
                    location,
                    app: None,
                    priority: Priority::inherit(),
                })
                .spawn_unchecked(
                    move |_| async {
                        let _notify_guard = NotifyOnDrop(pair);
                        future.await
                    },
                    move |runnable| dispatcher.dispatch(GpuiRunnable::GpuiSpawned(runnable), None),
                )
        };
        runnable.schedule();
        task.await
    }

    /// Enqueues the given future to be run to completion on a background thread.
    /// The given label can be used to control the priority of the task in tests.
    #[track_caller]
    pub fn spawn_labeled<R>(
        &self,
        label: TaskLabel,
        future: impl Future<Output = R> + Send + 'static,
    ) -> Task<R>
    where
        R: Send + 'static,
    {
        self.spawn_internal::<R>(Box::pin(future), Some(label), Priority::default())
    }

    #[track_caller]
    fn spawn_internal<R: Send + 'static>(
        &self,
        future: AnyFuture<R>,
        label: Option<TaskLabel>,
        priority: Priority,
    ) -> Task<R> {
        let dispatcher = self.dispatcher.clone();
        let (runnable, task) = if let Priority::Realtime(realtime) = priority {
            let location = core::panic::Location::caller();
            let (mut tx, rx) = flume::bounded::<Runnable<RunnableMeta>>(1);

            dispatcher.spawn_realtime(
                realtime,
                Box::new(move || {
                    while let Ok(runnable) = rx.recv() {
                        let start = Instant::now();
                        let location = runnable.metadata().location;
                        let mut timing = TaskTiming {
                            location,
                            start,
                            end: None,
                        };
                        profiler::add_task_timing(timing);

                        Priority::Realtime(realtime).set_as_default_for_spawns();
                        runnable.run();

                        let end = Instant::now();
                        timing.end = Some(end);
                        profiler::add_task_timing(timing);
                    }
                }),
            );

            async_task::Builder::new()
                .metadata(RunnableMeta {
                    location,
                    priority,
                    app: None,
                })
                .spawn(
                    move |_| future,
                    move |runnable| {
                        let _ = tx.send(runnable);
                    },
                )
        } else {
            let location = core::panic::Location::caller();
            async_task::Builder::new()
                .metadata(RunnableMeta {
                    location,
                    priority,
                    app: None,
                })
                .spawn(
                    move |_| future,
                    move |runnable| dispatcher.dispatch(GpuiRunnable::GpuiSpawned(runnable), label),
                )
        };

        runnable.schedule();
        Task(TaskState::Spawned(task))
    }

    /// Used by the test harness to run an async test in a synchronous fashion.
    #[cfg(any(test, feature = "test-support"))]
    #[track_caller]
    pub fn block_test<R>(&self, future: impl Future<Output = R>) -> R {
        if let Ok(value) = self.block_internal(false, future, None) {
            value
        } else {
            unreachable!()
        }
    }

    /// Block the current thread until the given future resolves.
    /// Consider using `block_with_timeout` instead.
    pub fn block<R>(&self, future: impl Future<Output = R>) -> R {
        if let Ok(value) = self.block_internal(true, future, None) {
            value
        } else {
            unreachable!()
        }
    }

    #[cfg(not(any(test, feature = "test-support")))]
    pub(crate) fn block_internal<Fut: Future>(
        &self,
        _background_only: bool,
        future: Fut,
        timeout: Option<Duration>,
    ) -> Result<Fut::Output, impl Future<Output = Fut::Output> + use<Fut>> {
        use std::time::Instant;

        let mut future = Box::pin(future);
        if timeout == Some(Duration::ZERO) {
            return Err(future);
        }
        let deadline = timeout.map(|timeout| Instant::now() + timeout);

        let parker = parking::Parker::new();
        let unparker = parker.unparker();
        let waker = waker_fn(move || {
            unparker.unpark();
        });
        let mut cx = std::task::Context::from_waker(&waker);

        loop {
            match future.as_mut().poll(&mut cx) {
                Poll::Ready(result) => return Ok(result),
                Poll::Pending => {
                    let timeout =
                        deadline.map(|deadline| deadline.saturating_duration_since(Instant::now()));
                    if let Some(timeout) = timeout {
                        if !parker.park_timeout(timeout)
                            && deadline.is_some_and(|deadline| deadline < Instant::now())
                        {
                            return Err(future);
                        }
                    } else {
                        parker.park();
                    }
                }
            }
        }
    }

    #[cfg(any(test, feature = "test-support"))]
    #[track_caller]
    pub(crate) fn block_internal<Fut: Future>(
        &self,
        background_only: bool,
        future: Fut,
        timeout: Option<Duration>,
    ) -> Result<Fut::Output, impl Future<Output = Fut::Output> + use<Fut>> {
        use std::sync::atomic::AtomicBool;
        use std::time::Instant;

        use parking::Parker;

        let mut future = Box::pin(future);
        if timeout == Some(Duration::ZERO) {
            return Err(future);
        }

        // When using a real platform (e.g., MacPlatform for visual tests that need actual
        // Metal rendering), there's no test dispatcher. In this case, we block the thread
        // directly by polling the future and parking until woken. This is required for
        // VisualTestAppContext which uses real platform rendering but still needs blocking
        // behavior for code paths like editor initialization that call block_with_timeout.
        let Some(dispatcher) = self.dispatcher.as_test() else {
            let deadline = timeout.map(|timeout| Instant::now() + timeout);

            let parker = Parker::new();
            let unparker = parker.unparker();
            let waker = waker_fn(move || {
                unparker.unpark();
            });
            let mut cx = std::task::Context::from_waker(&waker);

            loop {
                match future.as_mut().poll(&mut cx) {
                    Poll::Ready(result) => return Ok(result),
                    Poll::Pending => {
                        let timeout = deadline
                            .map(|deadline| deadline.saturating_duration_since(Instant::now()));
                        if let Some(timeout) = timeout {
                            if !parker.park_timeout(timeout)
                                && deadline.is_some_and(|deadline| deadline < Instant::now())
                            {
                                return Err(future);
                            }
                        } else {
                            parker.park();
                        }
                    }
                }
            }
        };

        let mut max_ticks = if timeout.is_some() {
            dispatcher.gen_block_on_ticks()
        } else {
            usize::MAX
        };

        let parker = Parker::new();
        let unparker = parker.unparker();

        let awoken = Arc::new(AtomicBool::new(false));
        let waker = waker_fn({
            let awoken = awoken.clone();
            let unparker = unparker.clone();
            move || {
                awoken.store(true, Ordering::SeqCst);
                unparker.unpark();
            }
        });
        let mut cx = std::task::Context::from_waker(&waker);

        let duration = Duration::from_secs(
            option_env!("GPUI_TEST_TIMEOUT")
                .and_then(|s| s.parse::<u64>().ok())
                .unwrap_or(180),
        );
        let mut test_should_end_by = Instant::now() + duration;

        loop {
            match future.as_mut().poll(&mut cx) {
                Poll::Ready(result) => return Ok(result),
                Poll::Pending => {
                    if max_ticks == 0 {
                        return Err(future);
                    }
                    max_ticks -= 1;

                    if !dispatcher.tick(background_only) {
                        if awoken.swap(false, Ordering::SeqCst) {
                            continue;
                        }

                        if !dispatcher.parking_allowed() {
                            if dispatcher.advance_clock_to_next_delayed() {
                                continue;
                            }
                            let mut backtrace_message = String::new();
                            let mut waiting_message = String::new();
                            if let Some(backtrace) = dispatcher.waiting_backtrace() {
                                backtrace_message =
                                    format!("\nbacktrace of waiting future:\n{:?}", backtrace);
                            }
                            if let Some(waiting_hint) = dispatcher.waiting_hint() {
                                waiting_message = format!("\n  waiting on: {}\n", waiting_hint);
                            }
                            panic!(
                                "parked with nothing left to run{waiting_message}{backtrace_message}",
                            )
                        }
                        dispatcher.push_unparker(unparker.clone());
                        parker.park_timeout(Duration::from_millis(1));
                        if Instant::now() > test_should_end_by {
                            panic!("test timed out after {duration:?} with allow_parking")
                        }
                    }
                }
            }
        }
    }

    /// Block the current thread until the given future resolves
    /// or `duration` has elapsed.
    pub fn block_with_timeout<Fut: Future>(
        &self,
        duration: Duration,
        future: Fut,
    ) -> Result<Fut::Output, impl Future<Output = Fut::Output> + use<Fut>> {
        self.block_internal(true, future, Some(duration))
    }

    /// Scoped lets you start a number of tasks and waits
    /// for all of them to complete before returning.
    pub async fn scoped<'scope, F>(&self, scheduler: F)
    where
        F: FnOnce(&mut Scope<'scope>),
    {
        let mut scope = Scope::new(self.clone(), Priority::default());
        (scheduler)(&mut scope);
        let spawned = mem::take(&mut scope.futures)
            .into_iter()
            .map(|f| self.spawn_with_priority(scope.priority, f))
            .collect::<Vec<_>>();
        for task in spawned {
            task.await;
        }
    }

    /// Scoped lets you start a number of tasks and waits
    /// for all of them to complete before returning.
    pub async fn scoped_priority<'scope, F>(&self, priority: Priority, scheduler: F)
    where
        F: FnOnce(&mut Scope<'scope>),
    {
        let mut scope = Scope::new(self.clone(), priority);
        (scheduler)(&mut scope);
        let spawned = mem::take(&mut scope.futures)
            .into_iter()
            .map(|f| self.spawn_with_priority(scope.priority, f))
            .collect::<Vec<_>>();
        for task in spawned {
            task.await;
        }
    }

    /// Get the current time.
    ///
    /// Calling this instead of `std::time::Instant::now` allows the use
    /// of fake timers in tests.
    pub fn now(&self) -> Instant {
        self.dispatcher.now()
    }

    /// Returns a task that will complete after the given duration.
    /// Depending on other concurrent tasks the elapsed duration may be longer
    /// than requested.
    pub fn timer(&self, duration: Duration) -> Task<()> {
        if duration.is_zero() {
            return Task::ready(());
        }
        let location = core::panic::Location::caller();
        let (runnable, task) = async_task::Builder::new()
            .metadata(RunnableMeta {
                location,
                priority: Priority::inherit(),
                app: None,
            })
            .spawn(move |_| async move {}, {
                let dispatcher = self.dispatcher.clone();
                move |runnable| {
                    dispatcher.dispatch_after(duration, GpuiRunnable::GpuiSpawned(runnable))
                }
            });
        runnable.schedule();
        Task(TaskState::Spawned(task))
    }

    /// in tests, start_waiting lets you indicate which task is waiting (for debugging only)
    #[cfg(any(test, feature = "test-support"))]
    pub fn start_waiting(&self) {
        self.dispatcher.as_test().unwrap().start_waiting();
    }

    /// in tests, removes the debugging data added by start_waiting
    #[cfg(any(test, feature = "test-support"))]
    pub fn finish_waiting(&self) {
        self.dispatcher.as_test().unwrap().finish_waiting();
    }

    /// in tests, run an arbitrary number of tasks (determined by the SEED environment variable)
    #[cfg(any(test, feature = "test-support"))]
    pub fn simulate_random_delay(&self) -> impl Future<Output = ()> + use<> {
        self.dispatcher.as_test().unwrap().simulate_random_delay()
    }

    /// in tests, indicate that a given task from `spawn_labeled` should run after everything else
    #[cfg(any(test, feature = "test-support"))]
    pub fn deprioritize(&self, task_label: TaskLabel) {
        self.dispatcher.as_test().unwrap().deprioritize(task_label)
    }

    /// in tests, move time forward. This does not run any tasks, but does make `timer`s ready.
    #[cfg(any(test, feature = "test-support"))]
    pub fn advance_clock(&self, duration: Duration) {
        self.dispatcher.as_test().unwrap().advance_clock(duration)
    }

    /// in tests, run one task.
    #[cfg(any(test, feature = "test-support"))]
    pub fn tick(&self) -> bool {
        self.dispatcher.as_test().unwrap().tick(false)
    }

    /// in tests, run all tasks that are ready to run. If after doing so
    /// the test still has outstanding tasks, this will panic. (See also [`Self::allow_parking`])
    #[cfg(any(test, feature = "test-support"))]
    pub fn run_until_parked(&self) {
        self.dispatcher.as_test().unwrap().run_until_parked()
    }

    /// in tests, prevents `run_until_parked` from panicking if there are outstanding tasks.
    /// This is useful when you are integrating other (non-GPUI) futures, like disk access, that
    /// do take real async time to run.
    #[cfg(any(test, feature = "test-support"))]
    pub fn allow_parking(&self) {
        self.dispatcher.as_test().unwrap().allow_parking();
    }

    /// undoes the effect of [`Self::allow_parking`].
    #[cfg(any(test, feature = "test-support"))]
    pub fn forbid_parking(&self) {
        self.dispatcher.as_test().unwrap().forbid_parking();
    }

    /// adds detail to the "parked with nothing let to run" message.
    #[cfg(any(test, feature = "test-support"))]
    pub fn set_waiting_hint(&self, msg: Option<String>) {
        self.dispatcher.as_test().unwrap().set_waiting_hint(msg);
    }

    /// in tests, returns the rng used by the dispatcher and seeded by the `SEED` environment variable
    #[cfg(any(test, feature = "test-support"))]
    pub fn rng(&self) -> StdRng {
        self.dispatcher.as_test().unwrap().rng()
    }

    /// How many CPUs are available to the dispatcher.
    pub fn num_cpus(&self) -> usize {
        #[cfg(any(test, feature = "test-support"))]
        return 4;

        #[cfg(not(any(test, feature = "test-support")))]
        return num_cpus::get();
    }

    /// Whether we're on the main thread.
    pub fn is_main_thread(&self) -> bool {
        self.dispatcher.is_main_thread()
    }

    #[cfg(any(test, feature = "test-support"))]
    /// in tests, control the number of ticks that `block_with_timeout` will run before timing out.
    pub fn set_block_on_ticks(&self, range: std::ops::RangeInclusive<usize>) {
        self.dispatcher.as_test().unwrap().set_block_on_ticks(range);
    }
}

/// ForegroundExecutor runs things on the main thread.
impl ForegroundExecutor {
    /// Creates a new ForegroundExecutor from the given PlatformDispatcher.
    pub fn new(dispatcher: Arc<dyn PlatformDispatcher>, liveness: std::sync::Weak<()>) -> Self {
        Self {
            dispatcher,
            liveness,
            not_send: PhantomData,
        }
    }

    /// Enqueues the given Task to run on the main thread at some point in the
    /// future. This inherits the priority of the caller. Use
    /// [`spawn_with_priority`](Self::spawn_with_priority) if you want to
    /// overwrite that.
    #[track_caller]
    pub fn spawn<R>(&self, future: impl Future<Output = R> + 'static) -> Task<R>
    where
        R: 'static,
    {
        self.inner_spawn(self.liveness.clone(), Priority::default(), future)
    }

    /// Enqueues the given Task to run on the main thread at some point in the future.
    #[track_caller]
    pub fn spawn_with_priority<R>(
        &self,
        priority: Priority,
        future: impl Future<Output = R> + 'static,
    ) -> Task<R>
    where
        R: 'static,
    {
        self.inner_spawn(self.liveness.clone(), priority, future)
    }

    #[track_caller]
    pub(crate) fn inner_spawn<R>(
        &self,
        app: std::sync::Weak<()>,
        priority: Priority,
        future: impl Future<Output = R> + 'static,
    ) -> Task<R>
    where
        R: 'static,
    {
        let dispatcher = self.dispatcher.clone();
        let location = core::panic::Location::caller();

        #[track_caller]
        fn inner<R: 'static>(
            dispatcher: Arc<dyn PlatformDispatcher>,
            future: AnyLocalFuture<R>,
            location: &'static core::panic::Location<'static>,
            app: std::sync::Weak<()>,
            priority: Priority,
        ) -> Task<R> {
            let (runnable, task) = spawn_local_with_source_location(
                future,
                move |runnable| {
                    dispatcher.dispatch_on_main_thread(GpuiRunnable::GpuiSpawned(runnable))
                },
                RunnableMeta {
                    location,
                    priority,
                    app: Some(app),
                },
            );
            runnable.schedule();
            Task(TaskState::Spawned(task))
        }
        inner::<R>(dispatcher, Box::pin(future), location, app, priority)
    }
}

/// Variant of `async_task::spawn_local` that includes the source location of the spawn in panics.
///
/// Copy-modified from:
/// <https://github.com/smol-rs/async-task/blob/ca9dbe1db9c422fd765847fa91306e30a6bb58a9/src/runnable.rs#L405>
#[track_caller]
fn spawn_local_with_source_location<Fut, S, M>(
    future: Fut,
    schedule: S,
    metadata: M,
) -> (Runnable<M>, async_task::Task<Fut::Output, M>)
where
    Fut: Future + 'static,
    Fut::Output: 'static,
    S: async_task::Schedule<M> + Send + Sync + 'static,
    M: 'static,
{
    #[inline]
    fn thread_id() -> ThreadId {
        std::thread_local! {
            static ID: ThreadId = thread::current().id();
        }
        ID.try_with(|id| *id)
            .unwrap_or_else(|_| thread::current().id())
    }

    struct Checked<F> {
        id: ThreadId,
        inner: ManuallyDrop<F>,
        location: &'static Location<'static>,
    }

    impl<F> Drop for Checked<F> {
        fn drop(&mut self) {
            assert!(
                self.id == thread_id(),
                "local task dropped by a thread that didn't spawn it. Task spawned at {}",
                self.location
            );
            unsafe { ManuallyDrop::drop(&mut self.inner) };
        }
    }

    impl<F: Future> Future for Checked<F> {
        type Output = F::Output;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            assert!(
                self.id == thread_id(),
                "local task polled by a thread that didn't spawn it. Task spawned at {}",
                self.location
            );
            unsafe { self.map_unchecked_mut(|c| &mut *c.inner).poll(cx) }
        }
    }

    // Wrap the future into one that checks which thread it's on.
    let future = Checked {
        id: thread_id(),
        inner: ManuallyDrop::new(future),
        location: Location::caller(),
    };

    unsafe {
        async_task::Builder::new()
            .metadata(metadata)
            .spawn_unchecked(move |_| future, schedule)
    }
}

/// Scope manages a set of tasks that are enqueued and waited on together. See [`BackgroundExecutor::scoped`].
pub struct Scope<'a> {
    executor: BackgroundExecutor,
    priority: Priority,
    futures: Vec<Pin<Box<dyn Future<Output = ()> + Send + 'static>>>,
    tx: Option<mpsc::Sender<()>>,
    rx: mpsc::Receiver<()>,
    lifetime: PhantomData<&'a ()>,
}

impl<'a> Scope<'a> {
    fn new(executor: BackgroundExecutor, priority: Priority) -> Self {
        let (tx, rx) = mpsc::channel(1);
        Self {
            executor,
            priority,
            tx: Some(tx),
            rx,
            futures: Default::default(),
            lifetime: PhantomData,
        }
    }

    /// How many CPUs are available to the dispatcher.
    pub fn num_cpus(&self) -> usize {
        self.executor.num_cpus()
    }

    /// Spawn a future into this scope.
    #[track_caller]
    pub fn spawn<F>(&mut self, f: F)
    where
        F: Future<Output = ()> + Send + 'a,
    {
        let tx = self.tx.clone().unwrap();

        // SAFETY: The 'a lifetime is guaranteed to outlive any of these futures because
        // dropping this `Scope` blocks until all of the futures have resolved.
        let f = unsafe {
            mem::transmute::<
                Pin<Box<dyn Future<Output = ()> + Send + 'a>>,
                Pin<Box<dyn Future<Output = ()> + Send + 'static>>,
            >(Box::pin(async move {
                f.await;
                drop(tx);
            }))
        };
        self.futures.push(f);
    }
}

impl Drop for Scope<'_> {
    fn drop(&mut self) {
        self.tx.take().unwrap();

        // Wait until the channel is closed, which means that all of the spawned
        // futures have resolved.
        self.executor.block(self.rx.next());
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use crate::{App, TestDispatcher, TestPlatform};
    use rand::SeedableRng;
    use std::cell::RefCell;

    /// Helper to create test infrastructure.
    /// Returns (dispatcher, background_executor, app) where app's foreground_executor has liveness.
    fn create_test_app() -> (TestDispatcher, BackgroundExecutor, Rc<crate::AppCell>) {
        let dispatcher = TestDispatcher::new(StdRng::seed_from_u64(0));
        let arc_dispatcher = Arc::new(dispatcher.clone());
        // Create liveness for task cancellation
        let liveness = std::sync::Arc::new(());
        let liveness_weak = std::sync::Arc::downgrade(&liveness);
        let background_executor = BackgroundExecutor::new(arc_dispatcher.clone());
        let foreground_executor = ForegroundExecutor::new(arc_dispatcher, liveness_weak);

        let platform = TestPlatform::new(background_executor.clone(), foreground_executor);
        let asset_source = Arc::new(());
        let http_client = http_client::FakeHttpClient::with_404_response();

        let app = App::new_app(platform, liveness, asset_source, http_client);
        (dispatcher, background_executor, app)
    }

    #[test]
    fn sanity_test_tasks_run() {
        let (dispatcher, _background_executor, app) = create_test_app();
        let foreground_executor = app.borrow().foreground_executor.clone();

        let task_ran = Rc::new(RefCell::new(false));

        foreground_executor
            .spawn({
                let task_ran = Rc::clone(&task_ran);
                async move {
                    *task_ran.borrow_mut() = true;
                }
            })
            .detach();

        // Run dispatcher while app is still alive
        dispatcher.run_until_parked();

        // Task should have run
        assert!(
            *task_ran.borrow(),
            "Task should run normally when app is alive"
        );
    }

    #[test]
    fn test_task_cancelled_when_app_dropped() {
        let (dispatcher, _background_executor, app) = create_test_app();
        let foreground_executor = app.borrow().foreground_executor.clone();
        let app_weak = Rc::downgrade(&app);

        let task_ran = Rc::new(RefCell::new(false));
        let task_ran_clone = Rc::clone(&task_ran);

        foreground_executor
            .spawn(async move {
                *task_ran_clone.borrow_mut() = true;
            })
            .detach();

        drop(app);

        assert!(app_weak.upgrade().is_none(), "App should have been dropped");

        dispatcher.run_until_parked();

        // The task should have been cancelled, not run
        assert!(
            !*task_ran.borrow(),
            "Task should have been cancelled when app was dropped, but it ran!"
        );
    }

    #[test]
    fn test_nested_tasks_both_cancel() {
        let (dispatcher, _background_executor, app) = create_test_app();
        let foreground_executor = app.borrow().foreground_executor.clone();
        let app_weak = Rc::downgrade(&app);

        let outer_completed = Rc::new(RefCell::new(false));
        let inner_completed = Rc::new(RefCell::new(false));
        let reached_await = Rc::new(RefCell::new(false));

        let outer_flag = Rc::clone(&outer_completed);
        let inner_flag = Rc::clone(&inner_completed);
        let await_flag = Rc::clone(&reached_await);

        // Channel to block the inner task until we're ready
        let (tx, rx) = futures::channel::oneshot::channel::<()>();

        let inner_executor = foreground_executor.clone();

        foreground_executor
            .spawn(async move {
                let inner_task = inner_executor.spawn({
                    let inner_flag = Rc::clone(&inner_flag);
                    async move {
                        rx.await.ok();
                        *inner_flag.borrow_mut() = true;
                    }
                });

                *await_flag.borrow_mut() = true;

                inner_task.await;

                *outer_flag.borrow_mut() = true;
            })
            .detach();

        // Run dispatcher until outer task reaches the await point
        // The inner task will be blocked on the channel
        dispatcher.run_until_parked();

        // Verify we actually reached the await point before dropping the app
        assert!(
            *reached_await.borrow(),
            "Outer task should have reached the await point"
        );

        // Neither task should have completed yet
        assert!(
            !*outer_completed.borrow(),
            "Outer task should not have completed yet"
        );
        assert!(
            !*inner_completed.borrow(),
            "Inner task should not have completed yet"
        );

        // Drop the channel sender and app while outer is awaiting inner
        drop(tx);
        drop(app);
        assert!(app_weak.upgrade().is_none(), "App should have been dropped");

        // Run dispatcher - both tasks should be cancelled
        dispatcher.run_until_parked();

        // Neither task should have completed (both were cancelled)
        assert!(
            !*outer_completed.borrow(),
            "Outer task should have been cancelled, not completed"
        );
        assert!(
            !*inner_completed.borrow(),
            "Inner task should have been cancelled, not completed"
        );
    }

    #[test]
    #[should_panic]
    fn test_polling_cancelled_task_panics() {
        let (dispatcher, background_executor, app) = create_test_app();
        let foreground_executor = app.borrow().foreground_executor.clone();
        let app_weak = Rc::downgrade(&app);

        let task = foreground_executor.spawn(async move { 42 });

        drop(app);

        assert!(app_weak.upgrade().is_none(), "App should have been dropped");

        dispatcher.run_until_parked();

        background_executor.block(task);
    }

    #[test]
    fn test_polling_cancelled_task_returns_none_with_fallible() {
        let (dispatcher, background_executor, app) = create_test_app();
        let foreground_executor = app.borrow().foreground_executor.clone();
        let app_weak = Rc::downgrade(&app);

        let task = foreground_executor.spawn(async move { 42 }).fallible();

        drop(app);

        assert!(app_weak.upgrade().is_none(), "App should have been dropped");

        dispatcher.run_until_parked();

        let result = background_executor.block(task);
        assert_eq!(result, None, "Cancelled task should return None");
    }
}