embassy-executor 0.1.1

//! Raw executor.
//!
//! This module exposes "raw" Executor and Task structs for more low level control.
//!
//! ## WARNING: here be dragons!
//!
//! Using this module requires respecting subtle safety contracts. If you can, prefer using the safe
//! [executor wrappers](crate::Executor) and the [`embassy_executor::task`](embassy_macros::task) macro, which are fully safe.

mod run_queue;
#[cfg(feature = "integrated-timers")]
mod timer_queue;
pub(crate) mod util;
mod waker;

use core::cell::Cell;
use core::future::Future;
use core::pin::Pin;
use core::ptr::NonNull;
use core::task::{Context, Poll};
use core::{mem, ptr};

use atomic_polyfill::{AtomicU32, Ordering};
use critical_section::CriticalSection;
#[cfg(feature = "integrated-timers")]
use embassy_time::driver::{self, AlarmHandle};
#[cfg(feature = "integrated-timers")]
use embassy_time::Instant;
#[cfg(feature = "rtos-trace")]
use rtos_trace::trace;

use self::run_queue::{RunQueue, RunQueueItem};
use self::util::UninitCell;
pub use self::waker::task_from_waker;
use super::SpawnToken;

/// Task is spawned (has a future)
pub(crate) const STATE_SPAWNED: u32 = 1 << 0;
/// Task is in the executor run queue
pub(crate) const STATE_RUN_QUEUED: u32 = 1 << 1;
/// Task is in the executor timer queue
#[cfg(feature = "integrated-timers")]
pub(crate) const STATE_TIMER_QUEUED: u32 = 1 << 2;

/// Raw task header for use in task pointers.
///
/// This is an opaque struct, used for raw pointers to tasks, for use
/// with funtions like [`wake_task`] and [`task_from_waker`].
pub struct TaskHeader {
    pub(crate) state: AtomicU32,
    pub(crate) run_queue_item: RunQueueItem,
    pub(crate) executor: Cell<*const Executor>, // Valid if state != 0
    pub(crate) poll_fn: UninitCell<unsafe fn(NonNull<TaskHeader>)>, // Valid if STATE_SPAWNED

    #[cfg(feature = "integrated-timers")]
    pub(crate) expires_at: Cell<Instant>,
    #[cfg(feature = "integrated-timers")]
    pub(crate) timer_queue_item: timer_queue::TimerQueueItem,
}

impl TaskHeader {
    pub(crate) const fn new() -> Self {
        Self {
            state: AtomicU32::new(0),
            run_queue_item: RunQueueItem::new(),
            executor: Cell::new(ptr::null()),
            poll_fn: UninitCell::uninit(),

            #[cfg(feature = "integrated-timers")]
            expires_at: Cell::new(Instant::from_ticks(0)),
            #[cfg(feature = "integrated-timers")]
            timer_queue_item: timer_queue::TimerQueueItem::new(),
        }
    }
}

/// Raw storage in which a task can be spawned.
///
/// This struct holds the necessary memory to spawn one task whose future is `F`.
/// At a given time, the `TaskStorage` may be in spawned or not-spawned state. You
/// may spawn it with [`TaskStorage::spawn()`], which will fail if it is already spawned.
///
/// A `TaskStorage` must live forever, it may not be deallocated even after the task has finished
/// running. Hence the relevant methods require `&'static self`. It may be reused, however.
///
/// Internally, the [embassy_executor::task](embassy_macros::task) macro allocates an array of `TaskStorage`s
/// in a `static`. The most common reason to use the raw `Task` is to have control of where
/// the memory for the task is allocated: on the stack, or on the heap with e.g. `Box::leak`, etc.

// repr(C) is needed to guarantee that the Task is located at offset 0
// This makes it safe to cast between TaskHeader and TaskStorage pointers.
#[repr(C)]
pub struct TaskStorage<F: Future + 'static> {
    raw: TaskHeader,
    future: UninitCell<F>, // Valid if STATE_SPAWNED
}

impl<F: Future + 'static> TaskStorage<F> {
    const NEW: Self = Self::new();

    /// Create a new TaskStorage, in not-spawned state.
    pub const fn new() -> Self {
        Self {
            raw: TaskHeader::new(),
            future: UninitCell::uninit(),
        }
    }

    /// Try to spawn the task.
    ///
    /// The `future` closure constructs the future. It's only called if spawning is
    /// actually possible. It is a closure instead of a simple `future: F` param to ensure
    /// the future is constructed in-place, avoiding a temporary copy in the stack thanks to
    /// NRVO optimizations.
    ///
    /// This function will fail if the task is already spawned and has not finished running.
    /// In this case, the error is delayed: a "poisoned" SpawnToken is returned, which will
    /// cause [`Spawner::spawn()`](super::Spawner::spawn) to return the error.
    ///
    /// Once the task has finished running, you may spawn it again. It is allowed to spawn it
    /// on a different executor.
    pub fn spawn(&'static self, future: impl FnOnce() -> F) -> SpawnToken<impl Sized> {
        if self.spawn_mark_used() {
            return unsafe { SpawnToken::<F>::new(self.spawn_initialize(future)) };
        }

        SpawnToken::<F>::new_failed()
    }

    fn spawn_mark_used(&'static self) -> bool {
        let state = STATE_SPAWNED | STATE_RUN_QUEUED;
        self.raw
            .state
            .compare_exchange(0, state, Ordering::AcqRel, Ordering::Acquire)
            .is_ok()
    }

    unsafe fn spawn_initialize(&'static self, future: impl FnOnce() -> F) -> NonNull<TaskHeader> {
        // Initialize the task
        self.raw.poll_fn.write(Self::poll);
        self.future.write(future());
        NonNull::new_unchecked(self as *const TaskStorage<F> as *const TaskHeader as *mut TaskHeader)
    }

    unsafe fn poll(p: NonNull<TaskHeader>) {
        let this = &*(p.as_ptr() as *const TaskStorage<F>);

        let future = Pin::new_unchecked(this.future.as_mut());
        let waker = waker::from_task(p);
        let mut cx = Context::from_waker(&waker);
        match future.poll(&mut cx) {
            Poll::Ready(_) => {
                this.future.drop_in_place();
                this.raw.state.fetch_and(!STATE_SPAWNED, Ordering::AcqRel);
            }
            Poll::Pending => {}
        }

        // the compiler is emitting a virtual call for waker drop, but we know
        // it's a noop for our waker.
        mem::forget(waker);
    }
}

unsafe impl<F: Future + 'static> Sync for TaskStorage<F> {}

/// Raw storage that can hold up to N tasks of the same type.
///
/// This is essentially a `[TaskStorage<F>; N]`.
pub struct TaskPool<F: Future + 'static, const N: usize> {
    pool: [TaskStorage<F>; N],
}

impl<F: Future + 'static, const N: usize> TaskPool<F, N> {
    /// Create a new TaskPool, with all tasks in non-spawned state.
    pub const fn new() -> Self {
        Self {
            pool: [TaskStorage::NEW; N],
        }
    }

    /// Try to spawn a task in the pool.
    ///
    /// See [`TaskStorage::spawn()`] for details.
    ///
    /// This will loop over the pool and spawn the task in the first storage that
    /// is currently free. If none is free, a "poisoned" SpawnToken is returned,
    /// which will cause [`Spawner::spawn()`](super::Spawner::spawn) to return the error.
    pub fn spawn(&'static self, future: impl FnOnce() -> F) -> SpawnToken<impl Sized> {
        for task in &self.pool {
            if task.spawn_mark_used() {
                return unsafe { SpawnToken::<F>::new(task.spawn_initialize(future)) };
            }
        }

        SpawnToken::<F>::new_failed()
    }

    /// Like spawn(), but allows the task to be send-spawned if the args are Send even if
    /// the future is !Send.
    ///
    /// Not covered by semver guarantees. DO NOT call this directly. Intended to be used
    /// by the Embassy macros ONLY.
    ///
    /// SAFETY: `future` must be a closure of the form `move || my_async_fn(args)`, where `my_async_fn`
    /// is an `async fn`, NOT a hand-written `Future`.
    #[doc(hidden)]
    pub unsafe fn _spawn_async_fn<FutFn>(&'static self, future: FutFn) -> SpawnToken<impl Sized>
    where
        FutFn: FnOnce() -> F,
    {
        // When send-spawning a task, we construct the future in this thread, and effectively
        // "send" it to the executor thread by enqueuing it in its queue. Therefore, in theory,
        // send-spawning should require the future `F` to be `Send`.
        //
        // The problem is this is more restrictive than needed. Once the future is executing,
        // it is never sent to another thread. It is only sent when spawning. It should be
        // enough for the task's arguments to be Send. (and in practice it's super easy to
        // accidentally make your futures !Send, for example by holding an `Rc` or a `&RefCell` across an `.await`.)
        //
        // We can do it by sending the task args and constructing the future in the executor thread
        // on first poll. However, this cannot be done in-place, so it'll waste stack space for a copy
        // of the args.
        //
        // Luckily, an `async fn` future contains just the args when freshly constructed. So, if the
        // args are Send, it's OK to send a !Send future, as long as we do it before first polling it.
        //
        // (Note: this is how the generators are implemented today, it's not officially guaranteed yet,
        // but it's possible it'll be guaranteed in the future. See zulip thread:
        // https://rust-lang.zulipchat.com/#narrow/stream/187312-wg-async/topic/.22only.20before.20poll.22.20Send.20futures )
        //
        // The `FutFn` captures all the args, so if it's Send, the task can be send-spawned.
        // This is why we return `SpawnToken<FutFn>` below.
        //
        // This ONLY holds for `async fn` futures. The other `spawn` methods can be called directly
        // by the user, with arbitrary hand-implemented futures. This is why these return `SpawnToken<F>`.

        for task in &self.pool {
            if task.spawn_mark_used() {
                return SpawnToken::<FutFn>::new(task.spawn_initialize(future));
            }
        }

        SpawnToken::<FutFn>::new_failed()
    }
}

/// Raw executor.
///
/// This is the core of the Embassy executor. It is low-level, requiring manual
/// handling of wakeups and task polling. If you can, prefer using one of the
/// [higher level executors](crate::Executor).
///
/// The raw executor leaves it up to you to handle wakeups and scheduling:
///
/// - To get the executor to do work, call `poll()`. This will poll all queued tasks (all tasks
///   that "want to run").
/// - You must supply a `signal_fn`. The executor will call it to notify you it has work
///   to do. You must arrange for `poll()` to be called as soon as possible.
///
/// `signal_fn` can be called from *any* context: any thread, any interrupt priority
/// level, etc. It may be called synchronously from any `Executor` method call as well.
/// You must deal with this correctly.
///
/// In particular, you must NOT call `poll` directly from `signal_fn`, as this violates
/// the requirement for `poll` to not be called reentrantly.
pub struct Executor {
    run_queue: RunQueue,
    signal_fn: fn(*mut ()),
    signal_ctx: *mut (),

    #[cfg(feature = "integrated-timers")]
    pub(crate) timer_queue: timer_queue::TimerQueue,
    #[cfg(feature = "integrated-timers")]
    alarm: AlarmHandle,
}

impl Executor {
    /// Create a new executor.
    ///
    /// When the executor has work to do, it will call `signal_fn` with
    /// `signal_ctx` as argument.
    ///
    /// See [`Executor`] docs for details on `signal_fn`.
    pub fn new(signal_fn: fn(*mut ()), signal_ctx: *mut ()) -> Self {
        #[cfg(feature = "integrated-timers")]
        let alarm = unsafe { unwrap!(driver::allocate_alarm()) };
        #[cfg(feature = "integrated-timers")]
        driver::set_alarm_callback(alarm, signal_fn, signal_ctx);

        Self {
            run_queue: RunQueue::new(),
            signal_fn,
            signal_ctx,

            #[cfg(feature = "integrated-timers")]
            timer_queue: timer_queue::TimerQueue::new(),
            #[cfg(feature = "integrated-timers")]
            alarm,
        }
    }

    /// Enqueue a task in the task queue
    ///
    /// # Safety
    /// - `task` must be a valid pointer to a spawned task.
    /// - `task` must be set up to run in this executor.
    /// - `task` must NOT be already enqueued (in this executor or another one).
    #[inline(always)]
    unsafe fn enqueue(&self, cs: CriticalSection, task: NonNull<TaskHeader>) {
        #[cfg(feature = "rtos-trace")]
        trace::task_ready_begin(task.as_ptr() as u32);

        if self.run_queue.enqueue(cs, task) {
            (self.signal_fn)(self.signal_ctx)
        }
    }

    /// Spawn a task in this executor.
    ///
    /// # Safety
    ///
    /// `task` must be a valid pointer to an initialized but not-already-spawned task.
    ///
    /// It is OK to use `unsafe` to call this from a thread that's not the executor thread.
    /// In this case, the task's Future must be Send. This is because this is effectively
    /// sending the task to the executor thread.
    pub(super) unsafe fn spawn(&'static self, task: NonNull<TaskHeader>) {
        task.as_ref().executor.set(self);

        #[cfg(feature = "rtos-trace")]
        trace::task_new(task.as_ptr() as u32);

        critical_section::with(|cs| {
            self.enqueue(cs, task);
        })
    }

    /// Poll all queued tasks in this executor.
    ///
    /// This loops over all tasks that are queued to be polled (i.e. they're
    /// freshly spawned or they've been woken). Other tasks are not polled.
    ///
    /// You must call `poll` after receiving a call to `signal_fn`. It is OK
    /// to call `poll` even when not requested by `signal_fn`, but it wastes
    /// energy.
    ///
    /// # Safety
    ///
    /// You must NOT call `poll` reentrantly on the same executor.
    ///
    /// In particular, note that `poll` may call `signal_fn` synchronously. Therefore, you
    /// must NOT directly call `poll()` from your `signal_fn`. Instead, `signal_fn` has to
    /// somehow schedule for `poll()` to be called later, at a time you know for sure there's
    /// no `poll()` already running.
    pub unsafe fn poll(&'static self) {
        loop {
            #[cfg(feature = "integrated-timers")]
            self.timer_queue.dequeue_expired(Instant::now(), |task| wake_task(task));

            self.run_queue.dequeue_all(|p| {
                let task = p.as_ref();

                #[cfg(feature = "integrated-timers")]
                task.expires_at.set(Instant::MAX);

                let state = task.state.fetch_and(!STATE_RUN_QUEUED, Ordering::AcqRel);
                if state & STATE_SPAWNED == 0 {
                    // If task is not running, ignore it. This can happen in the following scenario:
                    //   - Task gets dequeued, poll starts
                    //   - While task is being polled, it gets woken. It gets placed in the queue.
                    //   - Task poll finishes, returning done=true
                    //   - RUNNING bit is cleared, but the task is already in the queue.
                    return;
                }

                #[cfg(feature = "rtos-trace")]
                trace::task_exec_begin(p.as_ptr() as u32);

                // Run the task
                task.poll_fn.read()(p as _);

                #[cfg(feature = "rtos-trace")]
                trace::task_exec_end();

                // Enqueue or update into timer_queue
                #[cfg(feature = "integrated-timers")]
                self.timer_queue.update(p);
            });

            #[cfg(feature = "integrated-timers")]
            {
                // If this is already in the past, set_alarm might return false
                // In that case do another poll loop iteration.
                let next_expiration = self.timer_queue.next_expiration();
                if driver::set_alarm(self.alarm, next_expiration.as_ticks()) {
                    break;
                }
            }

            #[cfg(not(feature = "integrated-timers"))]
            {
                break;
            }
        }

        #[cfg(feature = "rtos-trace")]
        trace::system_idle();
    }

    /// Get a spawner that spawns tasks in this executor.
    ///
    /// It is OK to call this method multiple times to obtain multiple
    /// `Spawner`s. You may also copy `Spawner`s.
    pub fn spawner(&'static self) -> super::Spawner {
        super::Spawner::new(self)
    }
}

/// Wake a task by raw pointer.
///
/// You can obtain task pointers from `Waker`s using [`task_from_waker`].
///
/// # Safety
///
/// `task` must be a valid task pointer obtained from [`task_from_waker`].
pub unsafe fn wake_task(task: NonNull<TaskHeader>) {
    critical_section::with(|cs| {
        let header = task.as_ref();
        let state = header.state.load(Ordering::Relaxed);

        // If already scheduled, or if not started,
        if (state & STATE_RUN_QUEUED != 0) || (state & STATE_SPAWNED == 0) {
            return;
        }

        // Mark it as scheduled
        header.state.store(state | STATE_RUN_QUEUED, Ordering::Relaxed);

        // We have just marked the task as scheduled, so enqueue it.
        let executor = &*header.executor.get();
        executor.enqueue(cs, task);
    })
}

#[cfg(feature = "integrated-timers")]
struct TimerQueue;

#[cfg(feature = "integrated-timers")]
impl embassy_time::queue::TimerQueue for TimerQueue {
    fn schedule_wake(&'static self, at: Instant, waker: &core::task::Waker) {
        let task = waker::task_from_waker(waker);
        let task = unsafe { task.as_ref() };
        let expires_at = task.expires_at.get();
        task.expires_at.set(expires_at.min(at));
    }
}

#[cfg(feature = "integrated-timers")]
embassy_time::timer_queue_impl!(static TIMER_QUEUE: TimerQueue = TimerQueue);

#[cfg(feature = "rtos-trace")]
impl rtos_trace::RtosTraceOSCallbacks for Executor {
    fn task_list() {
        // We don't know what tasks exist, so we can't send them.
    }
    #[cfg(feature = "integrated-timers")]
    fn time() -> u64 {
        Instant::now().as_micros()
    }
    #[cfg(not(feature = "integrated-timers"))]
    fn time() -> u64 {
        0
    }
}

#[cfg(feature = "rtos-trace")]
rtos_trace::global_os_callbacks! {Executor}