src/header.rs

use core::alloc::Layout;
use core::cell::UnsafeCell;
use core::fmt;
use core::sync::atomic::{AtomicUsize, Ordering};
use core::task::Waker;

use crate::raw::TaskVTable;
use crate::state::*;
use crate::utils::{abort_on_panic, extend};

/// The header of a task.
///
/// This header is stored right at the beginning of every heap-allocated task.
pub(crate) struct Header {
    /// Current state of the task.
    ///
    /// Contains flags representing the current state and the reference count.
    pub(crate) state: AtomicUsize,

    /// The task that is blocked on the `JoinHandle`.
    ///
    /// This waker needs to be woken up once the task completes or is closed.
    pub(crate) awaiter: UnsafeCell<Option<Waker>>,

    /// The virtual table.
    ///
    /// In addition to the actual waker virtual table, it also contains pointers to several other
    /// methods necessary for bookkeeping the heap-allocated task.
    pub(crate) vtable: &'static TaskVTable,
}

impl Header {
    /// Cancels the task.
    ///
    /// This method will mark the task as closed, but it won't reschedule the task or drop its
    /// future.
    pub(crate) fn cancel(&self) {
        let mut state = self.state.load(Ordering::Acquire);

        loop {
            // If the task has been completed or closed, it can't be canceled.
            if state & (COMPLETED | CLOSED) != 0 {
                break;
            }

            // Mark the task as closed.
            match self.state.compare_exchange_weak(
                state,
                state | CLOSED,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => break,
                Err(s) => state = s,
            }
        }
    }

    /// Notifies the awaiter blocked on this task.
    ///
    /// If the awaiter is the same as the current waker, it will not be notified.
    #[inline]
    pub(crate) fn notify(&self, current: Option<&Waker>) {
        // Mark the awaiter as being notified.
        let state = self.state.fetch_or(NOTIFYING, Ordering::AcqRel);

        // If the awaiter was not being notified nor registered...
        if state & (NOTIFYING | REGISTERING) == 0 {
            // Take the waker out.
            let waker = unsafe { (*self.awaiter.get()).take() };

            // Mark the state as not being notified anymore nor containing an awaiter.
            self.state
                .fetch_and(!NOTIFYING & !AWAITER, Ordering::Release);

            if let Some(w) = waker {
                // We need a safeguard against panics because waking can panic.
                abort_on_panic(|| match current {
                    None => w.wake(),
                    Some(c) if !w.will_wake(c) => w.wake(),
                    Some(_) => {}
                });
            }
        }
    }

    /// Registers a new awaiter blocked on this task.
    ///
    /// This method is called when `JoinHandle` is polled and the task has not completed.
    #[inline]
    pub(crate) fn register(&self, waker: &Waker) {
        // Load the state and synchronize with it.
        let mut state = self.state.fetch_or(0, Ordering::Acquire);

        loop {
            // There can't be two concurrent registrations because `JoinHandle` can only be polled
            // by a unique pinned reference.
            debug_assert!(state & REGISTERING == 0);

            // If we're in the notifying state at this moment, just wake and return without
            // registering.
            if state & NOTIFYING != 0 {
                abort_on_panic(|| waker.wake_by_ref());
                return;
            }

            // Mark the state to let other threads know we're registering a new awaiter.
            match self.state.compare_exchange_weak(
                state,
                state | REGISTERING,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => {
                    state |= REGISTERING;
                    break;
                }
                Err(s) => state = s,
            }
        }

        // Put the waker into the awaiter field.
        unsafe {
            abort_on_panic(|| (*self.awaiter.get()) = Some(waker.clone()));
        }

        // This variable will contain the newly registered waker if a notification comes in before
        // we complete registration.
        let mut waker = None;

        loop {
            // If there was a notification, take the waker out of the awaiter field.
            if state & NOTIFYING != 0 {
                if let Some(w) = unsafe { (*self.awaiter.get()).take() } {
                    abort_on_panic(|| waker = Some(w));
                }
            }

            // The new state is not being notified nor registered, but there might or might not be
            // an awaiter depending on whether there was a concurrent notification.
            let new = if waker.is_none() {
                (state & !NOTIFYING & !REGISTERING) | AWAITER
            } else {
                state & !NOTIFYING & !REGISTERING & !AWAITER
            };

            match self
                .state
                .compare_exchange_weak(state, new, Ordering::AcqRel, Ordering::Acquire)
            {
                Ok(_) => break,
                Err(s) => state = s,
            }
        }

        // If there was a notification during registration, wake the awaiter now.
        if let Some(w) = waker {
            abort_on_panic(|| w.wake());
        }
    }

    /// Returns the offset at which the tag of type `T` is stored.
    #[inline]
    pub(crate) fn offset_tag<T>() -> usize {
        let layout_header = Layout::new::<Header>();
        let layout_t = Layout::new::<T>();
        let (_, offset_t) = extend(layout_header, layout_t);
        offset_t
    }
}

impl fmt::Debug for Header {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let state = self.state.load(Ordering::SeqCst);

        f.debug_struct("Header")
            .field("scheduled", &(state & SCHEDULED != 0))
            .field("running", &(state & RUNNING != 0))
            .field("completed", &(state & COMPLETED != 0))
            .field("closed", &(state & CLOSED != 0))
            .field("awaiter", &(state & AWAITER != 0))
            .field("handle", &(state & HANDLE != 0))
            .field("ref_count", &(state / REFERENCE))
            .finish()
    }
}