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use super::{OwnedList, Schedule, Task, TransferStack};
use std::{
cell::UnsafeCell,
collections::VecDeque,
fmt,
sync::{Mutex, MutexGuard},
};
/// A set of multi-producer, single consumer task queues, suitable for use by a
/// single-threaded scheduler.
///
/// This consists of a list of _all_ tasks bound to the scheduler, a run queue
/// of tasks notified from the thread the scheduler is running on (the "local
/// queue"), a run queue of tasks notified from another thread (the "remote
/// queue"), and a stack of tasks released from other threads which will
/// eventually need to be dropped by the scheduler on its own thread ("pending
/// drop").
///
/// Submitting tasks to or popping tasks from the local queue is unsafe, as it
/// must only be performed on the same thread as the scheduler.
pub(crate) struct MpscQueues<S: 'static> {
/// List of all active tasks spawned onto this executor.
///
/// # Safety
///
/// Must only be accessed from the primary thread
owned_tasks: UnsafeCell<OwnedList<S>>,
/// Local run queue.
///
/// Tasks notified from the current thread are pushed into this queue.
///
/// # Safety
///
/// References should not be handed out. Only call `push` / `pop` functions.
/// Only call from the owning thread.
local_queue: UnsafeCell<VecDeque<Task<S>>>,
/// Remote run queue.
///
/// Tasks notified from another thread are pushed into this queue.
remote_queue: Mutex<RemoteQueue<S>>,
/// Tasks pending drop
pending_drop: TransferStack<S>,
}
pub(crate) struct RemoteQueue<S: 'static> {
/// FIFO list of tasks
queue: VecDeque<Task<S>>,
/// `true` when a task can be pushed into the queue, `false` otherwise.
open: bool,
}
// === impl Queues ===
impl<S> MpscQueues<S>
where
S: Schedule + 'static,
{
pub(crate) const INITIAL_CAPACITY: usize = 64;
/// How often to check the remote queue first
pub(crate) const CHECK_REMOTE_INTERVAL: u8 = 13;
pub(crate) fn new() -> Self {
Self {
owned_tasks: UnsafeCell::new(OwnedList::new()),
local_queue: UnsafeCell::new(VecDeque::with_capacity(Self::INITIAL_CAPACITY)),
pending_drop: TransferStack::new(),
remote_queue: Mutex::new(RemoteQueue {
queue: VecDeque::with_capacity(Self::INITIAL_CAPACITY),
open: true,
}),
}
}
/// Adds a new task to the scheduler.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
pub(crate) unsafe fn add_task(&self, task: &Task<S>) {
(*self.owned_tasks.get()).insert(task);
}
/// Pushes a task to the local queue.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
pub(crate) unsafe fn push_local(&self, task: Task<S>) {
(*self.local_queue.get()).push_back(task);
}
/// Removes a task from the local queue.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
pub(crate) unsafe fn release_local(&self, task: &Task<S>) {
(*self.owned_tasks.get()).remove(task);
}
/// Locks the remote queue, returning a `MutexGuard`.
///
/// This can be used to push to the remote queue and perform other
/// operations while holding the lock.
///
/// # Panics
///
/// If the remote queue mutex is poisoned.
pub(crate) fn remote(&self) -> MutexGuard<'_, RemoteQueue<S>> {
self.remote_queue
.lock()
.expect("failed to lock remote queue")
}
/// Releases a task from outside of the thread that owns the scheduler.
///
/// This simply pushes the task to the pending drop queue.
pub(crate) fn release_remote(&self, task: Task<S>) {
self.pending_drop.push(task);
}
/// Returns the next task from the remote *or* local queue.
///
/// Typically, this checks the local queue before the remote queue, and only
/// checks the remote queue if the local queue is empty. However, to avoid
/// starving the remote queue, it is checked first every
/// `CHECK_REMOTE_INTERVAL` ticks.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
pub(crate) unsafe fn next_task(&self, tick: u8) -> Option<Task<S>> {
if 0 == tick % Self::CHECK_REMOTE_INTERVAL {
self.next_remote_task().or_else(|| self.next_local_task())
} else {
self.next_local_task().or_else(|| self.next_remote_task())
}
}
/// Returns the next task from the local queue.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
pub(crate) unsafe fn next_local_task(&self) -> Option<Task<S>> {
(*self.local_queue.get()).pop_front()
}
/// Returns the next task from the remote queue.
///
/// # Panics
///
/// If the mutex around the remote queue is poisoned _and_ the current
/// thread is not already panicking. This is safe to call in a `Drop` impl.
pub(crate) fn next_remote_task(&self) -> Option<Task<S>> {
// there is no semantic information in the `PoisonError`, and it
// doesn't implement `Debug`, but clippy thinks that it's bad to
// match all errors here...
#[allow(clippy::match_wild_err_arm)]
let mut lock = match self.remote_queue.lock() {
// If the lock is poisoned, but the thread is already panicking,
// avoid a double panic. This is necessary since `next_task` (which
// calls `next_remote_task`) can be called in the `Drop` impl.
Err(_) if std::thread::panicking() => return None,
Err(_) => panic!("mutex poisoned"),
Ok(lock) => lock,
};
lock.queue.pop_front()
}
/// Returns `true` if any owned tasks are still bound to this scheduler.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
pub(crate) unsafe fn has_tasks_remaining(&self) -> bool {
!(*self.owned_tasks.get()).is_empty()
}
/// Drains any tasks that have previously been released from other threads.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
pub(crate) unsafe fn drain_pending_drop(&self) {
for task in self.pending_drop.drain() {
(*self.owned_tasks.get()).remove(&task);
drop(task);
}
}
/// Shuts down the queues.
///
/// This performs the following operations:
///
/// 1. Close the remote queue (so that it will no longer accept new tasks).
/// 2. Drain the remote queue and shut down all tasks.
/// 3. Drain the local queue and shut down all tasks.
/// 4. Shut down the owned task list.
/// 5. Drain the list of tasks dropped externally and remove them from the
/// owned task list.
///
/// This method should be called before dropping a `Queues`. It is provided
/// as a method rather than a `Drop` impl because types that own a `Queues`
/// wish to perform other work in their `Drop` implementations _after_
/// shutting down the task queues.
///
/// # Safety
///
/// This method accesses the local task queue, and therefore *must* be
/// called only from the thread that owns the scheduler.
///
/// # Panics
///
/// If the mutex around the remote queue is poisoned _and_ the current
/// thread is not already panicking. This is safe to call in a `Drop` impl.
pub(crate) unsafe fn shutdown(&self) {
// Close and drain the remote queue.
self.close_remote();
// Drain the local queue.
self.close_local();
// Release owned tasks
self.shutdown_owned_tasks();
// Drain tasks pending drop.
self.drain_pending_drop();
}
/// Drains both the local and remote run queues, shutting down any tasks.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
pub(crate) unsafe fn drain_queues(&self) {
self.close_local();
self.close_remote();
}
/// Shuts down the scheduler's owned task list.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
unsafe fn shutdown_owned_tasks(&self) {
(*self.owned_tasks.get()).shutdown();
}
/// Drains the remote queue, and shut down its tasks.
///
/// This closes the remote queue. Any additional tasks added to it will be
/// shut down instead.
///
/// # Panics
/// If the mutex around the remote queue is poisoned _and_ the current
/// thread is not already panicking. This is safe to call in a `Drop` impl.
fn close_remote(&self) {
loop {
#[allow(clippy::match_wild_err_arm)]
let mut lock = match self.remote_queue.lock() {
// If the lock is poisoned, but the thread is already panicking,
// avoid a double panic. This is necessary since this fn can be
// called in a drop impl.
Err(_) if std::thread::panicking() => return,
Err(_) => panic!("mutex poisoned"),
Ok(lock) => lock,
};
lock.open = false;
if let Some(task) = lock.queue.pop_front() {
// Release lock before dropping task, in case
// task tries to re-schedule in its Drop.
drop(lock);
task.shutdown();
} else {
return;
}
}
}
/// Drains the local queue, and shut down its tasks.
///
/// # Safety
///
/// This *must* be called only from the thread that owns the scheduler.
unsafe fn close_local(&self) {
while let Some(task) = self.next_local_task() {
task.shutdown();
}
}
}
impl<S> fmt::Debug for MpscQueues<S> {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("MpscQueues")
.field("owned_tasks", &self.owned_tasks)
.field("remote_queue", &self.remote_queue)
.field("local_queue", &self.local_queue)
.finish()
}
}
// === impl RemoteQueue ===
impl<S> RemoteQueue<S>
where
S: Schedule,
{
/// Schedule a remote task.
///
/// If the queue is open to accept new tasks, the task is pushed to the back
/// of the queue. Otherwise, if the queue is closed (the scheduler is
/// shutting down), the new task will be shut down immediately.
///
/// `spawn` should be set if the caller is spawning a new task.
pub(crate) fn schedule(&mut self, task: Task<S>, spawn: bool) {
if !spawn || self.open {
self.queue.push_back(task);
} else {
task.shutdown();
}
}
}
impl<S> fmt::Debug for RemoteQueue<S> {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("RemoteQueue")
.field("queue", &self.queue)
.field("open", &self.open)
.finish()
}
}