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//! Implements a global task scheduler as a wrapper over Tokio.

//!

//! A few guarantees are made:

//! * The server will not shut down until all tasks complete.

//! * All scheduled tasks will complete at some point.

use crate::Game;

use fecs::World;

use once_cell::sync::OnceCell;

use parking_lot::Mutex;

use std::future::Future;

use std::mem::{transmute, MaybeUninit};

use std::sync::atomic::{AtomicUsize, Ordering};

use std::sync::Arc;

use tokio::runtime;

use tokio::sync::{oneshot, Notify};

use tokio::task::JoinHandle;

/// TODO: reduce allocation

enum SyncFn {

Owned(Box<dyn FnOnce(&mut Game, &mut World) + Send + 'static>),

Scoped(*mut (dyn FnMut(&mut Game, &mut World))),

}

unsafe impl Send for SyncFn {}

/// Fake wrapper which causes a value to become `Send` and `Sync`.

struct UnsafeSendSync<T>(T);

unsafe impl<T> Send for UnsafeSendSync<T> {}

unsafe impl<T> Sync for UnsafeSendSync<T> {}

/// Global task manager.

static TASK_MANAGER: OnceCell<TaskManager> = OnceCell::new();

/// Returns a reference to the global task manager.

pub fn tasks() -> &'static TaskManager {

TASK_MANAGER

.get()

.expect("task manager not initialized (call task::init() first)")

}

/// Intializes the global task manager.

pub fn init(runtime: runtime::Handle) {

TASK_MANAGER

.set(TaskManager::new(runtime))

.ok()

.expect("task manager already initialized");

}

pub struct TaskManager {

/// Number of currently running tasks.

running: Arc<AtomicUsize>,

/// Notify handle, woken every time an _async_ task stops.

notify: Arc<Notify>,

/// Handle to the Tokio runtime.

runtime: runtime::Handle,

/// Queue of tasks to run on the ticking thread.

sync_tx: flume::Sender<SyncFn>,

sync_rx: Mutex<flume::Receiver<SyncFn>>,

}

impl TaskManager {

fn new(runtime: runtime::Handle) -> Self {

let (sync_tx, sync_rx) = flume::bounded(16);

let sync_rx = Mutex::new(sync_rx);

Self {

running: Arc::new(AtomicUsize::new(0)),

notify: Arc::new(Notify::new()),

runtime,

sync_tx,

sync_rx,

}

/// Spawns an asynchronous task. The task is guaranteed

/// to finish before the server shuts down.

pub fn spawn<F>(&self, f: F) -> JoinHandle<F::Output>

where

F: Future + Send + 'static,

F::Output: Send + 'static,

{

self.running.fetch_add(1, Ordering::AcqRel);

let notify = Arc::clone(&self.notify);

let running = Arc::clone(&self.running);

self.runtime.spawn(async move {

let ret = f.await;

running.fetch_sub(1, Ordering::AcqRel);

notify.notify();

ret

})

}

/// Spawns a synchronous task with access to game state. The task

/// will run on the next tick cycle.

pub fn sync<F, R>(&self, f: F) -> oneshot::Receiver<R>

where

F: FnOnce(&mut Game, &mut World) -> R + Send + 'static,

R: Send + 'static,

{

let (tx, rx) = oneshot::channel();

self.sync_tx

.send(SyncFn::Owned(Box::new(move |game, world| {

let _ = tx.send(f(game, world));

})))

.ok()

.unwrap();

}

/// Runs a "scoped" synchronous task with access to game state.

///

/// The task will run on the next tick cycle. Afterward, the

/// returned future will complete with the return value.

#[allow(clippy::let_and_return)] // weird rustc bug causes compile error

pub async fn scoped<'a, F, R>(&self, mut f: F) -> R

where

F: FnMut(&mut Game, &mut World) -> R + Send + 'a,

R: Send + 'a,

{

// EXTREMELY UNSAFE IMPLEMENTATION.

// Please audit.

let notify = Notify::new();

let notify_ptr = UnsafeSendSync(&notify as *const Notify);

let mut return_value = MaybeUninit::<R>::uninit();

let return_value_ptr = UnsafeSendSync(return_value.as_mut_ptr());

// Dummy closure used to write the return value into `return_value`.

let mut dummy = move |game: &mut Game, world: &mut World| {

let ret = f(game, world);

unsafe {

return_value_ptr.0.write(ret);

(&*notify_ptr.0).notify();

}

};

// Erase the lifetime of `dummy`.

// This is legal because we ensure it

// is not dropped until it completes, and any

// references remain valid because this stack

// frame remains intact.

let dummy = UnsafeSendSync((&mut dummy) as *mut (dyn FnMut(&mut Game, &mut World) + 'a));

let dummy = unsafe {

transmute::<

UnsafeSendSync<*mut (dyn FnMut(&mut Game, &mut World) + 'a)>,

UnsafeSendSync<*mut (dyn FnMut(&mut Game, &mut World) + 'static)>,

>(dummy)

};

// Submit the dummy function to the queue.

self.sync_tx.send(SyncFn::Scoped(dummy.0)).ok().unwrap();

// Wait for the task to complete.

// This ensures that all the pointers above remain

// valid until `dummy` is called.

notify.notified().await;

// Return value was written by `dummy`.

unsafe { return_value.assume_init() }

}

/// Executes all queued sync tasks.

pub fn flush_sync(&self, game: &mut Game, world: &mut World) {

let sync_rx = self.sync_rx.lock();

while let Ok(task) = sync_rx.try_recv() {

match task {

SyncFn::Owned(f) => f(game, world),

SyncFn::Scoped(f) => {

let f = unsafe { &mut *f };

f(game, world);

}

/// Waits until all tasks have completed.

pub async fn wait(&self) {

loop {

// TODO: less naive approach?

if self.running.load(Ordering::Acquire) == 0 {

return;

}

self.notify.notified().await;

}

/// System to run sync-queued tasks.

#[fecs::system]

pub fn run_sync_tasks(game: &mut Game, world: &mut World) {

tasks().flush_sync(game, world);

}

#[cfg(test)]

mod tests {

use super::*;

#[tokio::test]

async fn task_spawning() {

init(runtime::Handle::current());

assert_eq!(tasks().spawn(async { 1 }).await.ok(), Some(1));

}

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