bevy/crates/bevy_tasks/src/task_pool.rs

use std::{
    future::Future,
    mem,
    pin::Pin,
    sync::Arc,
    thread::{self, JoinHandle},
};

use futures_lite::{future, pin};

use crate::Task;

/// Used to create a TaskPool
#[derive(Debug, Default, Clone)]
pub struct TaskPoolBuilder {
    /// If set, we'll set up the thread pool to use at most n threads. Otherwise use
    /// the logical core count of the system
    num_threads: Option<usize>,
    /// If set, we'll use the given stack size rather than the system default
    stack_size: Option<usize>,
    /// Allows customizing the name of the threads - helpful for debugging. If set, threads will
    /// be named <thread_name> (<thread_index>), i.e. "MyThreadPool (2)"
    thread_name: Option<String>,
}

impl TaskPoolBuilder {
    /// Creates a new TaskPoolBuilder instance
    pub fn new() -> Self {
        Self::default()
    }

    /// Override the number of threads created for the pool. If unset, we default to the number
    /// of logical cores of the system
    pub fn num_threads(mut self, num_threads: usize) -> Self {
        self.num_threads = Some(num_threads);
        self
    }

    /// Override the stack size of the threads created for the pool
    pub fn stack_size(mut self, stack_size: usize) -> Self {
        self.stack_size = Some(stack_size);
        self
    }

    /// Override the name of the threads created for the pool. If set, threads will
    /// be named <thread_name> (<thread_index>), i.e. "MyThreadPool (2)"
    pub fn thread_name(mut self, thread_name: String) -> Self {
        self.thread_name = Some(thread_name);
        self
    }

    /// Creates a new ThreadPoolBuilder based on the current options.
    pub fn build(self) -> TaskPool {
        TaskPool::new_internal(
            self.num_threads,
            self.stack_size,
            self.thread_name.as_deref(),
        )
    }
}

#[derive(Debug)]
struct TaskPoolInner {
    threads: Vec<JoinHandle<()>>,
    shutdown_tx: async_channel::Sender<()>,
}

impl Drop for TaskPoolInner {
    fn drop(&mut self) {
        self.shutdown_tx.close();

        for join_handle in self.threads.drain(..) {
            join_handle
                .join()
                .expect("task thread panicked while executing");
        }
    }
}

/// A thread pool for executing tasks. Tasks are futures that are being automatically driven by
/// the pool on threads owned by the pool.
#[derive(Debug, Clone)]
pub struct TaskPool {
    /// The executor for the pool
    ///
    /// This has to be separate from TaskPoolInner because we have to create an Arc<Executor> to
    /// pass into the worker threads, and we must create the worker threads before we can create the
    /// Vec<Task<T>> contained within TaskPoolInner
    executor: Arc<async_executor::Executor<'static>>,

    /// Inner state of the pool
    inner: Arc<TaskPoolInner>,
}

impl TaskPool {
    /// Create a `TaskPool` with the default configuration.
    pub fn new() -> Self {
        TaskPoolBuilder::new().build()
    }

    fn new_internal(
        num_threads: Option<usize>,
        stack_size: Option<usize>,
        thread_name: Option<&str>,
    ) -> Self {
        let (shutdown_tx, shutdown_rx) = async_channel::unbounded::<()>();

        let executor = Arc::new(async_executor::Executor::new());

        let num_threads = num_threads.unwrap_or_else(num_cpus::get);

        let threads = (0..num_threads)
            .map(|i| {
                let ex = Arc::clone(&executor);
                let shutdown_rx = shutdown_rx.clone();

                let thread_name = if let Some(thread_name) = thread_name {
                    format!("{} ({})", thread_name, i)
                } else {
                    format!("TaskPool ({})", i)
                };

                let mut thread_builder = thread::Builder::new().name(thread_name);

                if let Some(stack_size) = stack_size {
                    thread_builder = thread_builder.stack_size(stack_size);
                }

                thread_builder
                    .spawn(move || {
                        let shutdown_future = ex.run(shutdown_rx.recv());
                        // Use unwrap_err because we expect a Closed error
                        future::block_on(shutdown_future).unwrap_err();
                    })
                    .expect("failed to spawn thread")
            })
            .collect();

        Self {
            executor,
            inner: Arc::new(TaskPoolInner {
                threads,
                shutdown_tx,
            }),
        }
    }

    /// Return the number of threads owned by the task pool
    pub fn thread_num(&self) -> usize {
        self.inner.threads.len()
    }

    /// Allows spawning non-`static futures on the thread pool. The function takes a callback,
    /// passing a scope object into it. The scope object provided to the callback can be used
    /// to spawn tasks. This function will await the completion of all tasks before returning.
    ///
    /// This is similar to `rayon::scope` and `crossbeam::scope`
    pub fn scope<'scope, F, T>(&self, f: F) -> Vec<T>
    where
        F: FnOnce(&mut Scope<'scope, T>) + 'scope + Send,
        T: Send + 'static,
    {
        // SAFETY: This function blocks until all futures complete, so this future must return
        // before this function returns. However, rust has no way of knowing
        // this so we must convert to 'static here to appease the compiler as it is unable to
        // validate safety.
        let executor: &async_executor::Executor = &*self.executor;
        let executor: &'scope async_executor::Executor = unsafe { mem::transmute(executor) };

        let mut scope = Scope {
            executor,
            spawned: Vec::new(),
        };

        f(&mut scope);

        if scope.spawned.is_empty() {
            Vec::default()
        } else if scope.spawned.len() == 1 {
            vec![future::block_on(&mut scope.spawned[0])]
        } else {
            let fut = async move {
                let mut results = Vec::with_capacity(scope.spawned.len());
                for task in scope.spawned {
                    results.push(task.await);
                }

                results
            };

            // Pin the future on the stack.
            pin!(fut);

            // SAFETY: This function blocks until all futures complete, so we do not read/write the
            // data from futures outside of the 'scope lifetime. However, rust has no way of knowing
            // this so we must convert to 'static here to appease the compiler as it is unable to
            // validate safety.
            let fut: Pin<&mut (dyn Future<Output = Vec<T>> + Send)> = fut;
            let fut: Pin<&'static mut (dyn Future<Output = Vec<T>> + Send + 'static)> =
                unsafe { mem::transmute(fut) };

            // The thread that calls scope() will participate in driving tasks in the pool forward
            // until the tasks that are spawned by this scope() call complete. (If the caller of scope()
            // happens to be a thread in this thread pool, and we only have one thread in the pool, then
            // simply calling future::block_on(spawned) would deadlock.)
            let mut spawned = self.executor.spawn(fut);
            loop {
                if let Some(result) = future::block_on(future::poll_once(&mut spawned)) {
                    break result;
                }

                self.executor.try_tick();
            }
        }
    }

    /// Spawns a static future onto the thread pool. The returned Task is a future. It can also be
    /// cancelled and "detached" allowing it to continue running without having to be polled by the
    /// end-user.
    pub fn spawn<T>(&self, future: impl Future<Output = T> + Send + 'static) -> Task<T>
    where
        T: Send + 'static,
    {
        Task::new(self.executor.spawn(future))
    }
}

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

#[derive(Debug)]
pub struct Scope<'scope, T> {
    executor: &'scope async_executor::Executor<'scope>,
    spawned: Vec<async_executor::Task<T>>,
}

impl<'scope, T: Send + 'scope> Scope<'scope, T> {
    pub fn spawn<Fut: Future<Output = T> + 'scope + Send>(&mut self, f: Fut) {
        let task = self.executor.spawn(f);
        self.spawned.push(task);
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::sync::atomic::{AtomicI32, Ordering};

    #[test]
    pub fn test_spawn() {
        let pool = TaskPool::new();

        let foo = Box::new(42);
        let foo = &*foo;

        let count = Arc::new(AtomicI32::new(0));

        let outputs = pool.scope(|scope| {
            for _ in 0..100 {
                let count_clone = count.clone();
                scope.spawn(async move {
                    if *foo != 42 {
                        panic!("not 42!?!?")
                    } else {
                        count_clone.fetch_add(1, Ordering::Relaxed);
                        *foo
                    }
                });
            }
        });

        for output in &outputs {
            assert_eq!(*output, 42);
        }

        assert_eq!(outputs.len(), 100);
        assert_eq!(count.load(Ordering::Relaxed), 100);
    }
}