util: add spawn_pinned (#3370)

tokio-util/Cargo.toml

@@ -30,7 +30,7 @@ codec = []
 time = ["tokio/time","slab"]
 io = []
 io-util = ["io", "tokio/rt", "tokio/io-util"]
-rt = ["tokio/rt"]
+rt = ["tokio/rt", "tokio/sync", "futures-util"]
 
 __docs_rs = ["futures-util"]
 

tokio-util/src/lib.rs

@@ -43,6 +43,7 @@ cfg_io! {
 
 cfg_rt! {
     pub mod context;
+    pub mod task;
 }
 
 cfg_time! {

tokio-util/src/task/mod.rs

@@ -0,0 +1,4 @@
+//! Extra utilities for spawning tasks
+
+mod spawn_pinned;
+pub use spawn_pinned::LocalPoolHandle;

tokio-util/src/task/spawn_pinned.rs

@@ -0,0 +1,307 @@
+use futures_util::future::{AbortHandle, Abortable};
+use std::fmt;
+use std::fmt::{Debug, Formatter};
+use std::future::Future;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Arc;
+use tokio::runtime::Builder;
+use tokio::sync::mpsc::{unbounded_channel, UnboundedReceiver, UnboundedSender};
+use tokio::sync::oneshot;
+use tokio::task::{spawn_local, JoinHandle, LocalSet};
+
+/// A handle to a local pool, used for spawning `!Send` tasks.
+#[derive(Clone)]
+pub struct LocalPoolHandle {
+    pool: Arc<LocalPool>,
+}
+
+impl LocalPoolHandle {
+    /// Create a new pool of threads to handle `!Send` tasks. Spawn tasks onto this
+    /// pool via [`LocalPoolHandle::spawn_pinned`].
+    ///
+    /// # Panics
+    /// Panics if the pool size is less than one.
+    pub fn new(pool_size: usize) -> LocalPoolHandle {
+        assert!(pool_size > 0);
+
+        let workers = (0..pool_size)
+            .map(|_| LocalWorkerHandle::new_worker())
+            .collect();
+
+        let pool = Arc::new(LocalPool { workers });
+
+        LocalPoolHandle { pool }
+    }
+
+    /// Spawn a task onto a worker thread and pin it there so it can't be moved
+    /// off of the thread. Note that the future is not [`Send`], but the
+    /// [`FnOnce`] which creates it is.
+    ///
+    /// # Examples
+    /// ```
+    /// use std::rc::Rc;
+    /// use tokio_util::task::LocalPoolHandle;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     // Create the local pool
+    ///     let pool = LocalPoolHandle::new(1);
+    ///
+    ///     // Spawn a !Send future onto the pool and await it
+    ///     let output = pool
+    ///         .spawn_pinned(|| {
+    ///             // Rc is !Send + !Sync
+    ///             let local_data = Rc::new("test");
+    ///
+    ///             // This future holds an Rc, so it is !Send
+    ///             async move { local_data.to_string() }
+    ///         })
+    ///         .await
+    ///         .unwrap();
+    ///
+    ///     assert_eq!(output, "test");
+    /// }
+    /// ```
+    pub fn spawn_pinned<F, Fut>(&self, create_task: F) -> JoinHandle<Fut::Output>
+    where
+        F: FnOnce() -> Fut,
+        F: Send + 'static,
+        Fut: Future + 'static,
+        Fut::Output: Send + 'static,
+    {
+        self.pool.spawn_pinned(create_task)
+    }
+}
+
+impl Debug for LocalPoolHandle {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        f.write_str("LocalPoolHandle")
+    }
+}
+
+struct LocalPool {
+    workers: Vec<LocalWorkerHandle>,
+}
+
+impl LocalPool {
+    /// Spawn a `?Send` future onto a worker
+    fn spawn_pinned<F, Fut>(&self, create_task: F) -> JoinHandle<Fut::Output>
+    where
+        F: FnOnce() -> Fut,
+        F: Send + 'static,
+        Fut: Future + 'static,
+        Fut::Output: Send + 'static,
+    {
+        let (sender, receiver) = oneshot::channel();
+
+        let (worker, job_guard) = self.find_and_incr_least_burdened_worker();
+        let worker_spawner = worker.spawner.clone();
+
+        // Spawn a future onto the worker's runtime so we can immediately return
+        // a join handle.
+        worker.runtime_handle.spawn(async move {
+            // Move the job guard into the task
+            let _job_guard = job_guard;
+
+            // Propagate aborts via Abortable/AbortHandle
+            let (abort_handle, abort_registration) = AbortHandle::new_pair();
+            let _abort_guard = AbortGuard(abort_handle);
+
+            // Inside the future we can't run spawn_local yet because we're not
+            // in the context of a LocalSet. We need to send create_task to the
+            // LocalSet task for spawning.
+            let spawn_task = Box::new(move || {
+                // Once we're in the LocalSet context we can call spawn_local
+                let join_handle =
+                    spawn_local(
+                        async move { Abortable::new(create_task(), abort_registration).await },
+                    );
+
+                // Send the join handle back to the spawner. If sending fails,
+                // we assume the parent task was canceled, so cancel this task
+                // as well.
+                if let Err(join_handle) = sender.send(join_handle) {
+                    join_handle.abort()
+                }
+            });
+
+            // Send the callback to the LocalSet task
+            if let Err(e) = worker_spawner.send(spawn_task) {
+                // Propagate the error as a panic in the join handle.
+                panic!("Failed to send job to worker: {}", e);
+            }
+
+            // Wait for the task's join handle
+            let join_handle = match receiver.await {
+                Ok(handle) => handle,
+                Err(e) => {
+                    // We sent the task successfully, but failed to get its
+                    // join handle... We assume something happened to the worker
+                    // and the task was not spawned. Propagate the error as a
+                    // panic in the join handle.
+                    panic!("Worker failed to send join handle: {}", e);
+                }
+            };
+
+            // Wait for the task to complete
+            let join_result = join_handle.await;
+
+            match join_result {
+                Ok(Ok(output)) => output,
+                Ok(Err(_)) => {
+                    // Pinned task was aborted. But that only happens if this
+                    // task is aborted. So this is an impossible branch.
+                    unreachable!(
+                        "Reaching this branch means this task was previously \
+                         aborted but it continued running anyways"
+                    )
+                }
+                Err(e) => {
+                    if e.is_panic() {
+                        std::panic::resume_unwind(e.into_panic());
+                    } else if e.is_cancelled() {
+                        // No one else should have the join handle, so this is
+                        // unexpected. Forward this error as a panic in the join
+                        // handle.
+                        panic!("spawn_pinned task was canceled: {}", e);
+                    } else {
+                        // Something unknown happened (not a panic or
+                        // cancellation). Forward this error as a panic in the
+                        // join handle.
+                        panic!("spawn_pinned task failed: {}", e);
+                    }
+                }
+            }
+        })
+    }
+
+    /// Find the worker with the least number of tasks, increment its task
+    /// count, and return its handle. Make sure to actually spawn a task on
+    /// the worker so the task count is kept consistent with load.
+    ///
+    /// A job count guard is also returned to ensure the task count gets
+    /// decremented when the job is done.
+    fn find_and_incr_least_burdened_worker(&self) -> (&LocalWorkerHandle, JobCountGuard) {
+        loop {
+            let (worker, task_count) = self
+                .workers
+                .iter()
+                .map(|worker| (worker, worker.task_count.load(Ordering::SeqCst)))
+                .min_by_key(|&(_, count)| count)
+                .expect("There must be more than one worker");
+
+            // Make sure the task count hasn't changed since when we choose this
+            // worker. Otherwise, restart the search.
+            if worker
+                .task_count
+                .compare_exchange(
+                    task_count,
+                    task_count + 1,
+                    Ordering::SeqCst,
+                    Ordering::Relaxed,
+                )
+                .is_ok()
+            {
+                return (worker, JobCountGuard(Arc::clone(&worker.task_count)));
+            }
+        }
+    }
+}
+
+/// Automatically decrements a worker's job count when a job finishes (when
+/// this gets dropped).
+struct JobCountGuard(Arc<AtomicUsize>);
+
+impl Drop for JobCountGuard {
+    fn drop(&mut self) {
+        // Decrement the job count
+        let previous_value = self.0.fetch_sub(1, Ordering::SeqCst);
+        debug_assert!(previous_value >= 1);
+    }
+}
+
+/// Calls abort on the handle when dropped.
+struct AbortGuard(AbortHandle);
+
+impl Drop for AbortGuard {
+    fn drop(&mut self) {
+        self.0.abort();
+    }
+}
+
+type PinnedFutureSpawner = Box<dyn FnOnce() + Send + 'static>;
+
+struct LocalWorkerHandle {
+    runtime_handle: tokio::runtime::Handle,
+    spawner: UnboundedSender<PinnedFutureSpawner>,
+    task_count: Arc<AtomicUsize>,
+}
+
+impl LocalWorkerHandle {
+    /// Create a new worker for executing pinned tasks
+    fn new_worker() -> LocalWorkerHandle {
+        let (sender, receiver) = unbounded_channel();
+        let runtime = Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .expect("Failed to start a pinned worker thread runtime");
+        let runtime_handle = runtime.handle().clone();
+        let task_count = Arc::new(AtomicUsize::new(0));
+        let task_count_clone = Arc::clone(&task_count);
+
+        std::thread::spawn(|| Self::run(runtime, receiver, task_count_clone));
+
+        LocalWorkerHandle {
+            runtime_handle,
+            spawner: sender,
+            task_count,
+        }
+    }
+
+    fn run(
+        runtime: tokio::runtime::Runtime,
+        mut task_receiver: UnboundedReceiver<PinnedFutureSpawner>,
+        task_count: Arc<AtomicUsize>,
+    ) {
+        let local_set = LocalSet::new();
+        local_set.block_on(&runtime, async {
+            while let Some(spawn_task) = task_receiver.recv().await {
+                // Calls spawn_local(future)
+                (spawn_task)();
+            }
+        });
+
+        // If there are any tasks on the runtime associated with a LocalSet task
+        // that has already completed, but whose output has not yet been
+        // reported, let that task complete.
+        //
+        // Since the task_count is decremented when the runtime task exits,
+        // reading that counter lets us know if any such tasks completed during
+        // the call to `block_on`.
+        //
+        // Tasks on the LocalSet can't complete during this loop since they're
+        // stored on the LocalSet and we aren't accessing it.
+        let mut previous_task_count = task_count.load(Ordering::SeqCst);
+        loop {
+            // This call will also run tasks spawned on the runtime.
+            runtime.block_on(tokio::task::yield_now());
+            let new_task_count = task_count.load(Ordering::SeqCst);
+            if new_task_count == previous_task_count {
+                break;
+            } else {
+                previous_task_count = new_task_count;
+            }
+        }
+
+        // It's now no longer possible for a task on the runtime to be
+        // associated with a LocalSet task that has completed. Drop both the
+        // LocalSet and runtime to let tasks on the runtime be cancelled if and
+        // only if they are still on the LocalSet.
+        //
+        // Drop the LocalSet task first so that anyone awaiting the runtime
+        // JoinHandle will see the cancelled error after the LocalSet task
+        // destructor has completed.
+        drop(local_set);
+        drop(runtime);
+    }
+}