Import async-executor 1.5.0upstream/1.5.0upstream

12 files changed, 1887 insertions, 0 deletions

diff --git a/.cargo_vcs_info.json b/.cargo_vcs_info.json
new file mode 100644
index 0000000..46757e0
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\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..0105c9e
--- /dev/null
+++ b/CHANGELOG.md
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+# Version 1.5.0
+
+- Remove the dependency on the `once_cell` crate to restore the MSRV. (#29)
+- Update `concurrent-queue` to v2.
+
+# Version 1.4.1
+
+- Remove dependency on deprecated `vec-arena`. (#23)
+
+# Version 1.4.0
+
+- Add `Executor::is_empty()` and `LocalExecutor::is_empty()`.
+
+# Version 1.3.0
+
+- Parametrize executors over a lifetime to allow spawning non-`static` futures.
+
+# Version 1.2.0
+
+- Update `async-task` to v4.
+
+# Version 1.1.1
+
+- Replace `AtomicU64` with `AtomicUsize`.
+
+# Version 1.1.0
+
+- Use atomics to make `Executor::run()` and `Executor::tick()` futures `Send + Sync`.
+
+# Version 1.0.0
+
+- Stabilize.
+
+# Version 0.2.1
+
+- Add `try_tick()` and `tick()` methods.
+
+# Version 0.2.0
+
+- Redesign the whole API.
+
+# Version 0.1.2
+
+- Add the `Spawner` API.
+
+# Version 0.1.1
+
+- Initial version
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..4c46d67
--- /dev/null
+++ b/Cargo.lock
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new file mode 100644
index 0000000..2628cdf
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,66 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2018"
+rust-version = "1.47"
+name = "async-executor"
+version = "1.5.0"
+authors = ["Stjepan Glavina <stjepang@gmail.com>"]
+exclude = ["/.*"]
+description = "Async executor"
+readme = "README.md"
+keywords = [
+    "asynchronous",
+    "executor",
+    "single",
+    "multi",
+    "spawn",
+]
+categories = [
+    "asynchronous",
+    "concurrency",
+]
+license = "Apache-2.0 OR MIT"
+repository = "https://github.com/smol-rs/async-executor"
+
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+
+[dependencies.async-task]
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+
+[dependencies.concurrent-queue]
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+
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new file mode 100644
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--- /dev/null
+++ b/Cargo.toml.orig
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+[package]
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+# When publishing a new version:
+# - Update CHANGELOG.md
+# - Create "v1.x.y" git tag
+version = "1.5.0"
+authors = ["Stjepan Glavina <stjepang@gmail.com>"]
+edition = "2018"
+rust-version = "1.47"
+description = "Async executor"
+license = "Apache-2.0 OR MIT"
+repository = "https://github.com/smol-rs/async-executor"
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+
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+
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+easy-parallel = "3.1.0"
+num_cpus = "1.13.0"
+once_cell = "1.16.0"
diff --git a/LICENSE-APACHE b/LICENSE-APACHE
new file mode 100644
index 0000000..16fe87b
--- /dev/null
+++ b/LICENSE-APACHE
@@ -0,0 +1,201 @@
+                              Apache License
+                        Version 2.0, January 2004
+                     http://www.apache.org/licenses/
+
+TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+1. Definitions.
+
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+   Contributor provides its Contributions) on an "AS IS" BASIS,
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+
+8. Limitation of Liability. In no event and under no legal theory,
+   whether in tort (including negligence), contract, or otherwise,
+   unless required by applicable law (such as deliberate and grossly
+   negligent acts) or agreed to in writing, shall any Contributor be
+   liable to You for damages, including any direct, indirect, special,
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+   Work (including but not limited to damages for loss of goodwill,
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+   has been advised of the possibility of such damages.
+
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+   the Work or Derivative Works thereof, You may choose to offer,
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+   or other liability obligations and/or rights consistent with this
+   License. However, in accepting such obligations, You may act only
+   on Your own behalf and on Your sole responsibility, not on behalf
+   of any other Contributor, and only if You agree to indemnify,
+   defend, and hold each Contributor harmless for any liability
+   incurred by, or claims asserted against, such Contributor by reason
+   of your accepting any such warranty or additional liability.
+
+END OF TERMS AND CONDITIONS
+
+APPENDIX: How to apply the Apache License to your work.
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+   boilerplate notice, with the fields enclosed by brackets "[]"
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+Copyright [yyyy] [name of copyright owner]
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+	http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
diff --git a/LICENSE-MIT b/LICENSE-MIT
new file mode 100644
index 0000000..31aa793
--- /dev/null
+++ b/LICENSE-MIT
@@ -0,0 +1,23 @@
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..c330bd6
--- /dev/null
+++ b/README.md
@@ -0,0 +1,45 @@
+# async-executor
+
+[![Build](https://github.com/smol-rs/async-executor/workflows/Build%20and%20test/badge.svg)](
+https://github.com/smol-rs/async-executor/actions)
+[![License](https://img.shields.io/badge/license-Apache--2.0_OR_MIT-blue.svg)](
+https://github.com/smol-rs/async-executor)
+[![Cargo](https://img.shields.io/crates/v/async-executor.svg)](
+https://crates.io/crates/async-executor)
+[![Documentation](https://docs.rs/async-executor/badge.svg)](
+https://docs.rs/async-executor)
+
+Async executors.
+
+## Examples
+
+```rust
+use async_executor::Executor;
+use futures_lite::future;
+
+// Create a new executor.
+let ex = Executor::new();
+
+// Spawn a task.
+let task = ex.spawn(async {
+    println!("Hello world");
+});
+
+// Run the executor until the task completes.
+future::block_on(ex.run(task));
+```
+
+## License
+
+Licensed under either of
+
+ * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+#### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
diff --git a/benches/executor.rs b/benches/executor.rs
new file mode 100644
index 0000000..98f1cb5
--- /dev/null
+++ b/benches/executor.rs
@@ -0,0 +1,109 @@
+#![feature(test)]
+
+extern crate test;
+
+use std::future::Future;
+
+use async_executor::Executor;
+use futures_lite::{future, prelude::*};
+
+const TASKS: usize = 300;
+const STEPS: usize = 300;
+const LIGHT_TASKS: usize = 25_000;
+
+static EX: Executor<'_> = Executor::new();
+
+fn run(f: impl FnOnce()) {
+    let (s, r) = async_channel::bounded::<()>(1);
+    easy_parallel::Parallel::new()
+        .each(0..num_cpus::get(), |_| future::block_on(EX.run(r.recv())))
+        .finish(move || {
+            let _s = s;
+            f()
+        });
+}
+
+#[bench]
+fn create(b: &mut test::Bencher) {
+    b.iter(move || {
+        let ex = Executor::new();
+        let task = ex.spawn(async {});
+        future::block_on(ex.run(task));
+    });
+}
+
+#[bench]
+fn spawn_one(b: &mut test::Bencher) {
+    run(|| {
+        b.iter(move || {
+            future::block_on(async { EX.spawn(async {}).await });
+        });
+    });
+}
+
+#[bench]
+fn spawn_many(b: &mut test::Bencher) {
+    run(|| {
+        b.iter(move || {
+            future::block_on(async {
+                let mut tasks = Vec::new();
+                for _ in 0..LIGHT_TASKS {
+                    tasks.push(EX.spawn(async {}));
+                }
+                for task in tasks {
+                    task.await;
+                }
+            });
+        });
+    });
+}
+
+#[bench]
+fn spawn_recursively(b: &mut test::Bencher) {
+    fn go(i: usize) -> impl Future<Output = ()> + Send + 'static {
+        async move {
+            if i != 0 {
+                EX.spawn(async move {
+                    let fut = go(i - 1).boxed();
+                    fut.await;
+                })
+                .await;
+            }
+        }
+    }
+
+    run(|| {
+        b.iter(move || {
+            future::block_on(async {
+                let mut tasks = Vec::new();
+                for _ in 0..TASKS {
+                    tasks.push(EX.spawn(go(STEPS)));
+                }
+                for task in tasks {
+                    task.await;
+                }
+            });
+        });
+    });
+}
+
+#[bench]
+fn yield_now(b: &mut test::Bencher) {
+    run(|| {
+        b.iter(move || {
+            future::block_on(async {
+                let mut tasks = Vec::new();
+                for _ in 0..TASKS {
+                    tasks.push(EX.spawn(async move {
+                        for _ in 0..STEPS {
+                            future::yield_now().await;
+                        }
+                    }));
+                }
+                for task in tasks {
+                    task.await;
+                }
+            });
+        });
+    });
+}
diff --git a/examples/priority.rs b/examples/priority.rs
new file mode 100644
index 0000000..df77dd1
--- /dev/null
+++ b/examples/priority.rs
@@ -0,0 +1,85 @@
+//! An executor with task priorities.
+
+use std::future::Future;
+use std::thread;
+
+use async_executor::{Executor, Task};
+use futures_lite::{future, prelude::*};
+
+/// Task priority.
+#[repr(usize)]
+#[derive(Debug, Clone, Copy)]
+enum Priority {
+    High = 0,
+    Medium = 1,
+    Low = 2,
+}
+
+/// An executor with task priorities.
+///
+/// Tasks with lower priorities only get polled when there are no tasks with higher priorities.
+struct PriorityExecutor<'a> {
+    ex: [Executor<'a>; 3],
+}
+
+impl<'a> PriorityExecutor<'a> {
+    /// Creates a new executor.
+    const fn new() -> PriorityExecutor<'a> {
+        PriorityExecutor {
+            ex: [Executor::new(), Executor::new(), Executor::new()],
+        }
+    }
+
+    /// Spawns a task with the given priority.
+    fn spawn<T: Send + 'a>(
+        &self,
+        priority: Priority,
+        future: impl Future<Output = T> + Send + 'a,
+    ) -> Task<T> {
+        self.ex[priority as usize].spawn(future)
+    }
+
+    /// Runs the executor forever.
+    async fn run(&self) {
+        loop {
+            for _ in 0..200 {
+                let t0 = self.ex[0].tick();
+                let t1 = self.ex[1].tick();
+                let t2 = self.ex[2].tick();
+
+                // Wait until one of the ticks completes, trying them in order from highest
+                // priority to lowest priority.
+                t0.or(t1).or(t2).await;
+            }
+
+            // Yield every now and then.
+            future::yield_now().await;
+        }
+    }
+}
+
+fn main() {
+    static EX: PriorityExecutor<'_> = PriorityExecutor::new();
+
+    // Spawn a thread running the executor forever.
+    thread::spawn(|| future::block_on(EX.run()));
+
+    let mut tasks = Vec::new();
+
+    for _ in 0..20 {
+        // Choose a random priority.
+        let choice = [Priority::High, Priority::Medium, Priority::Low];
+        let priority = choice[fastrand::usize(..choice.len())];
+
+        // Spawn a task with this priority.
+        tasks.push(EX.spawn(priority, async move {
+            println!("{:?}", priority);
+            future::yield_now().await;
+            println!("{:?}", priority);
+        }));
+    }
+
+    for task in tasks {
+        future::block_on(task);
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..24fbfac
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,843 @@
+//! Async executors.
+//!
+//! # Examples
+//!
+//! ```
+//! use async_executor::Executor;
+//! use futures_lite::future;
+//!
+//! // Create a new executor.
+//! let ex = Executor::new();
+//!
+//! // Spawn a task.
+//! let task = ex.spawn(async {
+//!     println!("Hello world");
+//! });
+//!
+//! // Run the executor until the task completes.
+//! future::block_on(ex.run(task));
+//! ```
+
+#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
+
+use std::future::Future;
+use std::marker::PhantomData;
+use std::panic::{RefUnwindSafe, UnwindSafe};
+use std::rc::Rc;
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use std::sync::{Arc, Mutex, RwLock};
+use std::task::{Poll, Waker};
+
+use async_lock::OnceCell;
+use async_task::Runnable;
+use concurrent_queue::ConcurrentQueue;
+use futures_lite::{future, prelude::*};
+use slab::Slab;
+
+#[doc(no_inline)]
+pub use async_task::Task;
+
+/// An async executor.
+///
+/// # Examples
+///
+/// A multi-threaded executor:
+///
+/// ```
+/// use async_channel::unbounded;
+/// use async_executor::Executor;
+/// use easy_parallel::Parallel;
+/// use futures_lite::future;
+///
+/// let ex = Executor::new();
+/// let (signal, shutdown) = unbounded::<()>();
+///
+/// Parallel::new()
+///     // Run four executor threads.
+///     .each(0..4, |_| future::block_on(ex.run(shutdown.recv())))
+///     // Run the main future on the current thread.
+///     .finish(|| future::block_on(async {
+///         println!("Hello world!");
+///         drop(signal);
+///     }));
+/// ```
+#[derive(Debug)]
+pub struct Executor<'a> {
+    /// The executor state.
+    state: OnceCell<Arc<State>>,
+
+    /// Makes the `'a` lifetime invariant.
+    _marker: PhantomData<std::cell::UnsafeCell<&'a ()>>,
+}
+
+unsafe impl Send for Executor<'_> {}
+unsafe impl Sync for Executor<'_> {}
+
+impl UnwindSafe for Executor<'_> {}
+impl RefUnwindSafe for Executor<'_> {}
+
+impl<'a> Executor<'a> {
+    /// Creates a new executor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::Executor;
+    ///
+    /// let ex = Executor::new();
+    /// ```
+    pub const fn new() -> Executor<'a> {
+        Executor {
+            state: OnceCell::new(),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns `true` if there are no unfinished tasks.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::Executor;
+    ///
+    /// let ex = Executor::new();
+    /// assert!(ex.is_empty());
+    ///
+    /// let task = ex.spawn(async {
+    ///     println!("Hello world");
+    /// });
+    /// assert!(!ex.is_empty());
+    ///
+    /// assert!(ex.try_tick());
+    /// assert!(ex.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        self.state().active.lock().unwrap().is_empty()
+    }
+
+    /// Spawns a task onto the executor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::Executor;
+    ///
+    /// let ex = Executor::new();
+    ///
+    /// let task = ex.spawn(async {
+    ///     println!("Hello world");
+    /// });
+    /// ```
+    pub fn spawn<T: Send + 'a>(&self, future: impl Future<Output = T> + Send + 'a) -> Task<T> {
+        let mut active = self.state().active.lock().unwrap();
+
+        // Remove the task from the set of active tasks when the future finishes.
+        let index = active.vacant_entry().key();
+        let state = self.state().clone();
+        let future = async move {
+            let _guard = CallOnDrop(move || drop(state.active.lock().unwrap().try_remove(index)));
+            future.await
+        };
+
+        // Create the task and register it in the set of active tasks.
+        let (runnable, task) = unsafe { async_task::spawn_unchecked(future, self.schedule()) };
+        active.insert(runnable.waker());
+
+        runnable.schedule();
+        task
+    }
+
+    /// Attempts to run a task if at least one is scheduled.
+    ///
+    /// Running a scheduled task means simply polling its future once.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::Executor;
+    ///
+    /// let ex = Executor::new();
+    /// assert!(!ex.try_tick()); // no tasks to run
+    ///
+    /// let task = ex.spawn(async {
+    ///     println!("Hello world");
+    /// });
+    /// assert!(ex.try_tick()); // a task was found
+    /// ```
+    pub fn try_tick(&self) -> bool {
+        match self.state().queue.pop() {
+            Err(_) => false,
+            Ok(runnable) => {
+                // Notify another ticker now to pick up where this ticker left off, just in case
+                // running the task takes a long time.
+                self.state().notify();
+
+                // Run the task.
+                runnable.run();
+                true
+            }
+        }
+    }
+
+    /// Runs a single task.
+    ///
+    /// Running a task means simply polling its future once.
+    ///
+    /// If no tasks are scheduled when this method is called, it will wait until one is scheduled.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::Executor;
+    /// use futures_lite::future;
+    ///
+    /// let ex = Executor::new();
+    ///
+    /// let task = ex.spawn(async {
+    ///     println!("Hello world");
+    /// });
+    /// future::block_on(ex.tick()); // runs the task
+    /// ```
+    pub async fn tick(&self) {
+        let state = self.state();
+        let runnable = Ticker::new(state).runnable().await;
+        runnable.run();
+    }
+
+    /// Runs the executor until the given future completes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::Executor;
+    /// use futures_lite::future;
+    ///
+    /// let ex = Executor::new();
+    ///
+    /// let task = ex.spawn(async { 1 + 2 });
+    /// let res = future::block_on(ex.run(async { task.await * 2 }));
+    ///
+    /// assert_eq!(res, 6);
+    /// ```
+    pub async fn run<T>(&self, future: impl Future<Output = T>) -> T {
+        let runner = Runner::new(self.state());
+
+        // A future that runs tasks forever.
+        let run_forever = async {
+            loop {
+                for _ in 0..200 {
+                    let runnable = runner.runnable().await;
+                    runnable.run();
+                }
+                future::yield_now().await;
+            }
+        };
+
+        // Run `future` and `run_forever` concurrently until `future` completes.
+        future.or(run_forever).await
+    }
+
+    /// Returns a function that schedules a runnable task when it gets woken up.
+    fn schedule(&self) -> impl Fn(Runnable) + Send + Sync + 'static {
+        let state = self.state().clone();
+
+        // TODO(stjepang): If possible, push into the current local queue and notify the ticker.
+        move |runnable| {
+            state.queue.push(runnable).unwrap();
+            state.notify();
+        }
+    }
+
+    /// Returns a reference to the inner state.
+    fn state(&self) -> &Arc<State> {
+        self.state.get_or_init_blocking(|| Arc::new(State::new()))
+    }
+}
+
+impl Drop for Executor<'_> {
+    fn drop(&mut self) {
+        if let Some(state) = self.state.get() {
+            let mut active = state.active.lock().unwrap();
+            for w in active.drain() {
+                w.wake();
+            }
+            drop(active);
+
+            while state.queue.pop().is_ok() {}
+        }
+    }
+}
+
+impl<'a> Default for Executor<'a> {
+    fn default() -> Executor<'a> {
+        Executor::new()
+    }
+}
+
+/// A thread-local executor.
+///
+/// The executor can only be run on the thread that created it.
+///
+/// # Examples
+///
+/// ```
+/// use async_executor::LocalExecutor;
+/// use futures_lite::future;
+///
+/// let local_ex = LocalExecutor::new();
+///
+/// future::block_on(local_ex.run(async {
+///     println!("Hello world!");
+/// }));
+/// ```
+#[derive(Debug)]
+pub struct LocalExecutor<'a> {
+    /// The inner executor.
+    inner: Executor<'a>,
+
+    /// Makes the type `!Send` and `!Sync`.
+    _marker: PhantomData<Rc<()>>,
+}
+
+impl UnwindSafe for LocalExecutor<'_> {}
+impl RefUnwindSafe for LocalExecutor<'_> {}
+
+impl<'a> LocalExecutor<'a> {
+    /// Creates a single-threaded executor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::LocalExecutor;
+    ///
+    /// let local_ex = LocalExecutor::new();
+    /// ```
+    pub const fn new() -> LocalExecutor<'a> {
+        LocalExecutor {
+            inner: Executor::new(),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns `true` if there are no unfinished tasks.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::LocalExecutor;
+    ///
+    /// let local_ex = LocalExecutor::new();
+    /// assert!(local_ex.is_empty());
+    ///
+    /// let task = local_ex.spawn(async {
+    ///     println!("Hello world");
+    /// });
+    /// assert!(!local_ex.is_empty());
+    ///
+    /// assert!(local_ex.try_tick());
+    /// assert!(local_ex.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        self.inner().is_empty()
+    }
+
+    /// Spawns a task onto the executor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::LocalExecutor;
+    ///
+    /// let local_ex = LocalExecutor::new();
+    ///
+    /// let task = local_ex.spawn(async {
+    ///     println!("Hello world");
+    /// });
+    /// ```
+    pub fn spawn<T: 'a>(&self, future: impl Future<Output = T> + 'a) -> Task<T> {
+        let mut active = self.inner().state().active.lock().unwrap();
+
+        // Remove the task from the set of active tasks when the future finishes.
+        let index = active.vacant_entry().key();
+        let state = self.inner().state().clone();
+        let future = async move {
+            let _guard = CallOnDrop(move || drop(state.active.lock().unwrap().try_remove(index)));
+            future.await
+        };
+
+        // Create the task and register it in the set of active tasks.
+        let (runnable, task) = unsafe { async_task::spawn_unchecked(future, self.schedule()) };
+        active.insert(runnable.waker());
+
+        runnable.schedule();
+        task
+    }
+
+    /// Attempts to run a task if at least one is scheduled.
+    ///
+    /// Running a scheduled task means simply polling its future once.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::LocalExecutor;
+    ///
+    /// let ex = LocalExecutor::new();
+    /// assert!(!ex.try_tick()); // no tasks to run
+    ///
+    /// let task = ex.spawn(async {
+    ///     println!("Hello world");
+    /// });
+    /// assert!(ex.try_tick()); // a task was found
+    /// ```
+    pub fn try_tick(&self) -> bool {
+        self.inner().try_tick()
+    }
+
+    /// Runs a single task.
+    ///
+    /// Running a task means simply polling its future once.
+    ///
+    /// If no tasks are scheduled when this method is called, it will wait until one is scheduled.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::LocalExecutor;
+    /// use futures_lite::future;
+    ///
+    /// let ex = LocalExecutor::new();
+    ///
+    /// let task = ex.spawn(async {
+    ///     println!("Hello world");
+    /// });
+    /// future::block_on(ex.tick()); // runs the task
+    /// ```
+    pub async fn tick(&self) {
+        self.inner().tick().await
+    }
+
+    /// Runs the executor until the given future completes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_executor::LocalExecutor;
+    /// use futures_lite::future;
+    ///
+    /// let local_ex = LocalExecutor::new();
+    ///
+    /// let task = local_ex.spawn(async { 1 + 2 });
+    /// let res = future::block_on(local_ex.run(async { task.await * 2 }));
+    ///
+    /// assert_eq!(res, 6);
+    /// ```
+    pub async fn run<T>(&self, future: impl Future<Output = T>) -> T {
+        self.inner().run(future).await
+    }
+
+    /// Returns a function that schedules a runnable task when it gets woken up.
+    fn schedule(&self) -> impl Fn(Runnable) + Send + Sync + 'static {
+        let state = self.inner().state().clone();
+
+        move |runnable| {
+            state.queue.push(runnable).unwrap();
+            state.notify();
+        }
+    }
+
+    /// Returns a reference to the inner executor.
+    fn inner(&self) -> &Executor<'a> {
+        &self.inner
+    }
+}
+
+impl<'a> Default for LocalExecutor<'a> {
+    fn default() -> LocalExecutor<'a> {
+        LocalExecutor::new()
+    }
+}
+
+/// The state of a executor.
+#[derive(Debug)]
+struct State {
+    /// The global queue.
+    queue: ConcurrentQueue<Runnable>,
+
+    /// Local queues created by runners.
+    local_queues: RwLock<Vec<Arc<ConcurrentQueue<Runnable>>>>,
+
+    /// Set to `true` when a sleeping ticker is notified or no tickers are sleeping.
+    notified: AtomicBool,
+
+    /// A list of sleeping tickers.
+    sleepers: Mutex<Sleepers>,
+
+    /// Currently active tasks.
+    active: Mutex<Slab<Waker>>,
+}
+
+impl State {
+    /// Creates state for a new executor.
+    fn new() -> State {
+        State {
+            queue: ConcurrentQueue::unbounded(),
+            local_queues: RwLock::new(Vec::new()),
+            notified: AtomicBool::new(true),
+            sleepers: Mutex::new(Sleepers {
+                count: 0,
+                wakers: Vec::new(),
+                free_ids: Vec::new(),
+            }),
+            active: Mutex::new(Slab::new()),
+        }
+    }
+
+    /// Notifies a sleeping ticker.
+    #[inline]
+    fn notify(&self) {
+        if self
+            .notified
+            .compare_exchange(false, true, Ordering::SeqCst, Ordering::SeqCst)
+            .is_ok()
+        {
+            let waker = self.sleepers.lock().unwrap().notify();
+            if let Some(w) = waker {
+                w.wake();
+            }
+        }
+    }
+}
+
+/// A list of sleeping tickers.
+#[derive(Debug)]
+struct Sleepers {
+    /// Number of sleeping tickers (both notified and unnotified).
+    count: usize,
+
+    /// IDs and wakers of sleeping unnotified tickers.
+    ///
+    /// A sleeping ticker is notified when its waker is missing from this list.
+    wakers: Vec<(usize, Waker)>,
+
+    /// Reclaimed IDs.
+    free_ids: Vec<usize>,
+}
+
+impl Sleepers {
+    /// Inserts a new sleeping ticker.
+    fn insert(&mut self, waker: &Waker) -> usize {
+        let id = match self.free_ids.pop() {
+            Some(id) => id,
+            None => self.count + 1,
+        };
+        self.count += 1;
+        self.wakers.push((id, waker.clone()));
+        id
+    }
+
+    /// Re-inserts a sleeping ticker's waker if it was notified.
+    ///
+    /// Returns `true` if the ticker was notified.
+    fn update(&mut self, id: usize, waker: &Waker) -> bool {
+        for item in &mut self.wakers {
+            if item.0 == id {
+                if !item.1.will_wake(waker) {
+                    item.1 = waker.clone();
+                }
+                return false;
+            }
+        }
+
+        self.wakers.push((id, waker.clone()));
+        true
+    }
+
+    /// Removes a previously inserted sleeping ticker.
+    ///
+    /// Returns `true` if the ticker was notified.
+    fn remove(&mut self, id: usize) -> bool {
+        self.count -= 1;
+        self.free_ids.push(id);
+
+        for i in (0..self.wakers.len()).rev() {
+            if self.wakers[i].0 == id {
+                self.wakers.remove(i);
+                return false;
+            }
+        }
+        true
+    }
+
+    /// Returns `true` if a sleeping ticker is notified or no tickers are sleeping.
+    fn is_notified(&self) -> bool {
+        self.count == 0 || self.count > self.wakers.len()
+    }
+
+    /// Returns notification waker for a sleeping ticker.
+    ///
+    /// If a ticker was notified already or there are no tickers, `None` will be returned.
+    fn notify(&mut self) -> Option<Waker> {
+        if self.wakers.len() == self.count {
+            self.wakers.pop().map(|item| item.1)
+        } else {
+            None
+        }
+    }
+}
+
+/// Runs task one by one.
+#[derive(Debug)]
+struct Ticker<'a> {
+    /// The executor state.
+    state: &'a State,
+
+    /// Set to a non-zero sleeper ID when in sleeping state.
+    ///
+    /// States a ticker can be in:
+    /// 1) Woken.
+    /// 2a) Sleeping and unnotified.
+    /// 2b) Sleeping and notified.
+    sleeping: AtomicUsize,
+}
+
+impl Ticker<'_> {
+    /// Creates a ticker.
+    fn new(state: &State) -> Ticker<'_> {
+        Ticker {
+            state,
+            sleeping: AtomicUsize::new(0),
+        }
+    }
+
+    /// Moves the ticker into sleeping and unnotified state.
+    ///
+    /// Returns `false` if the ticker was already sleeping and unnotified.
+    fn sleep(&self, waker: &Waker) -> bool {
+        let mut sleepers = self.state.sleepers.lock().unwrap();
+
+        match self.sleeping.load(Ordering::SeqCst) {
+            // Move to sleeping state.
+            0 => self
+                .sleeping
+                .store(sleepers.insert(waker), Ordering::SeqCst),
+
+            // Already sleeping, check if notified.
+            id => {
+                if !sleepers.update(id, waker) {
+                    return false;
+                }
+            }
+        }
+
+        self.state
+            .notified
+            .swap(sleepers.is_notified(), Ordering::SeqCst);
+
+        true
+    }
+
+    /// Moves the ticker into woken state.
+    fn wake(&self) {
+        let id = self.sleeping.swap(0, Ordering::SeqCst);
+        if id != 0 {
+            let mut sleepers = self.state.sleepers.lock().unwrap();
+            sleepers.remove(id);
+
+            self.state
+                .notified
+                .swap(sleepers.is_notified(), Ordering::SeqCst);
+        }
+    }
+
+    /// Waits for the next runnable task to run.
+    async fn runnable(&self) -> Runnable {
+        self.runnable_with(|| self.state.queue.pop().ok()).await
+    }
+
+    /// Waits for the next runnable task to run, given a function that searches for a task.
+    async fn runnable_with(&self, mut search: impl FnMut() -> Option<Runnable>) -> Runnable {
+        future::poll_fn(|cx| {
+            loop {
+                match search() {
+                    None => {
+                        // Move to sleeping and unnotified state.
+                        if !self.sleep(cx.waker()) {
+                            // If already sleeping and unnotified, return.
+                            return Poll::Pending;
+                        }
+                    }
+                    Some(r) => {
+                        // Wake up.
+                        self.wake();
+
+                        // Notify another ticker now to pick up where this ticker left off, just in
+                        // case running the task takes a long time.
+                        self.state.notify();
+
+                        return Poll::Ready(r);
+                    }
+                }
+            }
+        })
+        .await
+    }
+}
+
+impl Drop for Ticker<'_> {
+    fn drop(&mut self) {
+        // If this ticker is in sleeping state, it must be removed from the sleepers list.
+        let id = self.sleeping.swap(0, Ordering::SeqCst);
+        if id != 0 {
+            let mut sleepers = self.state.sleepers.lock().unwrap();
+            let notified = sleepers.remove(id);
+
+            self.state
+                .notified
+                .swap(sleepers.is_notified(), Ordering::SeqCst);
+
+            // If this ticker was notified, then notify another ticker.
+            if notified {
+                drop(sleepers);
+                self.state.notify();
+            }
+        }
+    }
+}
+
+/// A worker in a work-stealing executor.
+///
+/// This is just a ticker that also has an associated local queue for improved cache locality.
+#[derive(Debug)]
+struct Runner<'a> {
+    /// The executor state.
+    state: &'a State,
+
+    /// Inner ticker.
+    ticker: Ticker<'a>,
+
+    /// The local queue.
+    local: Arc<ConcurrentQueue<Runnable>>,
+
+    /// Bumped every time a runnable task is found.
+    ticks: AtomicUsize,
+}
+
+impl Runner<'_> {
+    /// Creates a runner and registers it in the executor state.
+    fn new(state: &State) -> Runner<'_> {
+        let runner = Runner {
+            state,
+            ticker: Ticker::new(state),
+            local: Arc::new(ConcurrentQueue::bounded(512)),
+            ticks: AtomicUsize::new(0),
+        };
+        state
+            .local_queues
+            .write()
+            .unwrap()
+            .push(runner.local.clone());
+        runner
+    }
+
+    /// Waits for the next runnable task to run.
+    async fn runnable(&self) -> Runnable {
+        let runnable = self
+            .ticker
+            .runnable_with(|| {
+                // Try the local queue.
+                if let Ok(r) = self.local.pop() {
+                    return Some(r);
+                }
+
+                // Try stealing from the global queue.
+                if let Ok(r) = self.state.queue.pop() {
+                    steal(&self.state.queue, &self.local);
+                    return Some(r);
+                }
+
+                // Try stealing from other runners.
+                let local_queues = self.state.local_queues.read().unwrap();
+
+                // Pick a random starting point in the iterator list and rotate the list.
+                let n = local_queues.len();
+                let start = fastrand::usize(..n);
+                let iter = local_queues
+                    .iter()
+                    .chain(local_queues.iter())
+                    .skip(start)
+                    .take(n);
+
+                // Remove this runner's local queue.
+                let iter = iter.filter(|local| !Arc::ptr_eq(local, &self.local));
+
+                // Try stealing from each local queue in the list.
+                for local in iter {
+                    steal(local, &self.local);
+                    if let Ok(r) = self.local.pop() {
+                        return Some(r);
+                    }
+                }
+
+                None
+            })
+            .await;
+
+        // Bump the tick counter.
+        let ticks = self.ticks.fetch_add(1, Ordering::SeqCst);
+
+        if ticks % 64 == 0 {
+            // Steal tasks from the global queue to ensure fair task scheduling.
+            steal(&self.state.queue, &self.local);
+        }
+
+        runnable
+    }
+}
+
+impl Drop for Runner<'_> {
+    fn drop(&mut self) {
+        // Remove the local queue.
+        self.state
+            .local_queues
+            .write()
+            .unwrap()
+            .retain(|local| !Arc::ptr_eq(local, &self.local));
+
+        // Re-schedule remaining tasks in the local queue.
+        while let Ok(r) = self.local.pop() {
+            r.schedule();
+        }
+    }
+}
+
+/// Steals some items from one queue into another.
+fn steal<T>(src: &ConcurrentQueue<T>, dest: &ConcurrentQueue<T>) {
+    // Half of `src`'s length rounded up.
+    let mut count = (src.len() + 1) / 2;
+
+    if count > 0 {
+        // Don't steal more than fits into the queue.
+        if let Some(cap) = dest.capacity() {
+            count = count.min(cap - dest.len());
+        }
+
+        // Steal tasks.
+        for _ in 0..count {
+            if let Ok(t) = src.pop() {
+                assert!(dest.push(t).is_ok());
+            } else {
+                break;
+            }
+        }
+    }
+}
+
+/// Runs a closure when dropped.
+struct CallOnDrop<F: Fn()>(F);
+
+impl<F: Fn()> Drop for CallOnDrop<F> {
+    fn drop(&mut self) {
+        (self.0)();
+    }
+}
diff --git a/tests/drop.rs b/tests/drop.rs
new file mode 100644
index 0000000..dd20add
--- /dev/null
+++ b/tests/drop.rs
@@ -0,0 +1,128 @@
+use std::mem;
+use std::panic::catch_unwind;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Mutex;
+use std::task::{Poll, Waker};
+
+use async_executor::{Executor, Task};
+use futures_lite::future;
+use once_cell::sync::Lazy;
+
+#[test]
+fn executor_cancels_everything() {
+    static DROP: AtomicUsize = AtomicUsize::new(0);
+    static WAKER: Lazy<Mutex<Option<Waker>>> = Lazy::new(Default::default);
+
+    let ex = Executor::new();
+
+    let task = ex.spawn(async {
+        let _guard = CallOnDrop(|| {
+            DROP.fetch_add(1, Ordering::SeqCst);
+        });
+
+        future::poll_fn(|cx| {
+            *WAKER.lock().unwrap() = Some(cx.waker().clone());
+            Poll::Pending::<()>
+        })
+        .await;
+    });
+
+    future::block_on(ex.tick());
+    assert!(WAKER.lock().unwrap().is_some());
+    assert_eq!(DROP.load(Ordering::SeqCst), 0);
+
+    drop(ex);
+    assert_eq!(DROP.load(Ordering::SeqCst), 1);
+
+    assert!(catch_unwind(|| future::block_on(task)).is_err());
+    assert_eq!(DROP.load(Ordering::SeqCst), 1);
+}
+
+#[test]
+fn leaked_executor_leaks_everything() {
+    static DROP: AtomicUsize = AtomicUsize::new(0);
+    static WAKER: Lazy<Mutex<Option<Waker>>> = Lazy::new(Default::default);
+
+    let ex = Executor::new();
+
+    let task = ex.spawn(async {
+        let _guard = CallOnDrop(|| {
+            DROP.fetch_add(1, Ordering::SeqCst);
+        });
+
+        future::poll_fn(|cx| {
+            *WAKER.lock().unwrap() = Some(cx.waker().clone());
+            Poll::Pending::<()>
+        })
+        .await;
+    });
+
+    future::block_on(ex.tick());
+    assert!(WAKER.lock().unwrap().is_some());
+    assert_eq!(DROP.load(Ordering::SeqCst), 0);
+
+    mem::forget(ex);
+    assert_eq!(DROP.load(Ordering::SeqCst), 0);
+
+    assert!(future::block_on(future::poll_once(task)).is_none());
+    assert_eq!(DROP.load(Ordering::SeqCst), 0);
+}
+
+#[test]
+fn await_task_after_dropping_executor() {
+    let s: String = "hello".into();
+
+    let ex = Executor::new();
+    let task: Task<&str> = ex.spawn(async { &*s });
+    assert!(ex.try_tick());
+
+    drop(ex);
+    assert_eq!(future::block_on(task), "hello");
+    drop(s);
+}
+
+#[test]
+fn drop_executor_and_then_drop_finished_task() {
+    static DROP: AtomicUsize = AtomicUsize::new(0);
+
+    let ex = Executor::new();
+    let task = ex.spawn(async {
+        CallOnDrop(|| {
+            DROP.fetch_add(1, Ordering::SeqCst);
+        })
+    });
+    assert!(ex.try_tick());
+
+    assert_eq!(DROP.load(Ordering::SeqCst), 0);
+    drop(ex);
+    assert_eq!(DROP.load(Ordering::SeqCst), 0);
+    drop(task);
+    assert_eq!(DROP.load(Ordering::SeqCst), 1);
+}
+
+#[test]
+fn drop_finished_task_and_then_drop_executor() {
+    static DROP: AtomicUsize = AtomicUsize::new(0);
+
+    let ex = Executor::new();
+    let task = ex.spawn(async {
+        CallOnDrop(|| {
+            DROP.fetch_add(1, Ordering::SeqCst);
+        })
+    });
+    assert!(ex.try_tick());
+
+    assert_eq!(DROP.load(Ordering::SeqCst), 0);
+    drop(task);
+    assert_eq!(DROP.load(Ordering::SeqCst), 1);
+    drop(ex);
+    assert_eq!(DROP.load(Ordering::SeqCst), 1);
+}
+
+struct CallOnDrop<F: Fn()>(F);
+
+impl<F: Fn()> Drop for CallOnDrop<F> {
+    fn drop(&mut self) {
+        (self.0)();
+    }
+}