cros_async/src/sys/unix/executor.rs - platform/external/crosvm - Git at Google

 // Copyright 2020 The ChromiumOS Authors
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 use std::future::Future;
 use std::pin::Pin;
 use std::sync::Arc;

 use base::debug;
 use base::warn;
 use base::AsRawDescriptors;
 use base::RawDescriptor;
 use once_cell::sync::OnceCell;
 use serde::Deserialize;
 use serde::Serialize;
 use thiserror::Error as ThisError;

 use super::fd_executor::EpollReactor;
 use super::uring_executor::check_uring_availability;
 use super::uring_executor::is_uring_stable;
 use super::uring_executor::Error as UringError;
 use super::uring_executor::UringReactor;
 use crate::common_executor;
 use crate::common_executor::RawExecutor;
 use crate::AsyncResult;
 use crate::IntoAsync;
 use crate::IoSource;

 /// An executor for scheduling tasks that poll futures to completion.
 ///
 /// All asynchronous operations must run within an executor, which is capable of spawning futures as
 /// tasks. This executor also provides a mechanism for performing asynchronous I/O operations.
 ///
 /// The returned type is a cheap, clonable handle to the underlying executor. Cloning it will only
 /// create a new reference, not a new executor.
 ///
 /// Note that language limitations (trait objects can have <=1 non auto trait) require this to be
 /// represented on the POSIX side as an enum, rather than a trait. This leads to some code &
 /// interface duplication, but as far as we understand that is unavoidable.
 ///
 /// See <https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2571401/2..6/cros_async/src/executor.rs#b75>
 /// for further details.
 ///
 /// # Examples
 ///
 /// Concurrently wait for multiple files to become readable/writable and then read/write the data.
 ///
 /// ```
 /// use std::cmp::min;
 /// use std::error::Error;
 /// use std::fs::{File, OpenOptions};
 ///
 /// use cros_async::{AsyncResult, Executor, IoSource, complete3};
 /// const CHUNK_SIZE: usize = 32;
 ///
 /// // Write all bytes from `data` to `f`.
 /// async fn write_file(f: &IoSource<File>, mut data: Vec<u8>) -> AsyncResult<()> {
 ///     while data.len() > 0 {
 ///         let (count, mut buf) = f.write_from_vec(None, data).await?;
 ///
 ///         data = buf.split_off(count);
 ///     }
 ///
 ///     Ok(())
 /// }
 ///
 /// // Transfer `len` bytes of data from `from` to `to`.
 /// async fn transfer_data(
 ///     from: IoSource<File>,
 ///     to: IoSource<File>,
 ///     len: usize,
 /// ) -> AsyncResult<usize> {
 ///     let mut rem = len;
 ///
 ///     while rem > 0 {
 ///         let buf = vec![0u8; min(rem, CHUNK_SIZE)];
 ///         let (count, mut data) = from.read_to_vec(None, buf).await?;
 ///
 ///         if count == 0 {
 ///             // End of file. Return the number of bytes transferred.
 ///             return Ok(len - rem);
 ///         }
 ///
 ///         data.truncate(count);
 ///         write_file(&to, data).await?;
 ///
 ///         rem = rem.saturating_sub(count);
 ///     }
 ///
 ///     Ok(len)
 /// }
 ///
 /// #[cfg(any(target_os = "android", target_os = "linux"))]
 /// # fn do_it() -> Result<(), Box<dyn Error>> {
 ///     let ex = Executor::new()?;
 ///
 ///     let (rx, tx) = base::unix::pipe(true)?;
 ///     let zero = File::open("/dev/zero")?;
 ///     let zero_bytes = CHUNK_SIZE * 7;
 ///     let zero_to_pipe = transfer_data(
 ///         ex.async_from(zero)?,
 ///         ex.async_from(tx.try_clone()?)?,
 ///         zero_bytes,
 ///     );
 ///
 ///     let rand = File::open("/dev/urandom")?;
 ///     let rand_bytes = CHUNK_SIZE * 19;
 ///     let rand_to_pipe = transfer_data(ex.async_from(rand)?, ex.async_from(tx)?, rand_bytes);
 ///
 ///     let null = OpenOptions::new().write(true).open("/dev/null")?;
 ///     let null_bytes = zero_bytes + rand_bytes;
 ///     let pipe_to_null = transfer_data(ex.async_from(rx)?, ex.async_from(null)?, null_bytes);
 ///
 ///     ex.run_until(complete3(
 ///         async { assert_eq!(pipe_to_null.await.unwrap(), null_bytes) },
 ///         async { assert_eq!(zero_to_pipe.await.unwrap(), zero_bytes) },
 ///         async { assert_eq!(rand_to_pipe.await.unwrap(), rand_bytes) },
 ///     ))?;
 ///
 /// #     Ok(())
 /// # }
 /// #[cfg(any(target_os = "android", target_os = "linux"))]
 /// # do_it().unwrap();
 /// ```

 #[derive(Clone)]
 pub enum Executor {
     Uring(Arc<RawExecutor<UringReactor>>),
     Fd(Arc<RawExecutor<EpollReactor>>),
 }

 /// An enum to express the kind of the backend of `Executor`
 #[derive(
     Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize, serde_keyvalue::FromKeyValues,
 )]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 pub enum ExecutorKind {
     Uring,
     // For command-line parsing, user-friendly "epoll" is chosen instead of fd.
     #[serde(rename = "epoll")]
     Fd,
 }

 /// If set, [`ExecutorKind::default()`] returns the value of `DEFAULT_EXECUTOR_KIND`.
 /// If not set, [`ExecutorKind::default()`] returns a statically-chosen default value, and
 /// [`ExecutorKind::default()`] initializes `DEFAULT_EXECUTOR_KIND` with that value.
 static DEFAULT_EXECUTOR_KIND: OnceCell<ExecutorKind> = OnceCell::new();

 impl Default for ExecutorKind {
     fn default() -> Self {
         *DEFAULT_EXECUTOR_KIND.get_or_init(|| ExecutorKind::Fd)
     }
 }

 /// The error type for [`Executor::set_default_executor_kind()`].
 #[derive(Debug, ThisError)]
 pub enum SetDefaultExecutorKindError {
     /// The default executor kind is set more than once.
     #[error("The default executor kind is already set to {0:?}")]
     SetMoreThanOnce(ExecutorKind),

     /// io_uring is unavailable. The reason might be the lack of the kernel support,
     /// but is not limited to that.
     #[error("io_uring is unavailable: {0}")]
     UringUnavailable(UringError),
 }

 /// Reference to a task managed by the executor.
 ///
 /// Dropping a `TaskHandle` attempts to cancel the associated task. Call `detach` to allow it to
 /// continue running the background.
 ///
 /// `await`ing the `TaskHandle` waits for the task to finish and yields its result.
 pub enum TaskHandle<R> {
     Uring(common_executor::TaskHandle<UringReactor, R>),
     Fd(common_executor::TaskHandle<EpollReactor, R>),
 }

 impl<R: Send + 'static> TaskHandle<R> {
     pub fn detach(self) {
         match self {
             TaskHandle::Uring(x) => x.detach(),
             TaskHandle::Fd(x) => x.detach(),
         }
     }

     // Cancel the task and wait for it to stop. Returns the result of the task if it was already
     // finished.
     pub async fn cancel(self) -> Option<R> {
         match self {
             TaskHandle::Uring(x) => x.cancel().await,
             TaskHandle::Fd(x) => x.cancel().await,
         }
     }
 }

 impl<R: 'static> Future for TaskHandle<R> {
     type Output = R;

     fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context) -> std::task::Poll<Self::Output> {
         match self.get_mut() {
             TaskHandle::Uring(x) => Pin::new(x).poll(cx),
             TaskHandle::Fd(x) => Pin::new(x).poll(cx),
         }
     }
 }

 impl Executor {
     /// Create a new `Executor`.
     pub fn new() -> AsyncResult<Self> {
         Executor::with_executor_kind(ExecutorKind::default())
     }

     /// Create a new `Executor` of the given `ExecutorKind`.
     pub fn with_executor_kind(kind: ExecutorKind) -> AsyncResult<Self> {
         match kind {
             ExecutorKind::Uring => RawExecutor::new().map(Executor::Uring),
             ExecutorKind::Fd => RawExecutor::new().map(Executor::Fd),
         }
     }

     /// Set the default ExecutorKind for [`Self::new()`]. This call is effective only once.
     /// If a call is the first call, it sets the default, and `set_default_executor_kind`
     /// returns `Ok(())`. Otherwise, it returns `SetDefaultExecutorKindError::SetMoreThanOnce`
     /// which contains the existing ExecutorKind value configured by the first call.
     pub fn set_default_executor_kind(
         executor_kind: ExecutorKind,
     ) -> Result<(), SetDefaultExecutorKindError> {
         if executor_kind == ExecutorKind::Uring {
             check_uring_availability().map_err(SetDefaultExecutorKindError::UringUnavailable)?;
             if !is_uring_stable() {
                 warn!(
                     "Enabling io_uring executor on the kernel version where io_uring is unstable"
                 );
             }
         }

         debug!("setting the default executor to {:?}", executor_kind);
         DEFAULT_EXECUTOR_KIND.set(executor_kind).map_err(|_|
             // `expect` succeeds since this closure runs only when DEFAULT_EXECUTOR_KIND is set.
             SetDefaultExecutorKindError::SetMoreThanOnce(
                 *DEFAULT_EXECUTOR_KIND
                     .get()
                     .expect("Failed to get DEFAULT_EXECUTOR_KIND"),
             ))
     }

     /// Create a new `IoSource<F>` associated with `self`. Callers may then use the returned
     /// `IoSource` to directly start async operations without needing a separate reference to the
     /// executor.
     pub fn async_from<'a, F: IntoAsync + 'a>(&self, f: F) -> AsyncResult<IoSource<F>> {
         match self {
             Executor::Uring(ex) => ex.new_source(f),
             Executor::Fd(ex) => ex.new_source(f),
         }
     }

     /// Spawn a new future for this executor to run to completion. Callers may use the returned
     /// `TaskHandle` to await on the result of `f`. Dropping the returned `TaskHandle` will cancel
     /// `f`, preventing it from being polled again. To drop a `TaskHandle` without canceling the
     /// future associated with it use `TaskHandle::detach`.
     ///
     /// # Examples
     ///
     /// ```
     /// # use cros_async::AsyncResult;
     /// # fn example_spawn() -> AsyncResult<()> {
     /// #      use std::thread;
     ///
     /// #      use cros_async::Executor;
     ///       use futures::executor::block_on;
     ///
     /// #      let ex = Executor::new()?;
     ///
     /// #      // Spawn a thread that runs the executor.
     /// #      let ex2 = ex.clone();
     /// #      thread::spawn(move || ex2.run());
     ///
     ///       let task = ex.spawn(async { 7 + 13 });
     ///
     ///       let result = block_on(task);
     ///       assert_eq!(result, 20);
     /// #     Ok(())
     /// # }
     ///
     /// # example_spawn().unwrap();
     /// ```
     pub fn spawn<F>(&self, f: F) -> TaskHandle<F::Output>
     where
         F: Future + Send + 'static,
         F::Output: Send + 'static,
     {
         match self {
             Executor::Uring(ex) => TaskHandle::Uring(ex.spawn(f)),
             Executor::Fd(ex) => TaskHandle::Fd(ex.spawn(f)),
         }
     }

     /// Spawn a thread-local task for this executor to drive to completion. Like `spawn` but without
     /// requiring `Send` on `F` or `F::Output`. This method should only be called from the same
     /// thread where `run()` or `run_until()` is called.
     ///
     /// # Panics
     ///
     /// `Executor::run` and `Executor::run_until` will panic if they try to poll a future that was
     /// added by calling `spawn_local` from a different thread.
     ///
     /// # Examples
     ///
     /// ```
     /// # use cros_async::AsyncResult;
     /// # fn example_spawn_local() -> AsyncResult<()> {
     /// #      use cros_async::Executor;
     ///
     /// #      let ex = Executor::new()?;
     ///
     ///       let task = ex.spawn_local(async { 7 + 13 });
     ///
     ///       let result = ex.run_until(task)?;
     ///       assert_eq!(result, 20);
     /// #     Ok(())
     /// # }
     ///
     /// # example_spawn_local().unwrap();
     /// ```
     pub fn spawn_local<F>(&self, f: F) -> TaskHandle<F::Output>
     where
         F: Future + 'static,
         F::Output: 'static,
     {
         match self {
             Executor::Uring(ex) => TaskHandle::Uring(ex.spawn_local(f)),
             Executor::Fd(ex) => TaskHandle::Fd(ex.spawn_local(f)),
         }
     }

     /// Run the provided closure on a dedicated thread where blocking is allowed.
     ///
     /// Callers may `await` on the returned `TaskHandle` to wait for the result of `f`. Dropping
     /// the returned `TaskHandle` may not cancel the operation if it was already started on a
     /// worker thread.
     ///
     /// # Panics
     ///
     /// `await`ing the `TaskHandle` after the `Executor` is dropped will panic if the work was not
     /// already completed.
     ///
     /// # Examples
     ///
     /// ```edition2018
     /// # use cros_async::Executor;
     ///
     /// # async fn do_it(ex: &Executor) {
     ///     let res = ex.spawn_blocking(move || {
     ///         // Do some CPU-intensive or blocking work here.
     ///
     ///         42
     ///     }).await;
     ///
     ///     assert_eq!(res, 42);
     /// # }
     ///
     /// # let ex = Executor::new().unwrap();
     /// # ex.run_until(do_it(&ex)).unwrap();
     /// ```
     pub fn spawn_blocking<F, R>(&self, f: F) -> TaskHandle<R>
     where
         F: FnOnce() -> R + Send + 'static,
         R: Send + 'static,
     {
         match self {
             Executor::Uring(ex) => TaskHandle::Uring(ex.spawn_blocking(f)),
             Executor::Fd(ex) => TaskHandle::Fd(ex.spawn_blocking(f)),
         }
     }

     /// Run the executor indefinitely, driving all spawned futures to completion. This method will
     /// block the current thread and only return in the case of an error.
     ///
     /// # Panics
     ///
     /// Once this method has been called on a thread, it may only be called on that thread from that
     /// point on. Attempting to call it from another thread will panic.
     ///
     /// # Examples
     ///
     /// ```
     /// # use cros_async::AsyncResult;
     /// # fn example_run() -> AsyncResult<()> {
     ///       use std::thread;
     ///
     ///       use cros_async::Executor;
     ///       use futures::executor::block_on;
     ///
     ///       let ex = Executor::new()?;
     ///
     ///       // Spawn a thread that runs the executor.
     ///       let ex2 = ex.clone();
     ///       thread::spawn(move || ex2.run());
     ///
     ///       let task = ex.spawn(async { 7 + 13 });
     ///
     ///       let result = block_on(task);
     ///       assert_eq!(result, 20);
     /// #     Ok(())
     /// # }
     ///
     /// # example_run().unwrap();
     /// ```
     pub fn run(&self) -> AsyncResult<()> {
         self.run_until(std::future::pending())
     }

     /// Drive all futures spawned in this executor until `f` completes. This method will block the
     /// current thread only until `f` is complete and there may still be unfinished futures in the
     /// executor.
     ///
     /// # Panics
     ///
     /// Once this method has been called on a thread, from then onwards it may only be called on
     /// that thread. Attempting to call it from another thread will panic.
     ///
     /// # Examples
     ///
     /// ```
     /// # use cros_async::AsyncResult;
     /// # fn example_run_until() -> AsyncResult<()> {
     ///       use cros_async::Executor;
     ///
     ///       let ex = Executor::new()?;
     ///
     ///       let task = ex.spawn_local(async { 7 + 13 });
     ///
     ///       let result = ex.run_until(task)?;
     ///       assert_eq!(result, 20);
     /// #     Ok(())
     /// # }
     ///
     /// # example_run_until().unwrap();
     /// ```
     pub fn run_until<F: Future>(&self, f: F) -> AsyncResult<F::Output> {
         match self {
             Executor::Uring(ex) => Ok(ex.run_until(f)?),
             Executor::Fd(ex) => Ok(ex.run_until(f)?),
         }
     }
 }

 impl AsRawDescriptors for Executor {
     fn as_raw_descriptors(&self) -> Vec<RawDescriptor> {
         match self {
             Executor::Uring(ex) => ex.as_raw_descriptors(),
             Executor::Fd(ex) => ex.as_raw_descriptors(),
         }
     }
 }
	// Copyright 2020 The ChromiumOS Authors
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	use std::future::Future;
	use std::pin::Pin;
	use std::sync::Arc;

	use base::debug;
	use base::warn;
	use base::AsRawDescriptors;
	use base::RawDescriptor;
	use once_cell::sync::OnceCell;
	use serde::Deserialize;
	use serde::Serialize;
	use thiserror::Error as ThisError;

	use super::fd_executor::EpollReactor;
	use super::uring_executor::check_uring_availability;
	use super::uring_executor::is_uring_stable;
	use super::uring_executor::Error as UringError;
	use super::uring_executor::UringReactor;
	use crate::common_executor;
	use crate::common_executor::RawExecutor;
	use crate::AsyncResult;
	use crate::IntoAsync;
	use crate::IoSource;

	/// An executor for scheduling tasks that poll futures to completion.
	///
	/// All asynchronous operations must run within an executor, which is capable of spawning futures as
	/// tasks. This executor also provides a mechanism for performing asynchronous I/O operations.
	///
	/// The returned type is a cheap, clonable handle to the underlying executor. Cloning it will only
	/// create a new reference, not a new executor.
	///
	/// Note that language limitations (trait objects can have <=1 non auto trait) require this to be
	/// represented on the POSIX side as an enum, rather than a trait. This leads to some code &
	/// interface duplication, but as far as we understand that is unavoidable.
	///
	/// See <https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2571401/2..6/cros_async/src/executor.rs#b75>
	/// for further details.
	///
	/// # Examples
	///
	/// Concurrently wait for multiple files to become readable/writable and then read/write the data.
	///
	/// ```
	/// use std::cmp::min;
	/// use std::error::Error;
	/// use std::fs::{File, OpenOptions};
	///
	/// use cros_async::{AsyncResult, Executor, IoSource, complete3};
	/// const CHUNK_SIZE: usize = 32;
	///
	/// // Write all bytes from `data` to `f`.
	/// async fn write_file(f: &IoSource<File>, mut data: Vec<u8>) -> AsyncResult<()> {
	/// while data.len() > 0 {
	/// let (count, mut buf) = f.write_from_vec(None, data).await?;
	///
	/// data = buf.split_off(count);
	/// }
	///
	/// Ok(())
	/// }
	///
	/// // Transfer `len` bytes of data from `from` to `to`.
	/// async fn transfer_data(
	/// from: IoSource<File>,
	/// to: IoSource<File>,
	/// len: usize,
	/// ) -> AsyncResult<usize> {
	/// let mut rem = len;
	///
	/// while rem > 0 {
	/// let buf = vec![0u8; min(rem, CHUNK_SIZE)];
	/// let (count, mut data) = from.read_to_vec(None, buf).await?;
	///
	/// if count == 0 {
	/// // End of file. Return the number of bytes transferred.
	/// return Ok(len - rem);
	/// }
	///
	/// data.truncate(count);
	/// write_file(&to, data).await?;
	///
	/// rem = rem.saturating_sub(count);
	/// }
	///
	/// Ok(len)
	/// }
	///
	/// #[cfg(any(target_os = "android", target_os = "linux"))]
	/// # fn do_it() -> Result<(), Box<dyn Error>> {
	/// let ex = Executor::new()?;
	///
	/// let (rx, tx) = base::unix::pipe(true)?;
	/// let zero = File::open("/dev/zero")?;
	/// let zero_bytes = CHUNK_SIZE * 7;
	/// let zero_to_pipe = transfer_data(
	/// ex.async_from(zero)?,
	/// ex.async_from(tx.try_clone()?)?,
	/// zero_bytes,
	/// );
	///
	/// let rand = File::open("/dev/urandom")?;
	/// let rand_bytes = CHUNK_SIZE * 19;
	/// let rand_to_pipe = transfer_data(ex.async_from(rand)?, ex.async_from(tx)?, rand_bytes);
	///
	/// let null = OpenOptions::new().write(true).open("/dev/null")?;
	/// let null_bytes = zero_bytes + rand_bytes;
	/// let pipe_to_null = transfer_data(ex.async_from(rx)?, ex.async_from(null)?, null_bytes);
	///
	/// ex.run_until(complete3(
	/// async { assert_eq!(pipe_to_null.await.unwrap(), null_bytes) },
	/// async { assert_eq!(zero_to_pipe.await.unwrap(), zero_bytes) },
	/// async { assert_eq!(rand_to_pipe.await.unwrap(), rand_bytes) },
	/// ))?;
	///
	/// # Ok(())
	/// # }
	/// #[cfg(any(target_os = "android", target_os = "linux"))]
	/// # do_it().unwrap();
	/// ```

	#[derive(Clone)]
	pub enum Executor {
	Uring(Arc<RawExecutor<UringReactor>>),
	Fd(Arc<RawExecutor<EpollReactor>>),
	}

	/// An enum to express the kind of the backend of `Executor`
	#[derive(
	Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize, serde_keyvalue::FromKeyValues,
	)]
	#[serde(deny_unknown_fields, rename_all = "kebab-case")]
	pub enum ExecutorKind {
	Uring,
	// For command-line parsing, user-friendly "epoll" is chosen instead of fd.
	#[serde(rename = "epoll")]
	Fd,
	}

	/// If set, [`ExecutorKind::default()`] returns the value of `DEFAULT_EXECUTOR_KIND`.
	/// If not set, [`ExecutorKind::default()`] returns a statically-chosen default value, and
	/// [`ExecutorKind::default()`] initializes `DEFAULT_EXECUTOR_KIND` with that value.
	static DEFAULT_EXECUTOR_KIND: OnceCell<ExecutorKind> = OnceCell::new();

	impl Default for ExecutorKind {
	fn default() -> Self {
	*DEFAULT_EXECUTOR_KIND.get_or_init(\|\| ExecutorKind::Fd)
	}
	}

	/// The error type for [`Executor::set_default_executor_kind()`].
	#[derive(Debug, ThisError)]
	pub enum SetDefaultExecutorKindError {
	/// The default executor kind is set more than once.
	#[error("The default executor kind is already set to {0:?}")]
	SetMoreThanOnce(ExecutorKind),

	/// io_uring is unavailable. The reason might be the lack of the kernel support,
	/// but is not limited to that.
	#[error("io_uring is unavailable: {0}")]
	UringUnavailable(UringError),
	}

	/// Reference to a task managed by the executor.
	///
	/// Dropping a `TaskHandle` attempts to cancel the associated task. Call `detach` to allow it to
	/// continue running the background.
	///
	/// `await`ing the `TaskHandle` waits for the task to finish and yields its result.
	pub enum TaskHandle<R> {
	Uring(common_executor::TaskHandle<UringReactor, R>),
	Fd(common_executor::TaskHandle<EpollReactor, R>),
	}

	impl<R: Send + 'static> TaskHandle<R> {
	pub fn detach(self) {
	match self {
	TaskHandle::Uring(x) => x.detach(),
	TaskHandle::Fd(x) => x.detach(),
	}
	}

	// Cancel the task and wait for it to stop. Returns the result of the task if it was already
	// finished.
	pub async fn cancel(self) -> Option<R> {
	match self {
	TaskHandle::Uring(x) => x.cancel().await,
	TaskHandle::Fd(x) => x.cancel().await,
	}
	}
	}

	impl<R: 'static> Future for TaskHandle<R> {
	type Output = R;

	fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context) -> std::task::Poll<Self::Output> {
	match self.get_mut() {
	TaskHandle::Uring(x) => Pin::new(x).poll(cx),
	TaskHandle::Fd(x) => Pin::new(x).poll(cx),
	}
	}
	}

	impl Executor {
	/// Create a new `Executor`.
	pub fn new() -> AsyncResult<Self> {
	Executor::with_executor_kind(ExecutorKind::default())
	}

	/// Create a new `Executor` of the given `ExecutorKind`.
	pub fn with_executor_kind(kind: ExecutorKind) -> AsyncResult<Self> {
	match kind {
	ExecutorKind::Uring => RawExecutor::new().map(Executor::Uring),
	ExecutorKind::Fd => RawExecutor::new().map(Executor::Fd),
	}
	}

	/// Set the default ExecutorKind for [`Self::new()`]. This call is effective only once.
	/// If a call is the first call, it sets the default, and `set_default_executor_kind`
	/// returns `Ok(())`. Otherwise, it returns `SetDefaultExecutorKindError::SetMoreThanOnce`
	/// which contains the existing ExecutorKind value configured by the first call.
	pub fn set_default_executor_kind(
	executor_kind: ExecutorKind,
	) -> Result<(), SetDefaultExecutorKindError> {
	if executor_kind == ExecutorKind::Uring {
	check_uring_availability().map_err(SetDefaultExecutorKindError::UringUnavailable)?;
	if !is_uring_stable() {
	warn!(
	"Enabling io_uring executor on the kernel version where io_uring is unstable"
	);
	}
	}

	debug!("setting the default executor to {:?}", executor_kind);
	DEFAULT_EXECUTOR_KIND.set(executor_kind).map_err(\|_\|
	// `expect` succeeds since this closure runs only when DEFAULT_EXECUTOR_KIND is set.
	SetDefaultExecutorKindError::SetMoreThanOnce(
	*DEFAULT_EXECUTOR_KIND
	.get()
	.expect("Failed to get DEFAULT_EXECUTOR_KIND"),
	))
	}

	/// Create a new `IoSource<F>` associated with `self`. Callers may then use the returned
	/// `IoSource` to directly start async operations without needing a separate reference to the
	/// executor.
	pub fn async_from<'a, F: IntoAsync + 'a>(&self, f: F) -> AsyncResult<IoSource<F>> {
	match self {
	Executor::Uring(ex) => ex.new_source(f),
	Executor::Fd(ex) => ex.new_source(f),
	}
	}

	/// Spawn a new future for this executor to run to completion. Callers may use the returned
	/// `TaskHandle` to await on the result of `f`. Dropping the returned `TaskHandle` will cancel
	/// `f`, preventing it from being polled again. To drop a `TaskHandle` without canceling the
	/// future associated with it use `TaskHandle::detach`.
	///
	/// # Examples
	///
	/// ```
	/// # use cros_async::AsyncResult;
	/// # fn example_spawn() -> AsyncResult<()> {
	/// # use std::thread;
	///
	/// # use cros_async::Executor;
	/// use futures::executor::block_on;
	///
	/// # let ex = Executor::new()?;
	///
	/// # // Spawn a thread that runs the executor.
	/// # let ex2 = ex.clone();
	/// # thread::spawn(move \|\| ex2.run());
	///
	/// let task = ex.spawn(async { 7 + 13 });
	///
	/// let result = block_on(task);
	/// assert_eq!(result, 20);
	/// # Ok(())
	/// # }
	///
	/// # example_spawn().unwrap();
	/// ```
	pub fn spawn<F>(&self, f: F) -> TaskHandle<F::Output>
	where
	F: Future + Send + 'static,
	F::Output: Send + 'static,
	{
	match self {
	Executor::Uring(ex) => TaskHandle::Uring(ex.spawn(f)),
	Executor::Fd(ex) => TaskHandle::Fd(ex.spawn(f)),
	}
	}

	/// Spawn a thread-local task for this executor to drive to completion. Like `spawn` but without
	/// requiring `Send` on `F` or `F::Output`. This method should only be called from the same
	/// thread where `run()` or `run_until()` is called.
	///
	/// # Panics
	///
	/// `Executor::run` and `Executor::run_until` will panic if they try to poll a future that was
	/// added by calling `spawn_local` from a different thread.
	///
	/// # Examples
	///
	/// ```
	/// # use cros_async::AsyncResult;
	/// # fn example_spawn_local() -> AsyncResult<()> {
	/// # use cros_async::Executor;
	///
	/// # let ex = Executor::new()?;
	///
	/// let task = ex.spawn_local(async { 7 + 13 });
	///
	/// let result = ex.run_until(task)?;
	/// assert_eq!(result, 20);
	/// # Ok(())
	/// # }
	///
	/// # example_spawn_local().unwrap();
	/// ```
	pub fn spawn_local<F>(&self, f: F) -> TaskHandle<F::Output>
	where
	F: Future + 'static,
	F::Output: 'static,
	{
	match self {
	Executor::Uring(ex) => TaskHandle::Uring(ex.spawn_local(f)),
	Executor::Fd(ex) => TaskHandle::Fd(ex.spawn_local(f)),
	}
	}

	/// Run the provided closure on a dedicated thread where blocking is allowed.
	///
	/// Callers may `await` on the returned `TaskHandle` to wait for the result of `f`. Dropping
	/// the returned `TaskHandle` may not cancel the operation if it was already started on a
	/// worker thread.
	///
	/// # Panics
	///
	/// `await`ing the `TaskHandle` after the `Executor` is dropped will panic if the work was not
	/// already completed.
	///
	/// # Examples
	///
	/// ```edition2018
	/// # use cros_async::Executor;
	///
	/// # async fn do_it(ex: &Executor) {
	/// let res = ex.spawn_blocking(move \|\| {
	/// // Do some CPU-intensive or blocking work here.
	///
	/// 42
	/// }).await;
	///
	/// assert_eq!(res, 42);
	/// # }
	///
	/// # let ex = Executor::new().unwrap();
	/// # ex.run_until(do_it(&ex)).unwrap();
	/// ```
	pub fn spawn_blocking<F, R>(&self, f: F) -> TaskHandle<R>
	where
	F: FnOnce() -> R + Send + 'static,
	R: Send + 'static,
	{
	match self {
	Executor::Uring(ex) => TaskHandle::Uring(ex.spawn_blocking(f)),
	Executor::Fd(ex) => TaskHandle::Fd(ex.spawn_blocking(f)),
	}
	}

	/// Run the executor indefinitely, driving all spawned futures to completion. This method will
	/// block the current thread and only return in the case of an error.
	///
	/// # Panics
	///
	/// Once this method has been called on a thread, it may only be called on that thread from that
	/// point on. Attempting to call it from another thread will panic.
	///
	/// # Examples
	///
	/// ```
	/// # use cros_async::AsyncResult;
	/// # fn example_run() -> AsyncResult<()> {
	/// use std::thread;
	///
	/// use cros_async::Executor;
	/// use futures::executor::block_on;
	///
	/// let ex = Executor::new()?;
	///
	/// // Spawn a thread that runs the executor.
	/// let ex2 = ex.clone();
	/// thread::spawn(move \|\| ex2.run());
	///
	/// let task = ex.spawn(async { 7 + 13 });
	///
	/// let result = block_on(task);
	/// assert_eq!(result, 20);
	/// # Ok(())
	/// # }
	///
	/// # example_run().unwrap();
	/// ```
	pub fn run(&self) -> AsyncResult<()> {
	self.run_until(std::future::pending())
	}

	/// Drive all futures spawned in this executor until `f` completes. This method will block the
	/// current thread only until `f` is complete and there may still be unfinished futures in the
	/// executor.
	///
	/// # Panics
	///
	/// Once this method has been called on a thread, from then onwards it may only be called on
	/// that thread. Attempting to call it from another thread will panic.
	///
	/// # Examples
	///
	/// ```
	/// # use cros_async::AsyncResult;
	/// # fn example_run_until() -> AsyncResult<()> {
	/// use cros_async::Executor;
	///
	/// let ex = Executor::new()?;
	///
	/// let task = ex.spawn_local(async { 7 + 13 });
	///
	/// let result = ex.run_until(task)?;
	/// assert_eq!(result, 20);
	/// # Ok(())
	/// # }
	///
	/// # example_run_until().unwrap();
	/// ```
	pub fn run_until<F: Future>(&self, f: F) -> AsyncResult<F::Output> {
	match self {
	Executor::Uring(ex) => Ok(ex.run_until(f)?),
	Executor::Fd(ex) => Ok(ex.run_until(f)?),
	}
	}
	}

	impl AsRawDescriptors for Executor {
	fn as_raw_descriptors(&self) -> Vec<RawDescriptor> {
	match self {
	Executor::Uring(ex) => ex.as_raw_descriptors(),
	Executor::Fd(ex) => ex.as_raw_descriptors(),
	}
	}
	}