async_core: Add async TimerFd

An async timer FD will allow block to keep the flush timer when it is
converted to async.

Change-Id: I25759be0f969d1962fd13f33709166527d8f117d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2168644
Tested-by: Dylan Reid <dgreid@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Commit-Queue: Dylan Reid <dgreid@chromium.org>

async_core/src/lib.rs

@@ -7,5 +7,8 @@
 //! crate.
 
 mod eventfd;
+mod timerfd;
 
 pub use eventfd::EventFd;
+pub use timerfd::Error as TimerFdError;
+pub use timerfd::TimerFd;

async_core/src/timerfd.rs

@@ -0,0 +1,243 @@
+// Copyright 2019 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+use std::convert::TryFrom;
+use std::fmt::{self, Display};
+use std::future::Future;
+use std::os::unix::io::AsRawFd;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::time::Duration;
+
+use libc::{EWOULDBLOCK, O_NONBLOCK};
+
+use cros_async::Error as ExecutorError;
+use cros_async::{add_read_waker, cancel_waker, WakerToken};
+use sys_util::{self, add_fd_flags};
+
+/// Errors generated while polling for events.
+#[derive(Debug)]
+pub enum Error {
+    /// An error occurred attempting to register a waker with the executor.
+    AddingWaker(ExecutorError),
+    /// An error occurred when reading the timer FD.
+    TimerFdRead(sys_util::Error),
+    /// An error occurred when resetting the timer FD.
+    TimerFdReset(sys_util::Error),
+    /// An error occurred when setting the timer FD non-blocking.
+    SettingNonBlocking(sys_util::Error),
+}
+pub type Result<T> = std::result::Result<T, Error>;
+
+impl std::error::Error for Error {}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use self::Error::*;
+
+        match self {
+            AddingWaker(e) => write!(
+                f,
+                "An error occurred attempting to register a waker with the executor: {}.",
+                e
+            ),
+            TimerFdRead(e) => write!(f, "An error occurred when reading the timer FD: {}.", e),
+            TimerFdReset(e) => write!(f, "An error occurred when resetting the timer FD: {}.", e),
+            SettingNonBlocking(e) => {
+                write!(f, "An error occurred setting the FD non-blocking: {}.", e)
+            }
+        }
+    }
+}
+
+/// Asynchronous version of `sys_util::TimerFd`. Provides a method for asynchronously waiting for a
+/// timer to fire.
+///
+/// # Example
+///
+/// ```
+/// use std::convert::TryInto;
+///
+/// use async_core::TimerFd;
+/// use sys_util::{self, Result};
+/// async fn process_events() -> Result<()> {
+///     let mut async_timer: TimerFd = sys_util::TimerFd::new()?.try_into().unwrap();
+///     while let Ok(e) = async_timer.next_expiration().await {
+///         // Handle timer here.
+///     }
+///     Ok(())
+/// }
+/// ```
+pub struct TimerFd(sys_util::TimerFd);
+
+impl TimerFd {
+    /// Reset the inner timer to a new duration.
+    pub fn reset(&self, dur: Duration, interval: Option<Duration>) -> Result<()> {
+        self.0.reset(dur, interval).map_err(Error::TimerFdReset)
+    }
+
+    /// Asynchronously wait for the timer to fire.
+    /// Returns a Future that can be `awaited` for the next interval.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use async_core::TimerFd;
+    /// async fn print_events(mut timer_fd: TimerFd) {
+    ///     loop {
+    ///         match timer_fd.next_expiration().await {
+    ///             Ok(_) => println!("timer fired"),
+    ///             Err(e) => break,
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    pub fn next_expiration(&self) -> TimerFuture {
+        TimerFuture::new(&self.0)
+    }
+}
+
+impl TryFrom<sys_util::TimerFd> for TimerFd {
+    type Error = Error;
+
+    fn try_from(timerfd: sys_util::TimerFd) -> Result<TimerFd> {
+        let fd = timerfd.as_raw_fd();
+        add_fd_flags(fd, O_NONBLOCK).map_err(Error::SettingNonBlocking)?;
+        Ok(TimerFd(timerfd))
+    }
+}
+
+/// A Future that yields completes when the timer has fired the next time.
+pub struct TimerFuture<'a> {
+    inner: &'a sys_util::TimerFd,
+    waker_token: Option<WakerToken>,
+}
+
+impl<'a> TimerFuture<'a> {
+    fn new(inner: &'a sys_util::TimerFd) -> TimerFuture<'a> {
+        TimerFuture {
+            inner,
+            waker_token: None,
+        }
+    }
+}
+
+impl<'a> Future for TimerFuture<'a> {
+    type Output = Result<u64>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
+        if let Some(token) = self.waker_token.take() {
+            let _ = cancel_waker(token);
+        }
+
+        match self.inner.wait() {
+            Ok(count) => Poll::Ready(Ok(count)),
+            Err(e) => {
+                if e.errno() == EWOULDBLOCK {
+                    match add_read_waker(self.inner.as_raw_fd(), cx.waker().clone()) {
+                        Ok(token) => {
+                            self.waker_token = Some(token);
+                            Poll::Pending
+                        }
+                        Err(e) => Poll::Ready(Err(Error::AddingWaker(e))),
+                    }
+                } else {
+                    // Indicate something went wrong and no more events will be provided.
+                    Poll::Ready(Err(Error::TimerFdRead(e)))
+                }
+            }
+        }
+    }
+}
+
+impl<'a> Drop for TimerFuture<'a> {
+    fn drop(&mut self) {
+        if let Some(token) = self.waker_token.take() {
+            let _ = cancel_waker(token);
+        }
+    }
+}
+
+impl AsRef<sys_util::TimerFd> for TimerFd {
+    fn as_ref(&self) -> &sys_util::TimerFd {
+        &self.0
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::time::Duration;
+
+    use cros_async::{run_one, select2, SelectResult};
+    use futures::future::pending;
+    use futures::future::Either;
+    use futures::pin_mut;
+
+    use super::*;
+
+    #[test]
+    fn timer_timeout() {
+        async fn timeout(duration: Duration) -> u64 {
+            let t = sys_util::TimerFd::new().unwrap();
+            let async_timer = TimerFd::try_from(t).unwrap();
+            async_timer.reset(duration, None).unwrap();
+            async_timer.next_expiration().await.unwrap()
+        }
+
+        let to = timeout(Duration::from_millis(10));
+        pin_mut!(to);
+        if let Ok((SelectResult::Finished(timer_count), SelectResult::Pending(_pend_fut))) =
+            select2(to, pending::<()>())
+        {
+            assert_eq!(timer_count, 1);
+        } else {
+            panic!("wrong futures returned from select2");
+        }
+    }
+
+    #[test]
+    fn select_with_eventfd() {
+        async fn async_test() {
+            let mut e = crate::eventfd::EventFd::new().unwrap();
+            let t = sys_util::TimerFd::new().unwrap();
+            let async_timer = TimerFd::try_from(t).unwrap();
+            async_timer.reset(Duration::from_millis(10), None).unwrap();
+            let timer = async_timer.next_expiration();
+            let event = e.read_next();
+            pin_mut!(timer);
+            pin_mut!(event);
+            match futures::future::select(timer, event).await {
+                Either::Left((_timer, _event)) => (),
+                _ => panic!("unexpected select result"),
+            }
+        }
+
+        let fut = async_test();
+
+        run_one(Box::pin(fut)).unwrap();
+    }
+
+    #[test]
+    fn pend_unarmed_cancel() {
+        async fn async_test() {
+            let t = sys_util::TimerFd::new().unwrap();
+            let async_timer = TimerFd::try_from(t).unwrap();
+            let done = async {
+                std::thread::sleep(Duration::from_millis(10));
+                5usize
+            };
+            let timer = async_timer.next_expiration();
+            pin_mut!(done);
+            pin_mut!(timer);
+            match futures::future::select(timer, done).await {
+                Either::Right((_, _timer)) => (),
+                _ => panic!("unexpected select result"),
+            }
+        }
+
+        let fut = async_test();
+
+        run_one(Box::pin(fut)).unwrap();
+    }
+}