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Implement async_std::sync::Condvar (#369)

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* Implement async_std::sync::Condvar

Part of #217

* More rigourous detection of notification for condvar

* Use state of Waker instead of AtomicUsize to keep track of if task was
notified.

* Add test for notify_all

* Implement wait_timeout_until

And add warnings about spurious wakeups to wait and wait_timeout

* Use WakerSet for Condvar

This should also address concerns about spurious wakeups.

* Add test for wait_timeout with no lock held

* Add comments describing AwaitNotify struct

And remove an unnneded comment in a Debug implementation
pull/540/merge
Thayne McCombs 5 years ago committed by GitHub
parent aebba2bd95
commit db438abb8f
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417
src/sync/condvar.rs
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use std::fmt;
use std::pin::Pin;
use std::time::Duration;
use super::mutex::{guard_lock, MutexGuard};
use crate::future::{timeout, Future};
use crate::sync::WakerSet;
use crate::task::{Context, Poll};
#[derive(Debug, PartialEq, Eq, Copy, Clone)]
pub struct WaitTimeoutResult(bool);
/// A type indicating whether a timed wait on a condition variable returned due to a time out or
/// not
impl WaitTimeoutResult {
/// Returns `true` if the wait was known to have timed out.
pub fn timed_out(self) -> bool {
self.0
}
}
/// A Condition Variable
///
/// This type is an async version of [`std::sync::Mutex`].
///
/// [`std::sync::Condvar`]: https://doc.rust-lang.org/std/sync/struct.Condvar.html
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use std::sync::Arc;
///
/// use async_std::sync::{Mutex, Condvar};
/// use async_std::task;
///
/// let pair = Arc::new((Mutex::new(false), Condvar::new()));
/// let pair2 = pair.clone();
///
/// // Inside of our lock, spawn a new thread, and then wait for it to start.
/// task::spawn(async move {
/// let (lock, cvar) = &*pair2;
/// let mut started = lock.lock().await;
/// *started = true;
/// // We notify the condvar that the value has changed.
/// cvar.notify_one();
/// });
///
/// // Wait for the thread to start up.
/// let (lock, cvar) = &*pair;
/// let mut started = lock.lock().await;
/// while !*started {
/// started = cvar.wait(started).await;
/// }
///
/// # })
/// ```
pub struct Condvar {
wakers: WakerSet,
}
unsafe impl Send for Condvar {}
unsafe impl Sync for Condvar {}
impl Default for Condvar {
fn default() -> Self {
Condvar::new()
}
}
impl Condvar {
/// Creates a new condition variable
///
/// # Examples
///
/// ```
/// use async_std::sync::Condvar;
///
/// let cvar = Condvar::new();
/// ```
pub fn new() -> Self {
Condvar {
wakers: WakerSet::new(),
}
}
/// Blocks the current task until this condition variable receives a notification.
///
/// Unlike the std equivalent, this does not check that a single mutex is used at runtime.
/// However, as a best practice avoid using with multiple mutexes.
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// use std::sync::Arc;
///
/// use async_std::sync::{Mutex, Condvar};
/// use async_std::task;
///
/// let pair = Arc::new((Mutex::new(false), Condvar::new()));
/// let pair2 = pair.clone();
///
/// task::spawn(async move {
/// let (lock, cvar) = &*pair2;
/// let mut started = lock.lock().await;
/// *started = true;
/// // We notify the condvar that the value has changed.
/// cvar.notify_one();
/// });
///
/// // Wait for the thread to start up.
/// let (lock, cvar) = &*pair;
/// let mut started = lock.lock().await;
/// while !*started {
/// started = cvar.wait(started).await;
/// }
/// # })
/// ```
#[allow(clippy::needless_lifetimes)]
pub async fn wait<'a, T>(&self, guard: MutexGuard<'a, T>) -> MutexGuard<'a, T> {
let mutex = guard_lock(&guard);
self.await_notify(guard).await;
mutex.lock().await
}
fn await_notify<'a, T>(&self, guard: MutexGuard<'a, T>) -> AwaitNotify<'_, 'a, T> {
AwaitNotify {
cond: self,
guard: Some(guard),
key: None,
}
}
/// Blocks the current taks until this condition variable receives a notification and the
/// required condition is met. Spurious wakeups are ignored and this function will only
/// return once the condition has been met.
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use std::sync::Arc;
///
/// use async_std::sync::{Mutex, Condvar};
/// use async_std::task;
///
/// let pair = Arc::new((Mutex::new(false), Condvar::new()));
/// let pair2 = pair.clone();
///
/// task::spawn(async move {
/// let (lock, cvar) = &*pair2;
/// let mut started = lock.lock().await;
/// *started = true;
/// // We notify the condvar that the value has changed.
/// cvar.notify_one();
/// });
///
/// // Wait for the thread to start up.
/// let (lock, cvar) = &*pair;
/// // As long as the value inside the `Mutex<bool>` is `false`, we wait.
/// let _guard = cvar.wait_until(lock.lock().await, |started| { *started }).await;
/// #
/// # })
/// ```
#[allow(clippy::needless_lifetimes)]
pub async fn wait_until<'a, T, F>(
&self,
mut guard: MutexGuard<'a, T>,
mut condition: F,
) -> MutexGuard<'a, T>
where
F: FnMut(&mut T) -> bool,
{
while !condition(&mut *guard) {
guard = self.wait(guard).await;
}
guard
}
/// Waits on this condition variable for a notification, timing out after a specified duration.
///
/// For these reasons `Condvar::wait_timeout_until` is recommended in most cases.
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use std::sync::Arc;
/// use std::time::Duration;
///
/// use async_std::sync::{Mutex, Condvar};
/// use async_std::task;
///
/// let pair = Arc::new((Mutex::new(false), Condvar::new()));
/// let pair2 = pair.clone();
///
/// task::spawn(async move {
/// let (lock, cvar) = &*pair2;
/// let mut started = lock.lock().await;
/// *started = true;
/// // We notify the condvar that the value has changed.
/// cvar.notify_one();
/// });
///
/// // wait for the thread to start up
/// let (lock, cvar) = &*pair;
/// let mut started = lock.lock().await;
/// loop {
/// let result = cvar.wait_timeout(started, Duration::from_millis(10)).await;
/// started = result.0;
/// if *started == true {
/// // We received the notification and the value has been updated, we can leave.
/// break
/// }
/// }
/// #
/// # })
/// ```
#[allow(clippy::needless_lifetimes)]
pub async fn wait_timeout<'a, T>(
&self,
guard: MutexGuard<'a, T>,
dur: Duration,
) -> (MutexGuard<'a, T>, WaitTimeoutResult) {
let mutex = guard_lock(&guard);
match timeout(dur, self.wait(guard)).await {
Ok(guard) => (guard, WaitTimeoutResult(false)),
Err(_) => (mutex.lock().await, WaitTimeoutResult(true)),
}
}
/// Waits on this condition variable for a notification, timing out after a specified duration.
/// Spurious wakes will not cause this function to return.
///
/// # Examples
/// ```
/// # async_std::task::block_on(async {
/// use std::sync::Arc;
/// use std::time::Duration;
///
/// use async_std::sync::{Mutex, Condvar};
/// use async_std::task;
///
/// let pair = Arc::new((Mutex::new(false), Condvar::new()));
/// let pair2 = pair.clone();
///
/// task::spawn(async move {
/// let (lock, cvar) = &*pair2;
/// let mut started = lock.lock().await;
/// *started = true;
/// // We notify the condvar that the value has changed.
/// cvar.notify_one();
/// });
///
/// // wait for the thread to start up
/// let (lock, cvar) = &*pair;
/// let result = cvar.wait_timeout_until(
/// lock.lock().await,
/// Duration::from_millis(100),
/// |&mut started| started,
/// ).await;
/// if result.1.timed_out() {
/// // timed-out without the condition ever evaluating to true.
/// }
/// // access the locked mutex via result.0
/// # });
/// ```
#[allow(clippy::needless_lifetimes)]
pub async fn wait_timeout_until<'a, T, F>(
&self,
guard: MutexGuard<'a, T>,
dur: Duration,
condition: F,
) -> (MutexGuard<'a, T>, WaitTimeoutResult)
where
F: FnMut(&mut T) -> bool,
{
let mutex = guard_lock(&guard);
match timeout(dur, self.wait_until(guard, condition)).await {
Ok(guard) => (guard, WaitTimeoutResult(false)),
Err(_) => (mutex.lock().await, WaitTimeoutResult(true)),
}
}
/// Wakes up one blocked task on this condvar.
///
/// # Examples
///
/// ```
/// # fn main() { async_std::task::block_on(async {
/// use std::sync::Arc;
///
/// use async_std::sync::{Mutex, Condvar};
/// use async_std::task;
///
/// let pair = Arc::new((Mutex::new(false), Condvar::new()));
/// let pair2 = pair.clone();
///
/// task::spawn(async move {
/// let (lock, cvar) = &*pair2;
/// let mut started = lock.lock().await;
/// *started = true;
/// // We notify the condvar that the value has changed.
/// cvar.notify_one();
/// });
///
/// // Wait for the thread to start up.
/// let (lock, cvar) = &*pair;
/// let mut started = lock.lock().await;
/// while !*started {
/// started = cvar.wait(started).await;
/// }
/// # }) }
/// ```
pub fn notify_one(&self) {
self.wakers.notify_one();
}
/// Wakes up all blocked tasks on this condvar.
///
/// # Examples
/// ```
/// # fn main() { async_std::task::block_on(async {
/// #
/// use std::sync::Arc;
///
/// use async_std::sync::{Mutex, Condvar};
/// use async_std::task;
///
/// let pair = Arc::new((Mutex::new(false), Condvar::new()));
/// let pair2 = pair.clone();
///
/// task::spawn(async move {
/// let (lock, cvar) = &*pair2;
/// let mut started = lock.lock().await;
/// *started = true;
/// // We notify the condvar that the value has changed.
/// cvar.notify_all();
/// });
///
/// // Wait for the thread to start up.
/// let (lock, cvar) = &*pair;
/// let mut started = lock.lock().await;
/// // As long as the value inside the `Mutex<bool>` is `false`, we wait.
/// while !*started {
/// started = cvar.wait(started).await;
/// }
/// #
/// # }) }
/// ```
pub fn notify_all(&self) {
self.wakers.notify_all();
}
}
impl fmt::Debug for Condvar {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("Condvar { .. }")
}
}
/// A future that waits for another task to notify the condition variable.
///
/// This is an internal future that `wait` and `wait_until` await on.
struct AwaitNotify<'a, 'b, T> {
/// The condition variable that we are waiting on
cond: &'a Condvar,
/// The lock used with `cond`.
/// This will be released the first time the future is polled,
/// after registering the context to be notified.
guard: Option<MutexGuard<'b, T>>,
/// A key into the conditions variable's `WakerSet`.
/// This is set to the index of the `Waker` for the context each time
/// the future is polled and not completed.
key: Option<usize>,
}
impl<'a, 'b, T> Future for AwaitNotify<'a, 'b, T> {
type Output = ();
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
match self.guard.take() {
Some(_) => {
self.key = Some(self.cond.wakers.insert(cx));
// the guard is dropped when we return, which frees the lock
Poll::Pending
}
None => {
if let Some(key) = self.key {
if self.cond.wakers.remove_if_notified(key, cx) {
self.key = None;
Poll::Ready(())
} else {
Poll::Pending
}
} else {
// This should only happen if it is polled twice after receiving a notification
Poll::Ready(())
}
}
}
}
}
impl<'a, 'b, T> Drop for AwaitNotify<'a, 'b, T> {
fn drop(&mut self) {
if let Some(key) = self.key {
self.cond.wakers.cancel(key);
}
}
}

2
src/sync/mod.rs
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@ -185,8 +185,10 @@ mod rwlock;
cfg_unstable! {
pub use barrier::{Barrier, BarrierWaitResult};
pub use channel::{channel, Sender, Receiver, RecvError, TryRecvError, TrySendError};
pub use condvar::Condvar;
mod barrier;
mod condvar;
mod channel;
}

5
src/sync/mutex.rs
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@ -287,3 +287,8 @@ impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
unsafe { &mut *self.0.value.get() }
}
}
#[cfg(feature = "unstable")]
pub fn guard_lock<'a, T>(guard: &MutexGuard<'a, T>) -> &'a Mutex<T> {
guard.0
}

22
src/sync/waker_set.rs
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@ -80,6 +80,28 @@ impl WakerSet {
}
}
/// If the waker for this key is still waiting for a notification, then update
/// the waker for the entry, and return false. If the waker has been notified,
/// treat the entry as completed and return true.
#[cfg(feature = "unstable")]
pub fn remove_if_notified(&self, key: usize, cx: &Context<'_>) -> bool {
let mut inner = self.lock();
match &mut inner.entries[key] {
None => {
inner.entries.remove(key);
true
}
Some(w) => {
// We were never woken, so update instead
if !w.will_wake(cx.waker()) {
*w = cx.waker().clone();
}
false
}
}
}
/// Removes the waker of a cancelled operation.
///
/// Returns `true` if another blocked operation from the set was notified.

91
tests/condvar.rs
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@ -0,0 +1,91 @@
#![cfg(feature = "unstable")]
use std::sync::Arc;
use std::time::Duration;
use async_std::sync::{Condvar, Mutex};
use async_std::task::{self, JoinHandle};
#[test]
fn wait_timeout_with_lock() {
task::block_on(async {
let pair = Arc::new((Mutex::new(false), Condvar::new()));
let pair2 = pair.clone();
task::spawn(async move {
let (m, c) = &*pair2;
let _g = m.lock().await;
task::sleep(Duration::from_millis(20)).await;
c.notify_one();
});
let (m, c) = &*pair;
let (_, wait_result) = c
.wait_timeout(m.lock().await, Duration::from_millis(10))
.await;
assert!(wait_result.timed_out());
})
}
#[test]
fn wait_timeout_without_lock() {
task::block_on(async {
let m = Mutex::new(false);
let c = Condvar::new();
let (_, wait_result) = c
.wait_timeout(m.lock().await, Duration::from_millis(10))
.await;
assert!(wait_result.timed_out());
})
}
#[test]
fn wait_timeout_until_timed_out() {
task::block_on(async {
let m = Mutex::new(false);
let c = Condvar::new();
let (_, wait_result) = c
.wait_timeout_until(m.lock().await, Duration::from_millis(10), |&mut started| {
started
})
.await;
assert!(wait_result.timed_out());
})
}
#[test]
fn notify_all() {
task::block_on(async {
let mut tasks: Vec<JoinHandle<()>> = Vec::new();
let pair = Arc::new((Mutex::new(0u32), Condvar::new()));
for _ in 0..10 {
let pair = pair.clone();
tasks.push(task::spawn(async move {
let (m, c) = &*pair;
let mut count = m.lock().await;
while *count == 0 {
count = c.wait(count).await;
}
*count += 1;
}));
}
// Give some time for tasks to start up
task::sleep(Duration::from_millis(5)).await;
let (m, c) = &*pair;
{
let mut count = m.lock().await;
*count += 1;
c.notify_all();
}
for t in tasks {
t.await;
}
let count = m.lock().await;
assert_eq!(11, *count);
})
}
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