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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/git/checkouts/tokio-377c595163f99a10/dfe252d/tokio/src/sync/notify.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>notify.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../normalize.css"><link rel="stylesheet" href="../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../ayu.css" disabled><link rel="stylesheet" href="../../../dark.css" disabled><link rel="stylesheet" href="../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../storage.js"></script><script defer src="../../../source-script.js"></script><script defer src="../../../source-files.js"></script><script defer src="../../../main.js"></script><noscript><link rel="stylesheet" href="../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../tokio/index.html"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span>
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</pre><pre class="rust"><code><span class="comment">// Allow `unreachable_pub` warnings when sync is not enabled
// due to the usage of `Notify` within the `rt` feature set.
// When this module is compiled with `sync` enabled we will warn on
// this lint. When `rt` is enabled we use `pub(crate)` which
// triggers this warning but it is safe to ignore in this case.
</span><span class="attribute">#![cfg_attr(not(feature = <span class="string">&quot;sync&quot;</span>), allow(unreachable_pub, dead_code))]
</span><span class="kw">use </span><span class="kw">crate</span>::loom::sync::atomic::AtomicUsize;
<span class="kw">use </span><span class="kw">crate</span>::loom::sync::Mutex;
<span class="kw">use </span><span class="kw">crate</span>::util::linked_list::{<span class="self">self</span>, LinkedList};
<span class="kw">use </span><span class="kw">crate</span>::util::WakeList;
<span class="kw">use </span>std::cell::UnsafeCell;
<span class="kw">use </span>std::future::Future;
<span class="kw">use </span>std::marker::PhantomPinned;
<span class="kw">use </span>std::panic::{RefUnwindSafe, UnwindSafe};
<span class="kw">use </span>std::pin::Pin;
<span class="kw">use </span>std::ptr::NonNull;
<span class="kw">use </span>std::sync::atomic::Ordering::SeqCst;
<span class="kw">use </span>std::task::{Context, Poll, Waker};
<span class="kw">type </span>WaitList = LinkedList&lt;Waiter, &lt;Waiter <span class="kw">as </span>linked_list::Link&gt;::Target&gt;;
<span class="doccomment">/// Notifies a single task to wake up.
///
/// `Notify` provides a basic mechanism to notify a single task of an event.
/// `Notify` itself does not carry any data. Instead, it is to be used to signal
/// another task to perform an operation.
///
/// A `Notify` can be thought of as a [`Semaphore`] starting with 0 permits. The
/// [`notified().await`] method waits for a permit to become available, and
/// [`notify_one()`] sets a permit **if there currently are no available
/// permits**.
///
/// The synchronization details of `Notify` are similar to
/// [`thread::park`][park] and [`Thread::unpark`][unpark] from std. A [`Notify`]
/// value contains a single permit. [`notified().await`] waits for the permit to
/// be made available, consumes the permit, and resumes. [`notify_one()`] sets
/// the permit, waking a pending task if there is one.
///
/// If `notify_one()` is called **before** `notified().await`, then the next
/// call to `notified().await` will complete immediately, consuming the permit.
/// Any subsequent calls to `notified().await` will wait for a new permit.
///
/// If `notify_one()` is called **multiple** times before `notified().await`,
/// only a **single** permit is stored. The next call to `notified().await` will
/// complete immediately, but the one after will wait for a new permit.
///
/// # Examples
///
/// Basic usage.
///
/// ```
/// use tokio::sync::Notify;
/// use std::sync::Arc;
///
/// #[tokio::main]
/// async fn main() {
/// let notify = Arc::new(Notify::new());
/// let notify2 = notify.clone();
///
/// let handle = tokio::spawn(async move {
/// notify2.notified().await;
/// println!(&quot;received notification&quot;);
/// });
///
/// println!(&quot;sending notification&quot;);
/// notify.notify_one();
///
/// // Wait for task to receive notification.
/// handle.await.unwrap();
/// }
/// ```
///
/// Unbound multi-producer single-consumer (mpsc) channel.
///
/// No wakeups can be lost when using this channel because the call to
/// `notify_one()` will store a permit in the `Notify`, which the following call
/// to `notified()` will consume.
///
/// ```
/// use tokio::sync::Notify;
///
/// use std::collections::VecDeque;
/// use std::sync::Mutex;
///
/// struct Channel&lt;T&gt; {
/// values: Mutex&lt;VecDeque&lt;T&gt;&gt;,
/// notify: Notify,
/// }
///
/// impl&lt;T&gt; Channel&lt;T&gt; {
/// pub fn send(&amp;self, value: T) {
/// self.values.lock().unwrap()
/// .push_back(value);
///
/// // Notify the consumer a value is available
/// self.notify.notify_one();
/// }
///
/// // This is a single-consumer channel, so several concurrent calls to
/// // `recv` are not allowed.
/// pub async fn recv(&amp;self) -&gt; T {
/// loop {
/// // Drain values
/// if let Some(value) = self.values.lock().unwrap().pop_front() {
/// return value;
/// }
///
/// // Wait for values to be available
/// self.notify.notified().await;
/// }
/// }
/// }
/// ```
///
/// Unbound multi-producer multi-consumer (mpmc) channel.
///
/// The call to [`enable`] is important because otherwise if you have two
/// calls to `recv` and two calls to `send` in parallel, the following could
/// happen:
///
/// 1. Both calls to `try_recv` return `None`.
/// 2. Both new elements are added to the vector.
/// 3. The `notify_one` method is called twice, adding only a single
/// permit to the `Notify`.
/// 4. Both calls to `recv` reach the `Notified` future. One of them
/// consumes the permit, and the other sleeps forever.
///
/// By adding the `Notified` futures to the list by calling `enable` before
/// `try_recv`, the `notify_one` calls in step three would remove the
/// futures from the list and mark them notified instead of adding a permit
/// to the `Notify`. This ensures that both futures are woken.
///
/// Notice that this failure can only happen if there are two concurrent calls
/// to `recv`. This is why the mpsc example above does not require a call to
/// `enable`.
///
/// ```
/// use tokio::sync::Notify;
///
/// use std::collections::VecDeque;
/// use std::sync::Mutex;
///
/// struct Channel&lt;T&gt; {
/// messages: Mutex&lt;VecDeque&lt;T&gt;&gt;,
/// notify_on_sent: Notify,
/// }
///
/// impl&lt;T&gt; Channel&lt;T&gt; {
/// pub fn send(&amp;self, msg: T) {
/// let mut locked_queue = self.messages.lock().unwrap();
/// locked_queue.push_back(msg);
/// drop(locked_queue);
///
/// // Send a notification to one of the calls currently
/// // waiting in a call to `recv`.
/// self.notify_on_sent.notify_one();
/// }
///
/// pub fn try_recv(&amp;self) -&gt; Option&lt;T&gt; {
/// let mut locked_queue = self.messages.lock().unwrap();
/// locked_queue.pop_front()
/// }
///
/// pub async fn recv(&amp;self) -&gt; T {
/// let future = self.notify_on_sent.notified();
/// tokio::pin!(future);
///
/// loop {
/// // Make sure that no wakeup is lost if we get
/// // `None` from `try_recv`.
/// future.as_mut().enable();
///
/// if let Some(msg) = self.try_recv() {
/// return msg;
/// }
///
/// // Wait for a call to `notify_one`.
/// //
/// // This uses `.as_mut()` to avoid consuming the future,
/// // which lets us call `Pin::set` below.
/// future.as_mut().await;
///
/// // Reset the future in case another call to
/// // `try_recv` got the message before us.
/// future.set(self.notify_on_sent.notified());
/// }
/// }
/// }
/// ```
///
/// [park]: std::thread::park
/// [unpark]: std::thread::Thread::unpark
/// [`notified().await`]: Notify::notified()
/// [`notify_one()`]: Notify::notify_one()
/// [`enable`]: Notified::enable()
/// [`Semaphore`]: crate::sync::Semaphore
</span><span class="attribute">#[derive(Debug)]
</span><span class="kw">pub struct </span>Notify {
<span class="comment">// This uses 2 bits to store one of `EMPTY`,
// `WAITING` or `NOTIFIED`. The rest of the bits
// are used to store the number of times `notify_waiters`
// was called.
</span>state: AtomicUsize,
waiters: Mutex&lt;WaitList&gt;,
}
<span class="attribute">#[derive(Debug, Clone, Copy)]
</span><span class="kw">enum </span>NotificationType {
<span class="comment">// Notification triggered by calling `notify_waiters`
</span>AllWaiters,
<span class="comment">// Notification triggered by calling `notify_one`
</span>OneWaiter,
}
<span class="attribute">#[derive(Debug)]
</span><span class="kw">struct </span>Waiter {
<span class="doccomment">/// Intrusive linked-list pointers.
</span>pointers: linked_list::Pointers&lt;Waiter&gt;,
<span class="doccomment">/// Waiting task&#39;s waker.
</span>waker: <span class="prelude-ty">Option</span>&lt;Waker&gt;,
<span class="doccomment">/// `true` if the notification has been assigned to this waiter.
</span>notified: <span class="prelude-ty">Option</span>&lt;NotificationType&gt;,
<span class="doccomment">/// Should not be `Unpin`.
</span>_p: PhantomPinned,
}
<span class="macro">generate_addr_of_methods! </span>{
<span class="kw">impl</span>&lt;&gt; Waiter {
<span class="kw">unsafe fn </span>addr_of_pointers(<span class="self">self</span>: NonNull&lt;<span class="self">Self</span>&gt;) -&gt; NonNull&lt;linked_list::Pointers&lt;Waiter&gt;&gt; {
<span class="kw-2">&amp;</span><span class="self">self</span>.pointers
}
}
}
<span class="doccomment">/// Future returned from [`Notify::notified()`].
///
/// This future is fused, so once it has completed, any future calls to poll
/// will immediately return `Poll::Ready`.
</span><span class="attribute">#[derive(Debug)]
</span><span class="kw">pub struct </span>Notified&lt;<span class="lifetime">&#39;a</span>&gt; {
<span class="doccomment">/// The `Notify` being received on.
</span>notify: <span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>Notify,
<span class="doccomment">/// The current state of the receiving process.
</span>state: State,
<span class="doccomment">/// Entry in the waiter `LinkedList`.
</span>waiter: UnsafeCell&lt;Waiter&gt;,
}
<span class="kw">unsafe impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; Send <span class="kw">for </span>Notified&lt;<span class="lifetime">&#39;a</span>&gt; {}
<span class="kw">unsafe impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; Sync <span class="kw">for </span>Notified&lt;<span class="lifetime">&#39;a</span>&gt; {}
<span class="attribute">#[derive(Debug)]
</span><span class="kw">enum </span>State {
Init(usize),
Waiting,
Done,
}
<span class="kw">const </span>NOTIFY_WAITERS_SHIFT: usize = <span class="number">2</span>;
<span class="kw">const </span>STATE_MASK: usize = (<span class="number">1 </span>&lt;&lt; NOTIFY_WAITERS_SHIFT) - <span class="number">1</span>;
<span class="kw">const </span>NOTIFY_WAITERS_CALLS_MASK: usize = !STATE_MASK;
<span class="doccomment">/// Initial &quot;idle&quot; state.
</span><span class="kw">const </span>EMPTY: usize = <span class="number">0</span>;
<span class="doccomment">/// One or more threads are currently waiting to be notified.
</span><span class="kw">const </span>WAITING: usize = <span class="number">1</span>;
<span class="doccomment">/// Pending notification.
</span><span class="kw">const </span>NOTIFIED: usize = <span class="number">2</span>;
<span class="kw">fn </span>set_state(data: usize, state: usize) -&gt; usize {
(data &amp; NOTIFY_WAITERS_CALLS_MASK) | (state &amp; STATE_MASK)
}
<span class="kw">fn </span>get_state(data: usize) -&gt; usize {
data &amp; STATE_MASK
}
<span class="kw">fn </span>get_num_notify_waiters_calls(data: usize) -&gt; usize {
(data &amp; NOTIFY_WAITERS_CALLS_MASK) &gt;&gt; NOTIFY_WAITERS_SHIFT
}
<span class="kw">fn </span>inc_num_notify_waiters_calls(data: usize) -&gt; usize {
data + (<span class="number">1 </span>&lt;&lt; NOTIFY_WAITERS_SHIFT)
}
<span class="kw">fn </span>atomic_inc_num_notify_waiters_calls(data: <span class="kw-2">&amp;</span>AtomicUsize) {
data.fetch_add(<span class="number">1 </span>&lt;&lt; NOTIFY_WAITERS_SHIFT, SeqCst);
}
<span class="kw">impl </span>Notify {
<span class="doccomment">/// Create a new `Notify`, initialized without a permit.
///
/// # Examples
///
/// ```
/// use tokio::sync::Notify;
///
/// let notify = Notify::new();
/// ```
</span><span class="kw">pub fn </span>new() -&gt; Notify {
Notify {
state: AtomicUsize::new(<span class="number">0</span>),
waiters: Mutex::new(LinkedList::new()),
}
}
<span class="doccomment">/// Create a new `Notify`, initialized without a permit.
///
/// # Examples
///
/// ```
/// use tokio::sync::Notify;
///
/// static NOTIFY: Notify = Notify::const_new();
/// ```
</span><span class="attribute">#[cfg(all(feature = <span class="string">&quot;parking_lot&quot;</span>, not(all(loom, test))))]
#[cfg_attr(docsrs, doc(cfg(feature = <span class="string">&quot;parking_lot&quot;</span>)))]
</span><span class="kw">pub const fn </span>const_new() -&gt; Notify {
Notify {
state: AtomicUsize::new(<span class="number">0</span>),
waiters: Mutex::const_new(LinkedList::new()),
}
}
<span class="doccomment">/// Wait for a notification.
///
/// Equivalent to:
///
/// ```ignore
/// async fn notified(&amp;self);
/// ```
///
/// Each `Notify` value holds a single permit. If a permit is available from
/// an earlier call to [`notify_one()`], then `notified().await` will complete
/// immediately, consuming that permit. Otherwise, `notified().await` waits
/// for a permit to be made available by the next call to `notify_one()`.
///
/// The `Notified` future is not guaranteed to receive wakeups from calls to
/// `notify_one()` if it has not yet been polled. See the documentation for
/// [`Notified::enable()`] for more details.
///
/// The `Notified` future is guaranteed to receive wakeups from
/// `notify_waiters()` as soon as it has been created, even if it has not
/// yet been polled.
///
/// [`notify_one()`]: Notify::notify_one
/// [`Notified::enable()`]: Notified::enable
///
/// # Cancel safety
///
/// This method uses a queue to fairly distribute notifications in the order
/// they were requested. Cancelling a call to `notified` makes you lose your
/// place in the queue.
///
/// # Examples
///
/// ```
/// use tokio::sync::Notify;
/// use std::sync::Arc;
///
/// #[tokio::main]
/// async fn main() {
/// let notify = Arc::new(Notify::new());
/// let notify2 = notify.clone();
///
/// tokio::spawn(async move {
/// notify2.notified().await;
/// println!(&quot;received notification&quot;);
/// });
///
/// println!(&quot;sending notification&quot;);
/// notify.notify_one();
/// }
/// ```
</span><span class="kw">pub fn </span>notified(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; Notified&lt;<span class="lifetime">&#39;_</span>&gt; {
<span class="comment">// we load the number of times notify_waiters
// was called and store that in our initial state
</span><span class="kw">let </span>state = <span class="self">self</span>.state.load(SeqCst);
Notified {
notify: <span class="self">self</span>,
state: State::Init(state &gt;&gt; NOTIFY_WAITERS_SHIFT),
waiter: UnsafeCell::new(Waiter {
pointers: linked_list::Pointers::new(),
waker: <span class="prelude-val">None</span>,
notified: <span class="prelude-val">None</span>,
_p: PhantomPinned,
}),
}
}
<span class="doccomment">/// Notifies a waiting task.
///
/// If a task is currently waiting, that task is notified. Otherwise, a
/// permit is stored in this `Notify` value and the **next** call to
/// [`notified().await`] will complete immediately consuming the permit made
/// available by this call to `notify_one()`.
///
/// At most one permit may be stored by `Notify`. Many sequential calls to
/// `notify_one` will result in a single permit being stored. The next call to
/// `notified().await` will complete immediately, but the one after that
/// will wait.
///
/// [`notified().await`]: Notify::notified()
///
/// # Examples
///
/// ```
/// use tokio::sync::Notify;
/// use std::sync::Arc;
///
/// #[tokio::main]
/// async fn main() {
/// let notify = Arc::new(Notify::new());
/// let notify2 = notify.clone();
///
/// tokio::spawn(async move {
/// notify2.notified().await;
/// println!(&quot;received notification&quot;);
/// });
///
/// println!(&quot;sending notification&quot;);
/// notify.notify_one();
/// }
/// ```
</span><span class="comment">// Alias for old name in 0.x
</span><span class="attribute">#[cfg_attr(docsrs, doc(alias = <span class="string">&quot;notify&quot;</span>))]
</span><span class="kw">pub fn </span>notify_one(<span class="kw-2">&amp;</span><span class="self">self</span>) {
<span class="comment">// Load the current state
</span><span class="kw">let </span><span class="kw-2">mut </span>curr = <span class="self">self</span>.state.load(SeqCst);
<span class="comment">// If the state is `EMPTY`, transition to `NOTIFIED` and return.
</span><span class="kw">while let </span>EMPTY | NOTIFIED = get_state(curr) {
<span class="comment">// The compare-exchange from `NOTIFIED` -&gt; `NOTIFIED` is intended. A
// happens-before synchronization must happen between this atomic
// operation and a task calling `notified().await`.
</span><span class="kw">let </span>new = set_state(curr, NOTIFIED);
<span class="kw">let </span>res = <span class="self">self</span>.state.compare_exchange(curr, new, SeqCst, SeqCst);
<span class="kw">match </span>res {
<span class="comment">// No waiters, no further work to do
</span><span class="prelude-val">Ok</span>(<span class="kw">_</span>) =&gt; <span class="kw">return</span>,
<span class="prelude-val">Err</span>(actual) =&gt; {
curr = actual;
}
}
}
<span class="comment">// There are waiters, the lock must be acquired to notify.
</span><span class="kw">let </span><span class="kw-2">mut </span>waiters = <span class="self">self</span>.waiters.lock();
<span class="comment">// The state must be reloaded while the lock is held. The state may only
// transition out of WAITING while the lock is held.
</span>curr = <span class="self">self</span>.state.load(SeqCst);
<span class="kw">if let </span><span class="prelude-val">Some</span>(waker) = notify_locked(<span class="kw-2">&amp;mut </span>waiters, <span class="kw-2">&amp;</span><span class="self">self</span>.state, curr) {
drop(waiters);
waker.wake();
}
}
<span class="doccomment">/// Notifies all waiting tasks.
///
/// If a task is currently waiting, that task is notified. Unlike with
/// `notify_one()`, no permit is stored to be used by the next call to
/// `notified().await`. The purpose of this method is to notify all
/// already registered waiters. Registering for notification is done by
/// acquiring an instance of the `Notified` future via calling `notified()`.
///
/// # Examples
///
/// ```
/// use tokio::sync::Notify;
/// use std::sync::Arc;
///
/// #[tokio::main]
/// async fn main() {
/// let notify = Arc::new(Notify::new());
/// let notify2 = notify.clone();
///
/// let notified1 = notify.notified();
/// let notified2 = notify.notified();
///
/// let handle = tokio::spawn(async move {
/// println!(&quot;sending notifications&quot;);
/// notify2.notify_waiters();
/// });
///
/// notified1.await;
/// notified2.await;
/// println!(&quot;received notifications&quot;);
/// }
/// ```
</span><span class="kw">pub fn </span>notify_waiters(<span class="kw-2">&amp;</span><span class="self">self</span>) {
<span class="kw">let </span><span class="kw-2">mut </span>wakers = WakeList::new();
<span class="comment">// There are waiters, the lock must be acquired to notify.
</span><span class="kw">let </span><span class="kw-2">mut </span>waiters = <span class="self">self</span>.waiters.lock();
<span class="comment">// The state must be reloaded while the lock is held. The state may only
// transition out of WAITING while the lock is held.
</span><span class="kw">let </span>curr = <span class="self">self</span>.state.load(SeqCst);
<span class="kw">if </span><span class="macro">matches!</span>(get_state(curr), EMPTY | NOTIFIED) {
<span class="comment">// There are no waiting tasks. All we need to do is increment the
// number of times this method was called.
</span>atomic_inc_num_notify_waiters_calls(<span class="kw-2">&amp;</span><span class="self">self</span>.state);
<span class="kw">return</span>;
}
<span class="comment">// At this point, it is guaranteed that the state will not
// concurrently change, as holding the lock is required to
// transition **out** of `WAITING`.
</span><span class="lifetime">&#39;outer</span>: <span class="kw">loop </span>{
<span class="kw">while </span>wakers.can_push() {
<span class="kw">match </span>waiters.pop_back() {
<span class="prelude-val">Some</span>(<span class="kw-2">mut </span>waiter) =&gt; {
<span class="comment">// Safety: `waiters` lock is still held.
</span><span class="kw">let </span>waiter = <span class="kw">unsafe </span>{ waiter.as_mut() };
<span class="macro">assert!</span>(waiter.notified.is_none());
waiter.notified = <span class="prelude-val">Some</span>(NotificationType::AllWaiters);
<span class="kw">if let </span><span class="prelude-val">Some</span>(waker) = waiter.waker.take() {
wakers.push(waker);
}
}
<span class="prelude-val">None </span>=&gt; {
<span class="kw">break </span><span class="lifetime">&#39;outer</span>;
}
}
}
drop(waiters);
wakers.wake_all();
<span class="comment">// Acquire the lock again.
</span>waiters = <span class="self">self</span>.waiters.lock();
}
<span class="comment">// All waiters will be notified, the state must be transitioned to
// `EMPTY`. As transitioning **from** `WAITING` requires the lock to be
// held, a `store` is sufficient.
</span><span class="kw">let </span>new = set_state(inc_num_notify_waiters_calls(curr), EMPTY);
<span class="self">self</span>.state.store(new, SeqCst);
<span class="comment">// Release the lock before notifying
</span>drop(waiters);
wakers.wake_all();
}
}
<span class="kw">impl </span>Default <span class="kw">for </span>Notify {
<span class="kw">fn </span>default() -&gt; Notify {
Notify::new()
}
}
<span class="kw">impl </span>UnwindSafe <span class="kw">for </span>Notify {}
<span class="kw">impl </span>RefUnwindSafe <span class="kw">for </span>Notify {}
<span class="kw">fn </span>notify_locked(waiters: <span class="kw-2">&amp;mut </span>WaitList, state: <span class="kw-2">&amp;</span>AtomicUsize, curr: usize) -&gt; <span class="prelude-ty">Option</span>&lt;Waker&gt; {
<span class="kw">loop </span>{
<span class="kw">match </span>get_state(curr) {
EMPTY | NOTIFIED =&gt; {
<span class="kw">let </span>res = state.compare_exchange(curr, set_state(curr, NOTIFIED), SeqCst, SeqCst);
<span class="kw">match </span>res {
<span class="prelude-val">Ok</span>(<span class="kw">_</span>) =&gt; <span class="kw">return </span><span class="prelude-val">None</span>,
<span class="prelude-val">Err</span>(actual) =&gt; {
<span class="kw">let </span>actual_state = get_state(actual);
<span class="macro">assert!</span>(actual_state == EMPTY || actual_state == NOTIFIED);
state.store(set_state(actual, NOTIFIED), SeqCst);
<span class="kw">return </span><span class="prelude-val">None</span>;
}
}
}
WAITING =&gt; {
<span class="comment">// At this point, it is guaranteed that the state will not
// concurrently change as holding the lock is required to
// transition **out** of `WAITING`.
//
// Get a pending waiter
</span><span class="kw">let </span><span class="kw-2">mut </span>waiter = waiters.pop_back().unwrap();
<span class="comment">// Safety: `waiters` lock is still held.
</span><span class="kw">let </span>waiter = <span class="kw">unsafe </span>{ waiter.as_mut() };
<span class="macro">assert!</span>(waiter.notified.is_none());
waiter.notified = <span class="prelude-val">Some</span>(NotificationType::OneWaiter);
<span class="kw">let </span>waker = waiter.waker.take();
<span class="kw">if </span>waiters.is_empty() {
<span class="comment">// As this the **final** waiter in the list, the state
// must be transitioned to `EMPTY`. As transitioning
// **from** `WAITING` requires the lock to be held, a
// `store` is sufficient.
</span>state.store(set_state(curr, EMPTY), SeqCst);
}
<span class="kw">return </span>waker;
}
<span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
}
}
}
<span class="comment">// ===== impl Notified =====
</span><span class="kw">impl </span>Notified&lt;<span class="lifetime">&#39;_</span>&gt; {
<span class="doccomment">/// Adds this future to the list of futures that are ready to receive
/// wakeups from calls to [`notify_one`].
///
/// Polling the future also adds it to the list, so this method should only
/// be used if you want to add the future to the list before the first call
/// to `poll`. (In fact, this method is equivalent to calling `poll` except
/// that no `Waker` is registered.)
///
/// This has no effect on notifications sent using [`notify_waiters`], which
/// are received as long as they happen after the creation of the `Notified`
/// regardless of whether `enable` or `poll` has been called.
///
/// This method returns true if the `Notified` is ready. This happens in the
/// following situations:
///
/// 1. The `notify_waiters` method was called between the creation of the
/// `Notified` and the call to this method.
/// 2. This is the first call to `enable` or `poll` on this future, and the
/// `Notify` was holding a permit from a previous call to `notify_one`.
/// The call consumes the permit in that case.
/// 3. The future has previously been enabled or polled, and it has since
/// then been marked ready by either consuming a permit from the
/// `Notify`, or by a call to `notify_one` or `notify_waiters` that
/// removed it from the list of futures ready to receive wakeups.
///
/// If this method returns true, any future calls to poll on the same future
/// will immediately return `Poll::Ready`.
///
/// # Examples
///
/// Unbound multi-producer multi-consumer (mpmc) channel.
///
/// The call to `enable` is important because otherwise if you have two
/// calls to `recv` and two calls to `send` in parallel, the following could
/// happen:
///
/// 1. Both calls to `try_recv` return `None`.
/// 2. Both new elements are added to the vector.
/// 3. The `notify_one` method is called twice, adding only a single
/// permit to the `Notify`.
/// 4. Both calls to `recv` reach the `Notified` future. One of them
/// consumes the permit, and the other sleeps forever.
///
/// By adding the `Notified` futures to the list by calling `enable` before
/// `try_recv`, the `notify_one` calls in step three would remove the
/// futures from the list and mark them notified instead of adding a permit
/// to the `Notify`. This ensures that both futures are woken.
///
/// ```
/// use tokio::sync::Notify;
///
/// use std::collections::VecDeque;
/// use std::sync::Mutex;
///
/// struct Channel&lt;T&gt; {
/// messages: Mutex&lt;VecDeque&lt;T&gt;&gt;,
/// notify_on_sent: Notify,
/// }
///
/// impl&lt;T&gt; Channel&lt;T&gt; {
/// pub fn send(&amp;self, msg: T) {
/// let mut locked_queue = self.messages.lock().unwrap();
/// locked_queue.push_back(msg);
/// drop(locked_queue);
///
/// // Send a notification to one of the calls currently
/// // waiting in a call to `recv`.
/// self.notify_on_sent.notify_one();
/// }
///
/// pub fn try_recv(&amp;self) -&gt; Option&lt;T&gt; {
/// let mut locked_queue = self.messages.lock().unwrap();
/// locked_queue.pop_front()
/// }
///
/// pub async fn recv(&amp;self) -&gt; T {
/// let future = self.notify_on_sent.notified();
/// tokio::pin!(future);
///
/// loop {
/// // Make sure that no wakeup is lost if we get
/// // `None` from `try_recv`.
/// future.as_mut().enable();
///
/// if let Some(msg) = self.try_recv() {
/// return msg;
/// }
///
/// // Wait for a call to `notify_one`.
/// //
/// // This uses `.as_mut()` to avoid consuming the future,
/// // which lets us call `Pin::set` below.
/// future.as_mut().await;
///
/// // Reset the future in case another call to
/// // `try_recv` got the message before us.
/// future.set(self.notify_on_sent.notified());
/// }
/// }
/// }
/// ```
///
/// [`notify_one`]: Notify::notify_one()
/// [`notify_waiters`]: Notify::notify_waiters()
</span><span class="kw">pub fn </span>enable(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;) -&gt; bool {
<span class="self">self</span>.poll_notified(<span class="prelude-val">None</span>).is_ready()
}
<span class="doccomment">/// A custom `project` implementation is used in place of `pin-project-lite`
/// as a custom drop implementation is needed.
</span><span class="kw">fn </span>project(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;) -&gt; (<span class="kw-2">&amp;</span>Notify, <span class="kw-2">&amp;mut </span>State, <span class="kw-2">&amp;</span>UnsafeCell&lt;Waiter&gt;) {
<span class="kw">unsafe </span>{
<span class="comment">// Safety: both `notify` and `state` are `Unpin`.
</span>is_unpin::&lt;<span class="kw-2">&amp;</span>Notify&gt;();
is_unpin::&lt;AtomicUsize&gt;();
<span class="kw">let </span>me = <span class="self">self</span>.get_unchecked_mut();
(me.notify, <span class="kw-2">&amp;mut </span>me.state, <span class="kw-2">&amp;</span>me.waiter)
}
}
<span class="kw">fn </span>poll_notified(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, waker: <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;</span>Waker&gt;) -&gt; Poll&lt;()&gt; {
<span class="kw">use </span>State::<span class="kw-2">*</span>;
<span class="kw">let </span>(notify, state, waiter) = <span class="self">self</span>.project();
<span class="kw">loop </span>{
<span class="kw">match </span><span class="kw-2">*</span>state {
Init(initial_notify_waiters_calls) =&gt; {
<span class="kw">let </span>curr = notify.state.load(SeqCst);
<span class="comment">// Optimistically try acquiring a pending notification
</span><span class="kw">let </span>res = notify.state.compare_exchange(
set_state(curr, NOTIFIED),
set_state(curr, EMPTY),
SeqCst,
SeqCst,
);
<span class="kw">if </span>res.is_ok() {
<span class="comment">// Acquired the notification
</span><span class="kw-2">*</span>state = Done;
<span class="kw">return </span>Poll::Ready(());
}
<span class="comment">// Clone the waker before locking, a waker clone can be
// triggering arbitrary code.
</span><span class="kw">let </span>waker = waker.cloned();
<span class="comment">// Acquire the lock and attempt to transition to the waiting
// state.
</span><span class="kw">let </span><span class="kw-2">mut </span>waiters = notify.waiters.lock();
<span class="comment">// Reload the state with the lock held
</span><span class="kw">let </span><span class="kw-2">mut </span>curr = notify.state.load(SeqCst);
<span class="comment">// if notify_waiters has been called after the future
// was created, then we are done
</span><span class="kw">if </span>get_num_notify_waiters_calls(curr) != initial_notify_waiters_calls {
<span class="kw-2">*</span>state = Done;
<span class="kw">return </span>Poll::Ready(());
}
<span class="comment">// Transition the state to WAITING.
</span><span class="kw">loop </span>{
<span class="kw">match </span>get_state(curr) {
EMPTY =&gt; {
<span class="comment">// Transition to WAITING
</span><span class="kw">let </span>res = notify.state.compare_exchange(
set_state(curr, EMPTY),
set_state(curr, WAITING),
SeqCst,
SeqCst,
);
<span class="kw">if let </span><span class="prelude-val">Err</span>(actual) = res {
<span class="macro">assert_eq!</span>(get_state(actual), NOTIFIED);
curr = actual;
} <span class="kw">else </span>{
<span class="kw">break</span>;
}
}
WAITING =&gt; <span class="kw">break</span>,
NOTIFIED =&gt; {
<span class="comment">// Try consuming the notification
</span><span class="kw">let </span>res = notify.state.compare_exchange(
set_state(curr, NOTIFIED),
set_state(curr, EMPTY),
SeqCst,
SeqCst,
);
<span class="kw">match </span>res {
<span class="prelude-val">Ok</span>(<span class="kw">_</span>) =&gt; {
<span class="comment">// Acquired the notification
</span><span class="kw-2">*</span>state = Done;
<span class="kw">return </span>Poll::Ready(());
}
<span class="prelude-val">Err</span>(actual) =&gt; {
<span class="macro">assert_eq!</span>(get_state(actual), EMPTY);
curr = actual;
}
}
}
<span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
}
}
<span class="kw">if </span>waker.is_some() {
<span class="comment">// Safety: called while locked.
</span><span class="kw">unsafe </span>{
(<span class="kw-2">*</span>waiter.get()).waker = waker;
}
}
<span class="comment">// Insert the waiter into the linked list
//
// safety: pointers from `UnsafeCell` are never null.
</span>waiters.push_front(<span class="kw">unsafe </span>{ NonNull::new_unchecked(waiter.get()) });
<span class="kw-2">*</span>state = Waiting;
<span class="kw">return </span>Poll::Pending;
}
Waiting =&gt; {
<span class="comment">// Currently in the &quot;Waiting&quot; state, implying the caller has
// a waiter stored in the waiter list (guarded by
// `notify.waiters`). In order to access the waker fields,
// we must hold the lock.
</span><span class="kw">let </span>waiters = notify.waiters.lock();
<span class="comment">// Safety: called while locked
</span><span class="kw">let </span>w = <span class="kw">unsafe </span>{ <span class="kw-2">&amp;mut *</span>waiter.get() };
<span class="kw">if </span>w.notified.is_some() {
<span class="comment">// Our waker has been notified. Reset the fields and
// remove it from the list.
</span>w.waker = <span class="prelude-val">None</span>;
w.notified = <span class="prelude-val">None</span>;
<span class="kw-2">*</span>state = Done;
} <span class="kw">else </span>{
<span class="comment">// Update the waker, if necessary.
</span><span class="kw">if let </span><span class="prelude-val">Some</span>(waker) = waker {
<span class="kw">let </span>should_update = <span class="kw">match </span>w.waker.as_ref() {
<span class="prelude-val">Some</span>(current_waker) =&gt; !current_waker.will_wake(waker),
<span class="prelude-val">None </span>=&gt; <span class="bool-val">true</span>,
};
<span class="kw">if </span>should_update {
w.waker = <span class="prelude-val">Some</span>(waker.clone());
}
}
<span class="kw">return </span>Poll::Pending;
}
<span class="comment">// Explicit drop of the lock to indicate the scope that the
// lock is held. Because holding the lock is required to
// ensure safe access to fields not held within the lock, it
// is helpful to visualize the scope of the critical
// section.
</span>drop(waiters);
}
Done =&gt; {
<span class="kw">return </span>Poll::Ready(());
}
}
}
}
}
<span class="kw">impl </span>Future <span class="kw">for </span>Notified&lt;<span class="lifetime">&#39;_</span>&gt; {
<span class="kw">type </span>Output = ();
<span class="kw">fn </span>poll(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; Poll&lt;()&gt; {
<span class="self">self</span>.poll_notified(<span class="prelude-val">Some</span>(cx.waker()))
}
}
<span class="kw">impl </span>Drop <span class="kw">for </span>Notified&lt;<span class="lifetime">&#39;_</span>&gt; {
<span class="kw">fn </span>drop(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
<span class="kw">use </span>State::<span class="kw-2">*</span>;
<span class="comment">// Safety: The type only transitions to a &quot;Waiting&quot; state when pinned.
</span><span class="kw">let </span>(notify, state, waiter) = <span class="kw">unsafe </span>{ Pin::new_unchecked(<span class="self">self</span>).project() };
<span class="comment">// This is where we ensure safety. The `Notified` value is being
// dropped, which means we must ensure that the waiter entry is no
// longer stored in the linked list.
</span><span class="kw">if </span><span class="macro">matches!</span>(<span class="kw-2">*</span>state, Waiting) {
<span class="kw">let </span><span class="kw-2">mut </span>waiters = notify.waiters.lock();
<span class="kw">let </span><span class="kw-2">mut </span>notify_state = notify.state.load(SeqCst);
<span class="comment">// remove the entry from the list (if not already removed)
//
// safety: the waiter is only added to `waiters` by virtue of it
// being the only `LinkedList` available to the type.
</span><span class="kw">unsafe </span>{ waiters.remove(NonNull::new_unchecked(waiter.get())) };
<span class="kw">if </span>waiters.is_empty() &amp;&amp; get_state(notify_state) == WAITING {
notify_state = set_state(notify_state, EMPTY);
notify.state.store(notify_state, SeqCst);
}
<span class="comment">// See if the node was notified but not received. In this case, if
// the notification was triggered via `notify_one`, it must be sent
// to the next waiter.
//
// Safety: with the entry removed from the linked list, there can be
// no concurrent access to the entry
</span><span class="kw">if </span><span class="macro">matches!</span>(
<span class="kw">unsafe </span>{ (<span class="kw-2">*</span>waiter.get()).notified },
<span class="prelude-val">Some</span>(NotificationType::OneWaiter)
) {
<span class="kw">if let </span><span class="prelude-val">Some</span>(waker) = notify_locked(<span class="kw-2">&amp;mut </span>waiters, <span class="kw-2">&amp;</span>notify.state, notify_state) {
drop(waiters);
waker.wake();
}
}
}
}
}
<span class="doccomment">/// # Safety
///
/// `Waiter` is forced to be !Unpin.
</span><span class="kw">unsafe impl </span>linked_list::Link <span class="kw">for </span>Waiter {
<span class="kw">type </span>Handle = NonNull&lt;Waiter&gt;;
<span class="kw">type </span>Target = Waiter;
<span class="kw">fn </span>as_raw(handle: <span class="kw-2">&amp;</span>NonNull&lt;Waiter&gt;) -&gt; NonNull&lt;Waiter&gt; {
<span class="kw-2">*</span>handle
}
<span class="kw">unsafe fn </span>from_raw(ptr: NonNull&lt;Waiter&gt;) -&gt; NonNull&lt;Waiter&gt; {
ptr
}
<span class="kw">unsafe fn </span>pointers(target: NonNull&lt;Waiter&gt;) -&gt; NonNull&lt;linked_list::Pointers&lt;Waiter&gt;&gt; {
Waiter::addr_of_pointers(target)
}
}
<span class="kw">fn </span>is_unpin&lt;T: Unpin&gt;() {}
</code></pre></div>
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