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337 lines
9.7 KiB
Rust
337 lines
9.7 KiB
Rust
use std::cell::UnsafeCell;
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use std::fmt;
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use std::future::Future;
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use std::ops::{Deref, DerefMut};
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use std::pin::Pin;
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use std::sync::atomic::{AtomicUsize, Ordering};
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use std::task::{Context, Poll, Waker};
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use slab::Slab;
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/// Set if the mutex is locked.
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const LOCK: usize = 1 << 0;
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/// Set if there are tasks blocked on the mutex.
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const BLOCKED: usize = 1 << 1;
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/// A mutual exclusion primitive for protecting shared data.
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///
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/// This type is an async version of [`std::sync::Mutex`].
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///
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/// [`std::sync::Mutex`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html
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///
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/// # Examples
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///
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/// ```
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/// # #![feature(async_await)]
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/// use async_std::{sync::Mutex, task};
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/// use std::sync::Arc;
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///
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/// # futures::executor::block_on(async {
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/// let m = Arc::new(Mutex::new(0));
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/// let mut tasks = vec![];
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///
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/// for _ in 0..10 {
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/// let m = m.clone();
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/// tasks.push(task::spawn(async move {
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/// *m.lock().await += 1;
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/// }));
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/// }
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///
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/// for t in tasks {
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/// t.await;
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/// }
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/// assert_eq!(*m.lock().await, 10);
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/// # })
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/// ```
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pub struct Mutex<T> {
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state: AtomicUsize,
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blocked: std::sync::Mutex<Slab<Option<Waker>>>,
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value: UnsafeCell<T>,
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}
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unsafe impl<T: Send> Send for Mutex<T> {}
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unsafe impl<T: Send> Sync for Mutex<T> {}
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impl<T> Mutex<T> {
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/// Creates a new mutex.
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///
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/// # Examples
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///
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/// ```
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/// use async_std::sync::Mutex;
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///
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/// let mutex = Mutex::new(0);
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/// ```
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pub fn new(t: T) -> Mutex<T> {
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Mutex {
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state: AtomicUsize::new(0),
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blocked: std::sync::Mutex::new(Slab::new()),
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value: UnsafeCell::new(t),
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}
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}
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/// Acquires the lock.
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///
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/// Returns a guard that releases the lock when dropped.
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///
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/// # Examples
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///
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/// ```
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/// # #![feature(async_await)]
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/// use async_std::{sync::Mutex, task};
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/// use std::sync::Arc;
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///
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/// # futures::executor::block_on(async {
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/// let m1 = Arc::new(Mutex::new(10));
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/// let m2 = m1.clone();
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///
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/// task::spawn(async move {
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/// *m1.lock().await = 20;
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/// })
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/// .await;
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///
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/// assert_eq!(*m2.lock().await, 20);
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/// # })
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/// ```
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pub async fn lock(&self) -> MutexGuard<'_, T> {
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pub struct LockFuture<'a, T> {
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mutex: &'a Mutex<T>,
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opt_key: Option<usize>,
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acquired: bool,
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}
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impl<'a, T> Future for LockFuture<'a, T> {
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type Output = MutexGuard<'a, T>;
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fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
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match self.mutex.try_lock() {
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Some(guard) => {
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self.acquired = true;
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Poll::Ready(guard)
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}
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None => {
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let mut blocked = self.mutex.blocked.lock().unwrap();
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// Register the current task.
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match self.opt_key {
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None => {
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// Insert a new entry into the list of blocked tasks.
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let w = cx.waker().clone();
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let key = blocked.insert(Some(w));
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self.opt_key = Some(key);
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if blocked.len() == 1 {
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self.mutex.state.fetch_or(BLOCKED, Ordering::Relaxed);
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}
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}
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Some(key) => {
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// There is already an entry in the list of blocked tasks. Just
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// reset the waker if it was removed.
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if blocked[key].is_none() {
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let w = cx.waker().clone();
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blocked[key] = Some(w);
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}
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}
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}
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// Try locking again because it's possible the mutex got unlocked just
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// before the current task was registered as a blocked task.
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match self.mutex.try_lock() {
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Some(guard) => {
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self.acquired = true;
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Poll::Ready(guard)
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}
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None => Poll::Pending,
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}
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}
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}
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}
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}
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impl<T> Drop for LockFuture<'_, T> {
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fn drop(&mut self) {
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if let Some(key) = self.opt_key {
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let mut blocked = self.mutex.blocked.lock().unwrap();
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let opt_waker = blocked.remove(key);
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if opt_waker.is_none() && !self.acquired {
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// We were awoken but didn't acquire the lock. Wake up another task.
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if let Some((_, opt_waker)) = blocked.iter_mut().next() {
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if let Some(w) = opt_waker.take() {
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w.wake();
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}
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}
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}
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if blocked.is_empty() {
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self.mutex.state.fetch_and(!BLOCKED, Ordering::Relaxed);
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}
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}
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}
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}
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LockFuture {
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mutex: self,
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opt_key: None,
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acquired: false,
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}
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.await
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}
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/// Attempts to acquire the lock.
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///
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/// If the lock could not be acquired at this time, then [`None`] is returned. Otherwise, a
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/// guard is returned that releases the lock when dropped.
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///
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/// [`None`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.None
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///
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/// # Examples
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///
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/// ```
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/// # #![feature(async_await)]
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/// use async_std::{sync::Mutex, task};
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/// use std::sync::Arc;
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///
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/// # futures::executor::block_on(async {
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/// let m1 = Arc::new(Mutex::new(10));
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/// let m2 = m1.clone();
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///
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/// task::spawn(async move {
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/// if let Some(mut guard) = m1.try_lock() {
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/// *guard = 20;
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/// } else {
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/// println!("try_lock failed");
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/// }
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/// })
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/// .await;
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///
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/// assert_eq!(*m2.lock().await, 20);
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/// # })
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/// ```
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pub fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
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if self.state.fetch_or(LOCK, Ordering::Acquire) & LOCK == 0 {
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Some(MutexGuard(self))
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} else {
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None
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}
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}
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/// Consumes the mutex, returning the underlying data.
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///
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/// # Examples
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///
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/// ```
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/// # #![feature(async_await)]
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/// use async_std::sync::Mutex;
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///
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/// let mutex = Mutex::new(10);
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/// assert_eq!(mutex.into_inner(), 10);
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/// ```
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pub fn into_inner(self) -> T {
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self.value.into_inner()
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}
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/// Returns a mutable reference to the underlying data.
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///
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/// Since this call borrows the mutex mutably, no actual locking takes place -- the mutable
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/// borrow statically guarantees no locks exist.
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///
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/// # Examples
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///
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/// ```
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/// # #![feature(async_await)]
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/// use async_std::sync::Mutex;
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///
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/// # futures::executor::block_on(async {
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/// let mut mutex = Mutex::new(0);
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/// *mutex.get_mut() = 10;
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/// assert_eq!(*mutex.lock().await, 10);
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/// });
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/// ```
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pub fn get_mut(&mut self) -> &mut T {
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unsafe { &mut *self.value.get() }
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}
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}
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impl<T: fmt::Debug> fmt::Debug for Mutex<T> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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match self.try_lock() {
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None => {
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struct LockedPlaceholder;
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impl fmt::Debug for LockedPlaceholder {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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f.write_str("<locked>")
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}
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}
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f.debug_struct("Mutex")
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.field("data", &LockedPlaceholder)
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.finish()
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}
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Some(guard) => f.debug_struct("Mutex").field("data", &&*guard).finish(),
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}
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}
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}
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impl<T> From<T> for Mutex<T> {
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fn from(val: T) -> Mutex<T> {
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Mutex::new(val)
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}
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}
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impl<T: Default> Default for Mutex<T> {
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fn default() -> Mutex<T> {
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Mutex::new(Default::default())
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}
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}
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/// A guard that releases the lock when dropped.
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pub struct MutexGuard<'a, T>(&'a Mutex<T>);
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unsafe impl<T: Send> Send for MutexGuard<'_, T> {}
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unsafe impl<T: Sync> Sync for MutexGuard<'_, T> {}
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impl<T> Drop for MutexGuard<'_, T> {
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fn drop(&mut self) {
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let state = self.0.state.fetch_and(!LOCK, Ordering::AcqRel);
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// If there are any blocked tasks, wake one of them up.
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if state & BLOCKED != 0 {
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let mut blocked = self.0.blocked.lock().unwrap();
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if let Some((_, opt_waker)) = blocked.iter_mut().next() {
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// If there is no waker in this entry, that means it was already woken.
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if let Some(w) = opt_waker.take() {
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w.wake();
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}
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}
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}
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}
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}
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impl<T: fmt::Debug> fmt::Debug for MutexGuard<'_, T> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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fmt::Debug::fmt(&**self, f)
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}
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}
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impl<T: fmt::Display> fmt::Display for MutexGuard<'_, T> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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(**self).fmt(f)
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}
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}
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impl<T> Deref for MutexGuard<'_, T> {
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type Target = T;
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fn deref(&self) -> &T {
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unsafe { &*self.0.value.get() }
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}
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}
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impl<T> DerefMut for MutexGuard<'_, T> {
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fn deref_mut(&mut self) -> &mut T {
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unsafe { &mut *self.0.value.get() }
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}
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}
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