313 lines
9.8 KiB
Rust
313 lines
9.8 KiB
Rust
// Copyright 2016 Amanieu d'Antras
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//
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// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
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// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
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// http://opensource.org/licenses/MIT>, at your option. This file may not be
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// copied, modified, or distributed except according to those terms.
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use crate::raw_mutex::RawMutex;
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use lock_api;
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/// A mutual exclusion primitive useful for protecting shared data
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///
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/// This mutex will block threads waiting for the lock to become available. The
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/// mutex can be statically initialized or created by the `new`
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/// constructor. Each mutex has a type parameter which represents the data that
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/// it is protecting. The data can only be accessed through the RAII guards
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/// returned from `lock` and `try_lock`, which guarantees that the data is only
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/// ever accessed when the mutex is locked.
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///
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/// # Fairness
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///
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/// A typical unfair lock can often end up in a situation where a single thread
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/// quickly acquires and releases the same mutex in succession, which can starve
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/// other threads waiting to acquire the mutex. While this improves throughput
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/// because it doesn't force a context switch when a thread tries to re-acquire
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/// a mutex it has just released, this can starve other threads.
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///
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/// This mutex uses [eventual fairness](https://trac.webkit.org/changeset/203350)
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/// to ensure that the lock will be fair on average without sacrificing
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/// throughput. This is done by forcing a fair unlock on average every 0.5ms,
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/// which will force the lock to go to the next thread waiting for the mutex.
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///
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/// Additionally, any critical section longer than 1ms will always use a fair
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/// unlock, which has a negligible impact on throughput considering the length
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/// of the critical section.
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///
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/// You can also force a fair unlock by calling `MutexGuard::unlock_fair` when
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/// unlocking a mutex instead of simply dropping the `MutexGuard`.
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///
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/// # Differences from the standard library `Mutex`
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///
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/// - No poisoning, the lock is released normally on panic.
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/// - Only requires 1 byte of space, whereas the standard library boxes the
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/// `Mutex` due to platform limitations.
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/// - Can be statically constructed.
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/// - Does not require any drop glue when dropped.
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/// - Inline fast path for the uncontended case.
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/// - Efficient handling of micro-contention using adaptive spinning.
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/// - Allows raw locking & unlocking without a guard.
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/// - Supports eventual fairness so that the mutex is fair on average.
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/// - Optionally allows making the mutex fair by calling `MutexGuard::unlock_fair`.
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///
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/// # Examples
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///
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/// ```
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/// use parking_lot::Mutex;
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/// use std::sync::{Arc, mpsc::channel};
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/// use std::thread;
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///
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/// const N: usize = 10;
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///
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/// // Spawn a few threads to increment a shared variable (non-atomically), and
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/// // let the main thread know once all increments are done.
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/// //
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/// // Here we're using an Arc to share memory among threads, and the data inside
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/// // the Arc is protected with a mutex.
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/// let data = Arc::new(Mutex::new(0));
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///
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/// let (tx, rx) = channel();
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/// for _ in 0..10 {
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/// let (data, tx) = (Arc::clone(&data), tx.clone());
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/// thread::spawn(move || {
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/// // The shared state can only be accessed once the lock is held.
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/// // Our non-atomic increment is safe because we're the only thread
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/// // which can access the shared state when the lock is held.
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/// let mut data = data.lock();
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/// *data += 1;
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/// if *data == N {
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/// tx.send(()).unwrap();
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/// }
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/// // the lock is unlocked here when `data` goes out of scope.
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/// });
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/// }
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///
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/// rx.recv().unwrap();
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/// ```
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pub type Mutex<T> = lock_api::Mutex<RawMutex, T>;
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/// Creates a new mutex in an unlocked state ready for use.
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///
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/// This allows creating a mutex in a constant context on stable Rust.
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pub const fn const_mutex<T>(val: T) -> Mutex<T> {
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Mutex::const_new(<RawMutex as lock_api::RawMutex>::INIT, val)
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}
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/// An RAII implementation of a "scoped lock" of a mutex. When this structure is
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/// dropped (falls out of scope), the lock will be unlocked.
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///
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/// The data protected by the mutex can be accessed through this guard via its
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/// `Deref` and `DerefMut` implementations.
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pub type MutexGuard<'a, T> = lock_api::MutexGuard<'a, RawMutex, T>;
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/// An RAII mutex guard returned by `MutexGuard::map`, which can point to a
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/// subfield of the protected data.
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///
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/// The main difference between `MappedMutexGuard` and `MutexGuard` is that the
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/// former doesn't support temporarily unlocking and re-locking, since that
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/// could introduce soundness issues if the locked object is modified by another
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/// thread.
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pub type MappedMutexGuard<'a, T> = lock_api::MappedMutexGuard<'a, RawMutex, T>;
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#[cfg(test)]
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mod tests {
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use crate::{Condvar, Mutex};
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use std::sync::atomic::{AtomicUsize, Ordering};
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use std::sync::mpsc::channel;
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use std::sync::Arc;
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use std::thread;
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#[cfg(feature = "serde")]
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use bincode::{deserialize, serialize};
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struct Packet<T>(Arc<(Mutex<T>, Condvar)>);
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#[derive(Eq, PartialEq, Debug)]
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struct NonCopy(i32);
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unsafe impl<T: Send> Send for Packet<T> {}
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unsafe impl<T> Sync for Packet<T> {}
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#[test]
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fn smoke() {
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let m = Mutex::new(());
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drop(m.lock());
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drop(m.lock());
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}
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#[test]
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fn lots_and_lots() {
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const J: u32 = 1000;
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const K: u32 = 3;
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let m = Arc::new(Mutex::new(0));
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fn inc(m: &Mutex<u32>) {
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for _ in 0..J {
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*m.lock() += 1;
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}
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}
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let (tx, rx) = channel();
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for _ in 0..K {
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let tx2 = tx.clone();
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let m2 = m.clone();
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thread::spawn(move || {
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inc(&m2);
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tx2.send(()).unwrap();
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});
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let tx2 = tx.clone();
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let m2 = m.clone();
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thread::spawn(move || {
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inc(&m2);
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tx2.send(()).unwrap();
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});
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}
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drop(tx);
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for _ in 0..2 * K {
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rx.recv().unwrap();
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}
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assert_eq!(*m.lock(), J * K * 2);
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}
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#[test]
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fn try_lock() {
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let m = Mutex::new(());
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*m.try_lock().unwrap() = ();
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}
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#[test]
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fn test_into_inner() {
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let m = Mutex::new(NonCopy(10));
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assert_eq!(m.into_inner(), NonCopy(10));
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}
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#[test]
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fn test_into_inner_drop() {
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struct Foo(Arc<AtomicUsize>);
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impl Drop for Foo {
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fn drop(&mut self) {
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self.0.fetch_add(1, Ordering::SeqCst);
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}
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}
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let num_drops = Arc::new(AtomicUsize::new(0));
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let m = Mutex::new(Foo(num_drops.clone()));
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assert_eq!(num_drops.load(Ordering::SeqCst), 0);
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{
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let _inner = m.into_inner();
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assert_eq!(num_drops.load(Ordering::SeqCst), 0);
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}
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assert_eq!(num_drops.load(Ordering::SeqCst), 1);
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}
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#[test]
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fn test_get_mut() {
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let mut m = Mutex::new(NonCopy(10));
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*m.get_mut() = NonCopy(20);
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assert_eq!(m.into_inner(), NonCopy(20));
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}
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#[test]
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fn test_mutex_arc_condvar() {
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let packet = Packet(Arc::new((Mutex::new(false), Condvar::new())));
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let packet2 = Packet(packet.0.clone());
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let (tx, rx) = channel();
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let _t = thread::spawn(move || {
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// wait until parent gets in
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rx.recv().unwrap();
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let &(ref lock, ref cvar) = &*packet2.0;
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let mut lock = lock.lock();
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*lock = true;
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cvar.notify_one();
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});
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let &(ref lock, ref cvar) = &*packet.0;
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let mut lock = lock.lock();
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tx.send(()).unwrap();
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assert!(!*lock);
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while !*lock {
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cvar.wait(&mut lock);
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}
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}
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#[test]
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fn test_mutex_arc_nested() {
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// Tests nested mutexes and access
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// to underlying data.
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let arc = Arc::new(Mutex::new(1));
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let arc2 = Arc::new(Mutex::new(arc));
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let (tx, rx) = channel();
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let _t = thread::spawn(move || {
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let lock = arc2.lock();
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let lock2 = lock.lock();
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assert_eq!(*lock2, 1);
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tx.send(()).unwrap();
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});
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rx.recv().unwrap();
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}
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#[test]
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fn test_mutex_arc_access_in_unwind() {
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let arc = Arc::new(Mutex::new(1));
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let arc2 = arc.clone();
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let _ = thread::spawn(move || {
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struct Unwinder {
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i: Arc<Mutex<i32>>,
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}
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impl Drop for Unwinder {
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fn drop(&mut self) {
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*self.i.lock() += 1;
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}
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}
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let _u = Unwinder { i: arc2 };
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panic!();
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})
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.join();
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let lock = arc.lock();
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assert_eq!(*lock, 2);
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}
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#[test]
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fn test_mutex_unsized() {
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let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
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{
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let b = &mut *mutex.lock();
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b[0] = 4;
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b[2] = 5;
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}
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let comp: &[i32] = &[4, 2, 5];
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assert_eq!(&*mutex.lock(), comp);
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}
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#[test]
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fn test_mutexguard_sync() {
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fn sync<T: Sync>(_: T) {}
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let mutex = Mutex::new(());
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sync(mutex.lock());
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}
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#[test]
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fn test_mutex_debug() {
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let mutex = Mutex::new(vec![0u8, 10]);
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assert_eq!(format!("{:?}", mutex), "Mutex { data: [0, 10] }");
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let _lock = mutex.lock();
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assert_eq!(format!("{:?}", mutex), "Mutex { data: <locked> }");
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}
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#[cfg(feature = "serde")]
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#[test]
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fn test_serde() {
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let contents: Vec<u8> = vec![0, 1, 2];
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let mutex = Mutex::new(contents.clone());
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let serialized = serialize(&mutex).unwrap();
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let deserialized: Mutex<Vec<u8>> = deserialize(&serialized).unwrap();
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assert_eq!(*(mutex.lock()), *(deserialized.lock()));
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assert_eq!(contents, *(deserialized.lock()));
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}
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}
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