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use crate::cell::UnsafeCell;
use crate::fmt;
use crate::mem;
use crate::ops::{Deref, DerefMut};
use crate::ptr;
use crate::sys_common::mutex as sys;
use crate::sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
/// A mutual exclusion primitive useful for protecting shared data
///
/// This mutex will block threads waiting for the lock to become available. The
/// mutex can also be statically initialized or created via a [`new`]
/// constructor. Each mutex has a type parameter which represents the data that
/// it is protecting. The data can only be accessed through the RAII guards
/// returned from [`lock`] and [`try_lock`], which guarantees that the data is only
/// ever accessed when the mutex is locked.
///
/// # Poisoning
///
/// The mutexes in this module implement a strategy called "poisoning" where a
/// mutex is considered poisoned whenever a thread panics while holding the
/// mutex. Once a mutex is poisoned, all other threads are unable to access the
/// data by default as it is likely tainted (some invariant is not being
/// upheld).
///
/// For a mutex, this means that the [`lock`] and [`try_lock`] methods return a
/// [`Result`] which indicates whether a mutex has been poisoned or not. Most
/// usage of a mutex will simply [`unwrap()`] these results, propagating panics
/// among threads to ensure that a possibly invalid invariant is not witnessed.
///
/// A poisoned mutex, however, does not prevent all access to the underlying
/// data. The [`PoisonError`] type has an [`into_inner`] method which will return
/// the guard that would have otherwise been returned on a successful lock. This
/// allows access to the data, despite the lock being poisoned.
///
/// [`new`]: #method.new
/// [`lock`]: #method.lock
/// [`try_lock`]: #method.try_lock
/// [`Result`]: ../../std/result/enum.Result.html
/// [`unwrap()`]: ../../std/result/enum.Result.html#method.unwrap
/// [`PoisonError`]: ../../std/sync/struct.PoisonError.html
/// [`into_inner`]: ../../std/sync/struct.PoisonError.html#method.into_inner
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
/// use std::sync::mpsc::channel;
///
/// const N: usize = 10;
///
/// // Spawn a few threads to increment a shared variable (non-atomically), and
/// // let the main thread know once all increments are done.
/// //
/// // Here we're using an Arc to share memory among threads, and the data inside
/// // the Arc is protected with a mutex.
/// let data = Arc::new(Mutex::new(0));
///
/// let (tx, rx) = channel();
/// for _ in 0..N {
/// let (data, tx) = (Arc::clone(&data), tx.clone());
/// thread::spawn(move || {
/// // The shared state can only be accessed once the lock is held.
/// // Our non-atomic increment is safe because we're the only thread
/// // which can access the shared state when the lock is held.
/// //
/// // We unwrap() the return value to assert that we are not expecting
/// // threads to ever fail while holding the lock.
/// let mut data = data.lock().unwrap();
/// *data += 1;
/// if *data == N {
/// tx.send(()).unwrap();
/// }
/// // the lock is unlocked here when `data` goes out of scope.
/// });
/// }
///
/// rx.recv().unwrap();
/// ```
///
/// To recover from a poisoned mutex:
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
///
/// let lock = Arc::new(Mutex::new(0_u32));
/// let lock2 = lock.clone();
///
/// let _ = thread::spawn(move || -> () {
/// // This thread will acquire the mutex first, unwrapping the result of
/// // `lock` because the lock has not been poisoned.
/// let _guard = lock2.lock().unwrap();
///
/// // This panic while holding the lock (`_guard` is in scope) will poison
/// // the mutex.
/// panic!();
/// }).join();
///
/// // The lock is poisoned by this point, but the returned result can be
/// // pattern matched on to return the underlying guard on both branches.
/// let mut guard = match lock.lock() {
/// Ok(guard) => guard,
/// Err(poisoned) => poisoned.into_inner(),
/// };
///
/// *guard += 1;
/// ```
///
/// It is sometimes necessary to manually drop the mutex guard to unlock it
/// sooner than the end of the enclosing scope.
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
///
/// const N: usize = 3;
///
/// let data_mutex = Arc::new(Mutex::new(vec![1, 2, 3, 4]));
/// let res_mutex = Arc::new(Mutex::new(0));
///
/// let mut threads = Vec::with_capacity(N);
/// (0..N).for_each(|_| {
/// let data_mutex_clone = Arc::clone(&data_mutex);
/// let res_mutex_clone = Arc::clone(&res_mutex);
///
/// threads.push(thread::spawn(move || {
/// let mut data = data_mutex_clone.lock().unwrap();
/// // This is the result of some important and long-ish work.
/// let result = data.iter().fold(0, |acc, x| acc + x * 2);
/// data.push(result);
/// drop(data);
/// *res_mutex_clone.lock().unwrap() += result;
/// }));
/// });
///
/// let mut data = data_mutex.lock().unwrap();
/// // This is the result of some important and long-ish work.
/// let result = data.iter().fold(0, |acc, x| acc + x * 2);
/// data.push(result);
/// // We drop the `data` explicitly because it's not necessary anymore and the
/// // thread still has work to do. This allow other threads to start working on
/// // the data immediately, without waiting for the rest of the unrelated work
/// // to be done here.
/// //
/// // It's even more important here than in the threads because we `.join` the
/// // threads after that. If we had not dropped the mutex guard, a thread could
/// // be waiting forever for it, causing a deadlock.
/// drop(data);
/// // Here the mutex guard is not assigned to a variable and so, even if the
/// // scope does not end after this line, the mutex is still released: there is
/// // no deadlock.
/// *res_mutex.lock().unwrap() += result;
///
/// threads.into_iter().for_each(|thread| {
/// thread
/// .join()
/// .expect("The thread creating or execution failed !")
/// });
///
/// assert_eq!(*res_mutex.lock().unwrap(), 800);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "mutex_type")]
pub struct Mutex<T: ?Sized> {
// Note that this mutex is in a *box*, not inlined into the struct itself.
// Once a native mutex has been used once, its address can never change (it
// can't be moved). This mutex type can be safely moved at any time, so to
// ensure that the native mutex is used correctly we box the inner mutex to
// give it a constant address.
inner: Box<sys::Mutex>,
poison: poison::Flag,
data: UnsafeCell<T>,
}
// these are the only places where `T: Send` matters; all other
// functionality works fine on a single thread.
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
/// An RAII implementation of a "scoped lock" of a mutex. When this structure is
/// dropped (falls out of scope), the lock will be unlocked.
///
/// The data protected by the mutex can be accessed through this guard via its
/// [`Deref`] and [`DerefMut`] implementations.
///
/// This structure is created by the [`lock`] and [`try_lock`] methods on
/// [`Mutex`].
///
/// [`Deref`]: ../../std/ops/trait.Deref.html
/// [`DerefMut`]: ../../std/ops/trait.DerefMut.html
/// [`lock`]: struct.Mutex.html#method.lock
/// [`try_lock`]: struct.Mutex.html#method.try_lock
/// [`Mutex`]: struct.Mutex.html
#[must_use = "if unused the Mutex will immediately unlock"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct MutexGuard<'a, T: ?Sized + 'a> {
lock: &'a Mutex<T>,
poison: poison::Guard,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Send for MutexGuard<'_, T> {}
#[stable(feature = "mutexguard", since = "1.19.0")]
unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}
impl<T> Mutex<T> {
/// Creates a new mutex in an unlocked state ready for use.
///
/// # Examples
///
/// ```
/// use std::sync::Mutex;
///
/// let mutex = Mutex::new(0);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn new(t: T) -> Mutex<T> {
let mut m = Mutex {
inner: box sys::Mutex::new(),
poison: poison::Flag::new(),
data: UnsafeCell::new(t),
};
unsafe {
m.inner.init();
}
m
}
}
impl<T: ?Sized> Mutex<T> {
/// Acquires a mutex, blocking the current thread until it is able to do so.
///
/// This function will block the local thread until it is available to acquire
/// the mutex. Upon returning, the thread is the only thread with the lock
/// held. An RAII guard is returned to allow scoped unlock of the lock. When
/// the guard goes out of scope, the mutex will be unlocked.
///
/// The exact behavior on locking a mutex in the thread which already holds
/// the lock is left unspecified. However, this function will not return on
/// the second call (it might panic or deadlock, for example).
///
/// # Errors
///
/// If another user of this mutex panicked while holding the mutex, then
/// this call will return an error once the mutex is acquired.
///
/// # Panics
///
/// This function might panic when called if the lock is already held by
/// the current thread.
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
///
/// let mutex = Arc::new(Mutex::new(0));
/// let c_mutex = mutex.clone();
///
/// thread::spawn(move || {
/// *c_mutex.lock().unwrap() = 10;
/// }).join().expect("thread::spawn failed");
/// assert_eq!(*mutex.lock().unwrap(), 10);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn lock(&self) -> LockResult<MutexGuard<'_, T>> {
unsafe {
self.inner.raw_lock();
MutexGuard::new(self)
}
}
/// Attempts to acquire this lock.
///
/// If the lock could not be acquired at this time, then [`Err`] is returned.
/// Otherwise, an RAII guard is returned. The lock will be unlocked when the
/// guard is dropped.
///
/// This function does not block.
///
/// # Errors
///
/// If another user of this mutex panicked while holding the mutex, then
/// this call will return failure if the mutex would otherwise be
/// acquired.
///
/// [`Err`]: ../../std/result/enum.Result.html#variant.Err
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
///
/// let mutex = Arc::new(Mutex::new(0));
/// let c_mutex = mutex.clone();
///
/// thread::spawn(move || {
/// let mut lock = c_mutex.try_lock();
/// if let Ok(ref mut mutex) = lock {
/// **mutex = 10;
/// } else {
/// println!("try_lock failed");
/// }
/// }).join().expect("thread::spawn failed");
/// assert_eq!(*mutex.lock().unwrap(), 10);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn try_lock(&self) -> TryLockResult<MutexGuard<'_, T>> {
unsafe {
if self.inner.try_lock() {
Ok(MutexGuard::new(self)?)
} else {
Err(TryLockError::WouldBlock)
}
}
}
/// Determines whether the mutex is poisoned.
///
/// If another thread is active, the mutex can still become poisoned at any
/// time. You should not trust a `false` value for program correctness
/// without additional synchronization.
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
///
/// let mutex = Arc::new(Mutex::new(0));
/// let c_mutex = mutex.clone();
///
/// let _ = thread::spawn(move || {
/// let _lock = c_mutex.lock().unwrap();
/// panic!(); // the mutex gets poisoned
/// }).join();
/// assert_eq!(mutex.is_poisoned(), true);
/// ```
#[inline]
#[stable(feature = "sync_poison", since = "1.2.0")]
pub fn is_poisoned(&self) -> bool {
self.poison.get()
}
/// Consumes this mutex, returning the underlying data.
///
/// # Errors
///
/// If another user of this mutex panicked while holding the mutex, then
/// this call will return an error instead.
///
/// # Examples
///
/// ```
/// use std::sync::Mutex;
///
/// let mutex = Mutex::new(0);
/// assert_eq!(mutex.into_inner().unwrap(), 0);
/// ```
#[stable(feature = "mutex_into_inner", since = "1.6.0")]
pub fn into_inner(self) -> LockResult<T>
where
T: Sized,
{
// We know statically that there are no outstanding references to
// `self` so there's no need to lock the inner mutex.
//
// To get the inner value, we'd like to call `data.into_inner()`,
// but because `Mutex` impl-s `Drop`, we can't move out of it, so
// we'll have to destructure it manually instead.
unsafe {
// Like `let Mutex { inner, poison, data } = self`.
let (inner, poison, data) = {
let Mutex { ref inner, ref poison, ref data } = self;
(ptr::read(inner), ptr::read(poison), ptr::read(data))
};
mem::forget(self);
inner.destroy(); // Keep in sync with the `Drop` impl.
drop(inner);
poison::map_result(poison.borrow(), |_| data.into_inner())
}
}
/// Returns a mutable reference to the underlying data.
///
/// Since this call borrows the `Mutex` mutably, no actual locking needs to
/// take place -- the mutable borrow statically guarantees no locks exist.
///
/// # Errors
///
/// If another user of this mutex panicked while holding the mutex, then
/// this call will return an error instead.
///
/// # Examples
///
/// ```
/// use std::sync::Mutex;
///
/// let mut mutex = Mutex::new(0);
/// *mutex.get_mut().unwrap() = 10;
/// assert_eq!(*mutex.lock().unwrap(), 10);
/// ```
#[stable(feature = "mutex_get_mut", since = "1.6.0")]
pub fn get_mut(&mut self) -> LockResult<&mut T> {
// We know statically that there are no other references to `self`, so
// there's no need to lock the inner mutex.
let data = unsafe { &mut *self.data.get() };
poison::map_result(self.poison.borrow(), |_| data)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for Mutex<T> {
fn drop(&mut self) {
// This is actually safe b/c we know that there is no further usage of
// this mutex (it's up to the user to arrange for a mutex to get
// dropped, that's not our job)
//
// IMPORTANT: This code must be kept in sync with `Mutex::into_inner`.
unsafe { self.inner.destroy() }
}
}
#[stable(feature = "mutex_from", since = "1.24.0")]
impl<T> From<T> for Mutex<T> {
/// Creates a new mutex in an unlocked state ready for use.
/// This is equivalent to [`Mutex::new`].
///
/// [`Mutex::new`]: ../../std/sync/struct.Mutex.html#method.new
fn from(t: T) -> Self {
Mutex::new(t)
}
}
#[stable(feature = "mutex_default", since = "1.10.0")]
impl<T: ?Sized + Default> Default for Mutex<T> {
/// Creates a `Mutex<T>`, with the `Default` value for T.
fn default() -> Mutex<T> {
Mutex::new(Default::default())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.try_lock() {
Ok(guard) => f.debug_struct("Mutex").field("data", &&*guard).finish(),
Err(TryLockError::Poisoned(err)) => {
f.debug_struct("Mutex").field("data", &&**err.get_ref()).finish()
}
Err(TryLockError::WouldBlock) => {
struct LockedPlaceholder;
impl fmt::Debug for LockedPlaceholder {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("<locked>")
}
}
f.debug_struct("Mutex").field("data", &LockedPlaceholder).finish()
}
}
}
}
impl<'mutex, T: ?Sized> MutexGuard<'mutex, T> {
unsafe fn new(lock: &'mutex Mutex<T>) -> LockResult<MutexGuard<'mutex, T>> {
poison::map_result(lock.poison.borrow(), |guard| MutexGuard { lock, poison: guard })
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for MutexGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.lock.data.get() }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.lock.data.get() }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Drop for MutexGuard<'_, T> {
#[inline]
fn drop(&mut self) {
unsafe {
self.lock.poison.done(&self.poison);
self.lock.inner.raw_unlock();
}
}
}
#[stable(feature = "std_debug", since = "1.16.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
pub fn guard_lock<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a sys::Mutex {
&guard.lock.inner
}
pub fn guard_poison<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a poison::Flag {
&guard.lock.poison
}
#[cfg(all(test, not(target_os = "emscripten")))]
mod tests {
use crate::sync::atomic::{AtomicUsize, Ordering};
use crate::sync::mpsc::channel;
use crate::sync::{Arc, Condvar, Mutex};
use crate::thread;
struct Packet<T>(Arc<(Mutex<T>, Condvar)>);
#[derive(Eq, PartialEq, Debug)]
struct NonCopy(i32);
#[test]
fn smoke() {
let m = Mutex::new(());
drop(m.lock().unwrap());
drop(m.lock().unwrap());
}
#[test]
fn lots_and_lots() {
const J: u32 = 1000;
const K: u32 = 3;
let m = Arc::new(Mutex::new(0));
fn inc(m: &Mutex<u32>) {
for _ in 0..J {
*m.lock().unwrap() += 1;
}
}
let (tx, rx) = channel();
for _ in 0..K {
let tx2 = tx.clone();
let m2 = m.clone();
thread::spawn(move || {
inc(&m2);
tx2.send(()).unwrap();
});
let tx2 = tx.clone();
let m2 = m.clone();
thread::spawn(move || {
inc(&m2);
tx2.send(()).unwrap();
});
}
drop(tx);
for _ in 0..2 * K {
rx.recv().unwrap();
}
assert_eq!(*m.lock().unwrap(), J * K * 2);
}
#[test]
fn try_lock() {
let m = Mutex::new(());
*m.try_lock().unwrap() = ();
}
#[test]
fn test_into_inner() {
let m = Mutex::new(NonCopy(10));
assert_eq!(m.into_inner().unwrap(), NonCopy(10));
}
#[test]
fn test_into_inner_drop() {
struct Foo(Arc<AtomicUsize>);
impl Drop for Foo {
fn drop(&mut self) {
self.0.fetch_add(1, Ordering::SeqCst);
}
}
let num_drops = Arc::new(AtomicUsize::new(0));
let m = Mutex::new(Foo(num_drops.clone()));
assert_eq!(num_drops.load(Ordering::SeqCst), 0);
{
let _inner = m.into_inner().unwrap();
assert_eq!(num_drops.load(Ordering::SeqCst), 0);
}
assert_eq!(num_drops.load(Ordering::SeqCst), 1);
}
#[test]
fn test_into_inner_poison() {
let m = Arc::new(Mutex::new(NonCopy(10)));
let m2 = m.clone();
let _ = thread::spawn(move || {
let _lock = m2.lock().unwrap();
panic!("test panic in inner thread to poison mutex");
})
.join();
assert!(m.is_poisoned());
match Arc::try_unwrap(m).unwrap().into_inner() {
Err(e) => assert_eq!(e.into_inner(), NonCopy(10)),
Ok(x) => panic!("into_inner of poisoned Mutex is Ok: {:?}", x),
}
}
#[test]
fn test_get_mut() {
let mut m = Mutex::new(NonCopy(10));
*m.get_mut().unwrap() = NonCopy(20);
assert_eq!(m.into_inner().unwrap(), NonCopy(20));
}
#[test]
fn test_get_mut_poison() {
let m = Arc::new(Mutex::new(NonCopy(10)));
let m2 = m.clone();
let _ = thread::spawn(move || {
let _lock = m2.lock().unwrap();
panic!("test panic in inner thread to poison mutex");
})
.join();
assert!(m.is_poisoned());
match Arc::try_unwrap(m).unwrap().get_mut() {
Err(e) => assert_eq!(*e.into_inner(), NonCopy(10)),
Ok(x) => panic!("get_mut of poisoned Mutex is Ok: {:?}", x),
}
}
#[test]
fn test_mutex_arc_condvar() {
let packet = Packet(Arc::new((Mutex::new(false), Condvar::new())));
let packet2 = Packet(packet.0.clone());
let (tx, rx) = channel();
let _t = thread::spawn(move || {
// wait until parent gets in
rx.recv().unwrap();
let &(ref lock, ref cvar) = &*packet2.0;
let mut lock = lock.lock().unwrap();
*lock = true;
cvar.notify_one();
});
let &(ref lock, ref cvar) = &*packet.0;
let mut lock = lock.lock().unwrap();
tx.send(()).unwrap();
assert!(!*lock);
while !*lock {
lock = cvar.wait(lock).unwrap();
}
}
#[test]
fn test_arc_condvar_poison() {
let packet = Packet(Arc::new((Mutex::new(1), Condvar::new())));
let packet2 = Packet(packet.0.clone());
let (tx, rx) = channel();
let _t = thread::spawn(move || -> () {
rx.recv().unwrap();
let &(ref lock, ref cvar) = &*packet2.0;
let _g = lock.lock().unwrap();
cvar.notify_one();
// Parent should fail when it wakes up.
panic!();
});
let &(ref lock, ref cvar) = &*packet.0;
let mut lock = lock.lock().unwrap();
tx.send(()).unwrap();
while *lock == 1 {
match cvar.wait(lock) {
Ok(l) => {
lock = l;
assert_eq!(*lock, 1);
}
Err(..) => break,
}
}
}
#[test]
fn test_mutex_arc_poison() {
let arc = Arc::new(Mutex::new(1));
assert!(!arc.is_poisoned());
let arc2 = arc.clone();
let _ = thread::spawn(move || {
let lock = arc2.lock().unwrap();
assert_eq!(*lock, 2);
})
.join();
assert!(arc.lock().is_err());
assert!(arc.is_poisoned());
}
#[test]
fn test_mutex_arc_nested() {
// Tests nested mutexes and access
// to underlying data.
let arc = Arc::new(Mutex::new(1));
let arc2 = Arc::new(Mutex::new(arc));
let (tx, rx) = channel();
let _t = thread::spawn(move || {
let lock = arc2.lock().unwrap();
let lock2 = lock.lock().unwrap();
assert_eq!(*lock2, 1);
tx.send(()).unwrap();
});
rx.recv().unwrap();
}
#[test]
fn test_mutex_arc_access_in_unwind() {
let arc = Arc::new(Mutex::new(1));
let arc2 = arc.clone();
let _ = thread::spawn(move || -> () {
struct Unwinder {
i: Arc<Mutex<i32>>,
}
impl Drop for Unwinder {
fn drop(&mut self) {
*self.i.lock().unwrap() += 1;
}
}
let _u = Unwinder { i: arc2 };
panic!();
})
.join();
let lock = arc.lock().unwrap();
assert_eq!(*lock, 2);
}
#[test]
fn test_mutex_unsized() {
let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
{
let b = &mut *mutex.lock().unwrap();
b[0] = 4;
b[2] = 5;
}
let comp: &[i32] = &[4, 2, 5];
assert_eq!(&*mutex.lock().unwrap(), comp);
}
}