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https://github.com/tokio-rs/tokio.git
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tokio-sync: Add async mutual exclusion primitive (#964)
This PR introduces `Lock`: A concurrency primitive built on top of `Semaphore` that provides a `Mutex`-like primitive that interacts nicely with futures. Specifically, `LockGuard` (in contrast to `MutexGuard`) does _not_ borrow the `Lock`, and can thus be passed into a future where it will later be unlocked. This replaces #958, which attempted to introduce a less generic version. The primitive proposed there will instead live in [`async-lease`](https://github.com/jonhoo/async-lease).
This commit is contained in:
Jon Gjengset
committed by
Lucio Franco
Lucio Franco
parent
7e51ab05e9
commit
cf06621998
@@ -24,6 +24,7 @@ macro_rules! if_fuzz {
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}}
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}
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pub mod lock;
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mod loom;
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pub mod mpsc;
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pub mod oneshot;
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@@ -0,0 +1,182 @@
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//! An asynchronous `Mutex`-like type.
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//!
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//! This module provides [`Lock`], a type that acts similarly to an asynchronous `Mutex`, with one
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//! major difference: the [`LockGuard`] returned by `poll_lock` is not tied to the lifetime of the
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//! `Mutex`. This enables you to acquire a lock, and then pass that guard into a future, and then
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//! release it at some later point in time.
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//!
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//! This allows you to do something along the lines of:
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//!
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//! ```rust,no_run
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//! # #[macro_use]
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//! # extern crate futures;
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//! # extern crate tokio;
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//! # use futures::{future, Poll, Async, Future, Stream};
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//! use tokio::sync::lock::{Lock, LockGuard};
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//! struct MyType<S> {
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//! lock: Lock<S>,
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//! }
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//!
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//! impl<S> Future for MyType<S>
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//! where S: Stream<Item = u32> + Send + 'static
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//! {
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//! type Item = ();
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//! type Error = ();
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//!
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//! fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
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//! match self.lock.poll_lock() {
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//! Async::Ready(mut guard) => {
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//! tokio::spawn(future::poll_fn(move || {
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//! let item = try_ready!(guard.poll().map_err(|_| ()));
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//! println!("item = {:?}", item);
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//! Ok(().into())
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//! }));
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//! Ok(().into())
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//! },
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//! Async::NotReady => Ok(Async::NotReady)
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//! }
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//! }
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//! }
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//! # fn main() {}
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//! ```
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//!
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//! [`Lock`]: struct.Lock.html
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//! [`LockGuard`]: struct.LockGuard.html
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use futures::Async;
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use semaphore;
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use std::cell::UnsafeCell;
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use std::fmt;
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use std::ops::{Deref, DerefMut};
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use std::sync::Arc;
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/// An asynchronous mutual exclusion primitive useful for protecting shared data
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///
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/// Each mutex has a type parameter (`T`) which represents the data that it is protecting. The data
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/// can only be accessed through the RAII guards returned from `poll_lock`, which guarantees that
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/// the data is only ever accessed when the mutex is locked.
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#[derive(Debug)]
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pub struct Lock<T> {
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inner: Arc<State<T>>,
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permit: semaphore::Permit,
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}
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/// A handle to a held `Lock`.
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///
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/// As long as you have this guard, you have exclusive access to the underlying `T`. The guard
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/// internally keeps a reference-couned pointer to the original `Lock`, so even if the lock goes
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/// away, the guard remains valid.
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///
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/// The lock is automatically released whenever the guard is dropped, at which point `poll_lock`
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/// will succeed yet again.
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#[derive(Debug)]
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pub struct LockGuard<T>(Lock<T>);
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// As long as T: Send, it's fine to send Lock<T> to other threads.
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// If T was not Send, sending a Lock<T> would be bad, since you can access T through Lock<T>.
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unsafe impl<T> Send for Lock<T> where T: Send {}
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unsafe impl<T> Sync for LockGuard<T> where T: Send + Sync {}
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#[derive(Debug)]
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struct State<T> {
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c: UnsafeCell<T>,
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s: semaphore::Semaphore,
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}
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#[test]
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fn bounds() {
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fn check<T: Send>() {}
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check::<LockGuard<u32>>();
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}
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impl<T> Lock<T> {
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/// Creates a new lock in an unlocked state ready for use.
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pub fn new(t: T) -> Self {
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Self {
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inner: Arc::new(State {
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c: UnsafeCell::new(t),
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s: semaphore::Semaphore::new(1),
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}),
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permit: semaphore::Permit::new(),
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}
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}
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/// Try to acquire the lock.
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///
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/// If the lock is already held, the current task is notified when it is released.
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pub fn poll_lock(&mut self) -> Async<LockGuard<T>> {
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if let Async::NotReady = self.permit.poll_acquire(&self.inner.s).unwrap_or_else(|_| {
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// The semaphore was closed. but, we never explicitly close it, and we have a
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// handle to it through the Arc, which means that this can never happen.
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unreachable!()
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}) {
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return Async::NotReady;
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}
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// We want to move the acquired permit into the guard,
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// and leave an unacquired one in self.
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let acquired = Self {
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inner: self.inner.clone(),
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permit: ::std::mem::replace(&mut self.permit, semaphore::Permit::new()),
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};
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Async::Ready(LockGuard(acquired))
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}
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}
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impl<T> Drop for LockGuard<T> {
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fn drop(&mut self) {
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if self.0.permit.is_acquired() {
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self.0.permit.release(&self.0.inner.s);
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} else if ::std::thread::panicking() {
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// A guard _should_ always hold its permit, but if the thread is already panicking,
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// we don't want to generate a panic-while-panicing, since that's just unhelpful!
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} else {
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unreachable!("Permit not held when LockGuard was dropped")
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}
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}
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}
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impl<T> From<T> for Lock<T> {
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fn from(s: T) -> Self {
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Self::new(s)
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}
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}
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impl<T> Clone for Lock<T> {
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fn clone(&self) -> Self {
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Self {
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inner: self.inner.clone(),
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permit: semaphore::Permit::new(),
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}
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}
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}
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impl<T> Default for Lock<T>
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where
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T: Default,
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{
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fn default() -> Self {
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Self::new(T::default())
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}
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}
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impl<T> Deref for LockGuard<T> {
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type Target = T;
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fn deref(&self) -> &Self::Target {
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assert!(self.0.permit.is_acquired());
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unsafe { &*self.0.inner.c.get() }
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}
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}
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impl<T> DerefMut for LockGuard<T> {
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fn deref_mut(&mut self) -> &mut Self::Target {
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assert!(self.0.permit.is_acquired());
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unsafe { &mut *self.0.inner.c.get() }
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}
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}
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impl<T: fmt::Display> fmt::Display for LockGuard<T> {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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fmt::Display::fmt(&**self, f)
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}
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}
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