blob: af9e1eeb2ccea57188d526c57b573500421297d6 [file] [log] [blame]
use std::any::Any;
use std::boxed::Box;
use std::cell::UnsafeCell;
use std::error::Error;
use std::fmt;
use std::mem;
use std::ops::{Deref, DerefMut};
use std::sync::Arc;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::SeqCst;
use {Async, Future, Poll};
use task::{self, Task};
/// A type of futures-powered synchronization primitive which is a mutex between
/// two possible owners.
///
/// This primitive is not as generic as a full-blown mutex but is sufficient for
/// many use cases where there are only two possible owners of a resource. The
/// implementation of `BiLock` can be more optimized for just the two possible
/// owners.
///
/// Note that it's possible to use this lock through a poll-style interface with
/// the `poll_lock` method but you can also use it as a future with the `lock`
/// method that consumes a `BiLock` and returns a future that will resolve when
/// it's locked.
///
/// A `BiLock` is typically used for "split" operations where data which serves
/// two purposes wants to be split into two to be worked with separately. For
/// example a TCP stream could be both a reader and a writer or a framing layer
/// could be both a stream and a sink for messages. A `BiLock` enables splitting
/// these two and then using each independently in a futures-powered fashion.
#[derive(Debug)]
pub struct BiLock<T> {
inner: Arc<Inner<T>>,
}
#[derive(Debug)]
struct Inner<T> {
state: AtomicUsize,
inner: Option<UnsafeCell<T>>,
}
unsafe impl<T: Send> Send for Inner<T> {}
unsafe impl<T: Send> Sync for Inner<T> {}
impl<T> BiLock<T> {
/// Creates a new `BiLock` protecting the provided data.
///
/// Two handles to the lock are returned, and these are the only two handles
/// that will ever be available to the lock. These can then be sent to separate
/// tasks to be managed there.
pub fn new(t: T) -> (BiLock<T>, BiLock<T>) {
let inner = Arc::new(Inner {
state: AtomicUsize::new(0),
inner: Some(UnsafeCell::new(t)),
});
(BiLock { inner: inner.clone() }, BiLock { inner: inner })
}
/// Attempt to acquire this lock, returning `NotReady` if it can't be
/// acquired.
///
/// This function will acquire the lock in a nonblocking fashion, returning
/// immediately if the lock is already held. If the lock is successfully
/// acquired then `Async::Ready` is returned with a value that represents
/// the locked value (and can be used to access the protected data). The
/// lock is unlocked when the returned `BiLockGuard` is dropped.
///
/// If the lock is already held then this function will return
/// `Async::NotReady`. In this case the current task will also be scheduled
/// to receive a notification when the lock would otherwise become
/// available.
///
/// # Panics
///
/// This function will panic if called outside the context of a future's
/// task.
pub fn poll_lock(&self) -> Async<BiLockGuard<T>> {
loop {
match self.inner.state.swap(1, SeqCst) {
// Woohoo, we grabbed the lock!
0 => return Async::Ready(BiLockGuard { inner: self }),
// Oops, someone else has locked the lock
1 => {}
// A task was previously blocked on this lock, likely our task,
// so we need to update that task.
n => unsafe {
drop(Box::from_raw(n as *mut Task));
}
}
let me = Box::new(task::current());
let me = Box::into_raw(me) as usize;
match self.inner.state.compare_exchange(1, me, SeqCst, SeqCst) {
// The lock is still locked, but we've now parked ourselves, so
// just report that we're scheduled to receive a notification.
Ok(_) => return Async::NotReady,
// Oops, looks like the lock was unlocked after our swap above
// and before the compare_exchange. Deallocate what we just
// allocated and go through the loop again.
Err(0) => unsafe {
drop(Box::from_raw(me as *mut Task));
},
// The top of this loop set the previous state to 1, so if we
// failed the CAS above then it's because the previous value was
// *not* zero or one. This indicates that a task was blocked,
// but we're trying to acquire the lock and there's only one
// other reference of the lock, so it should be impossible for
// that task to ever block itself.
Err(n) => panic!("invalid state: {}", n),
}
}
}
/// Perform a "blocking lock" of this lock, consuming this lock handle and
/// returning a future to the acquired lock.
///
/// This function consumes the `BiLock<T>` and returns a sentinel future,
/// `BiLockAcquire<T>`. The returned future will resolve to
/// `BiLockAcquired<T>` which represents a locked lock similarly to
/// `BiLockGuard<T>`.
///
/// Note that the returned future will never resolve to an error.
pub fn lock(self) -> BiLockAcquire<T> {
BiLockAcquire {
inner: Some(self),
}
}
/// Attempts to put the two "halves" of a `BiLock<T>` back together and
/// recover the original value. Succeeds only if the two `BiLock<T>`s
/// originated from the same call to `BiLock::new`.
pub fn reunite(self, other: Self) -> Result<T, ReuniteError<T>> {
if &*self.inner as *const _ == &*other.inner as *const _ {
drop(other);
let inner = Arc::try_unwrap(self.inner)
.ok()
.expect("futures: try_unwrap failed in BiLock<T>::reunite");
Ok(unsafe { inner.into_inner() })
} else {
Err(ReuniteError(self, other))
}
}
fn unlock(&self) {
match self.inner.state.swap(0, SeqCst) {
// we've locked the lock, shouldn't be possible for us to see an
// unlocked lock.
0 => panic!("invalid unlocked state"),
// Ok, no one else tried to get the lock, we're done.
1 => {}
// Another task has parked themselves on this lock, let's wake them
// up as its now their turn.
n => unsafe {
Box::from_raw(n as *mut Task).notify();
}
}
}
}
impl<T> Inner<T> {
unsafe fn into_inner(mut self) -> T {
mem::replace(&mut self.inner, None).unwrap().into_inner()
}
}
impl<T> Drop for Inner<T> {
fn drop(&mut self) {
assert_eq!(self.state.load(SeqCst), 0);
}
}
/// Error indicating two `BiLock<T>`s were not two halves of a whole, and
/// thus could not be `reunite`d.
pub struct ReuniteError<T>(pub BiLock<T>, pub BiLock<T>);
impl<T> fmt::Debug for ReuniteError<T> {
fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
fmt.debug_tuple("ReuniteError")
.field(&"...")
.finish()
}
}
impl<T> fmt::Display for ReuniteError<T> {
fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
write!(fmt, "tried to reunite two BiLocks that don't form a pair")
}
}
impl<T: Any> Error for ReuniteError<T> {
fn description(&self) -> &str {
"tried to reunite two BiLocks that don't form a pair"
}
}
/// Returned RAII guard from the `poll_lock` method.
///
/// This structure acts as a sentinel to the data in the `BiLock<T>` itself,
/// implementing `Deref` and `DerefMut` to `T`. When dropped, the lock will be
/// unlocked.
#[derive(Debug)]
pub struct BiLockGuard<'a, T: 'a> {
inner: &'a BiLock<T>,
}
impl<'a, T> Deref for BiLockGuard<'a, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.inner.inner.inner.as_ref().unwrap().get() }
}
}
impl<'a, T> DerefMut for BiLockGuard<'a, T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.inner.inner.inner.as_ref().unwrap().get() }
}
}
impl<'a, T> Drop for BiLockGuard<'a, T> {
fn drop(&mut self) {
self.inner.unlock();
}
}
/// Future returned by `BiLock::lock` which will resolve when the lock is
/// acquired.
#[derive(Debug)]
pub struct BiLockAcquire<T> {
inner: Option<BiLock<T>>,
}
impl<T> Future for BiLockAcquire<T> {
type Item = BiLockAcquired<T>;
type Error = ();
fn poll(&mut self) -> Poll<BiLockAcquired<T>, ()> {
match self.inner.as_ref().expect("cannot poll after Ready").poll_lock() {
Async::Ready(r) => {
mem::forget(r);
}
Async::NotReady => return Ok(Async::NotReady),
}
Ok(Async::Ready(BiLockAcquired { inner: self.inner.take() }))
}
}
/// Resolved value of the `BiLockAcquire<T>` future.
///
/// This value, like `BiLockGuard<T>`, is a sentinel to the value `T` through
/// implementations of `Deref` and `DerefMut`. When dropped will unlock the
/// lock, and the original unlocked `BiLock<T>` can be recovered through the
/// `unlock` method.
#[derive(Debug)]
pub struct BiLockAcquired<T> {
inner: Option<BiLock<T>>,
}
impl<T> BiLockAcquired<T> {
/// Recovers the original `BiLock<T>`, unlocking this lock.
pub fn unlock(mut self) -> BiLock<T> {
let bi_lock = self.inner.take().unwrap();
bi_lock.unlock();
bi_lock
}
}
impl<T> Deref for BiLockAcquired<T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.inner.as_ref().unwrap().inner.inner.as_ref().unwrap().get() }
}
}
impl<T> DerefMut for BiLockAcquired<T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.inner.as_mut().unwrap().inner.inner.as_ref().unwrap().get() }
}
}
impl<T> Drop for BiLockAcquired<T> {
fn drop(&mut self) {
if let Some(ref bi_lock) = self.inner {
bi_lock.unlock();
}
}
}