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/* Copyright (c) 2010-2011 Dmitry Vyukov. All rights reserved.
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY DMITRY VYUKOV "AS IS" AND ANY EXPRESS OR IMPLIED
* WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
* SHALL DMITRY VYUKOV OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
* OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
* ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* The views and conclusions contained in the software and documentation are
* those of the authors and should not be interpreted as representing official
* policies, either expressed or implied, of Dmitry Vyukov.
*/
//! A mostly lock-free multi-producer, single consumer queue for sending
//! messages between asynchronous tasks.
//!
//! The queue implementation is essentially the same one used for mpsc channels
//! in the standard library.
//!
//! Note that the current implementation of this queue has a caveat of the `pop`
//! method, and see the method for more information about it. Due to this
//! caveat, this queue may not be appropriate for all use-cases.
// http://www.1024cores.net/home/lock-free-algorithms
// /queues/non-intrusive-mpsc-node-based-queue
// NOTE: this implementation is lifted from the standard library and only
// slightly modified
pub use self::PopResult::*;
use std::prelude::v1::*;
use std::cell::UnsafeCell;
use std::ptr;
use std::sync::atomic::{AtomicPtr, Ordering};
/// A result of the `pop` function.
pub enum PopResult<T> {
/// Some data has been popped
Data(T),
/// The queue is empty
Empty,
/// The queue is in an inconsistent state. Popping data should succeed, but
/// some pushers have yet to make enough progress in order allow a pop to
/// succeed. It is recommended that a pop() occur "in the near future" in
/// order to see if the sender has made progress or not
Inconsistent,
}
#[derive(Debug)]
struct Node<T> {
next: AtomicPtr<Node<T>>,
value: Option<T>,
}
/// The multi-producer single-consumer structure. This is not cloneable, but it
/// may be safely shared so long as it is guaranteed that there is only one
/// popper at a time (many pushers are allowed).
#[derive(Debug)]
pub struct Queue<T> {
head: AtomicPtr<Node<T>>,
tail: UnsafeCell<*mut Node<T>>,
}
unsafe impl<T: Send> Send for Queue<T> { }
unsafe impl<T: Send> Sync for Queue<T> { }
impl<T> Node<T> {
#[allow(clippy::new_ret_no_self)]
unsafe fn new(v: Option<T>) -> *mut Node<T> {
Box::into_raw(Box::new(Node {
next: AtomicPtr::new(ptr::null_mut()),
value: v,
}))
}
}
impl<T> Queue<T> {
/// Creates a new queue that is safe to share among multiple producers and
/// one consumer.
pub fn new() -> Queue<T> {
let stub = unsafe { Node::new(None) };
Queue {
head: AtomicPtr::new(stub),
tail: UnsafeCell::new(stub),
}
}
/// Pushes a new value onto this queue.
pub fn push(&self, t: T) {
unsafe {
let n = Node::new(Some(t));
let prev = self.head.swap(n, Ordering::AcqRel);
(*prev).next.store(n, Ordering::Release);
}
}
/// Pops some data from this queue.
///
/// Note that the current implementation means that this function cannot
/// return `Option<T>`. It is possible for this queue to be in an
/// inconsistent state where many pushes have succeeded and completely
/// finished, but pops cannot return `Some(t)`. This inconsistent state
/// happens when a pusher is preempted at an inopportune moment.
///
/// This inconsistent state means that this queue does indeed have data, but
/// it does not currently have access to it at this time.
///
/// This function is unsafe because only one thread can call it at a time.
pub unsafe fn pop(&self) -> PopResult<T> {
let tail = *self.tail.get();
let next = (*tail).next.load(Ordering::Acquire);
if !next.is_null() {
*self.tail.get() = next;
assert!((*tail).value.is_none());
assert!((*next).value.is_some());
let ret = (*next).value.take().unwrap();
drop(Box::from_raw(tail));
return Data(ret);
}
if self.head.load(Ordering::Acquire) == tail {Empty} else {Inconsistent}
}
}
impl<T> Drop for Queue<T> {
fn drop(&mut self) {
unsafe {
let mut cur = *self.tail.get();
while !cur.is_null() {
let next = (*cur).next.load(Ordering::Relaxed);
drop(Box::from_raw(cur));
cur = next;
}
}
}
}