Implementing LoopData

This type acts for a handle to storage of non-`Send` data. The handle itself is
sendable across threads and is therefore suitable for storage in a `Future`.
This data uses communication internally and a new method on `Task` to ensure
that when the data needs to be accessed the future will find its way to the
right thread.

More on this type coming soon!

src/mpsc_queue.rs

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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.
+//!
+//! This module contains an implementation of a concurrent MPSC queue. This
+//! queue can be used to share data between threads, and is also used as the
+//! building block of channels in rust.
+//!
+//! 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::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,
+}
+
+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).
+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> {
+    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 pre-empted 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;
+            }
+        }
+    }
+}