Add DelayQueue implementation to tokio-timer (#550)

This patch adds a `DelayQueue` to tokio_timer. The `DelayQueue` allows
inserting elements as well as specifying a time at which the element
should be returned to the user. This allows handling more complex
timeout situations.

tokio-timer/src/timer/atomic_stack.rs

@@ -0,0 +1,122 @@
+use Error;
+use super::Entry;
+
+use std::ptr;
+use std::sync::Arc;
+use std::sync::atomic::AtomicPtr;
+use std::sync::atomic::Ordering::SeqCst;
+
+/// A stack of `Entry` nodes
+#[derive(Debug)]
+pub(crate) struct AtomicStack {
+    /// Stack head
+    head: AtomicPtr<Entry>,
+}
+
+/// Entries that were removed from the stack
+#[derive(Debug)]
+pub(crate) struct AtomicStackEntries {
+    ptr: *mut Entry,
+}
+
+/// Used to indicate that the timer has shutdown.
+const SHUTDOWN: *mut Entry = 1 as *mut _;
+
+impl AtomicStack {
+    pub fn new() -> AtomicStack {
+        AtomicStack { head: AtomicPtr::new(ptr::null_mut()) }
+    }
+
+    /// Push an entry onto the stack.
+    ///
+    /// Returns `true` if the entry was pushed, `false` if the entry is already
+    /// on the stack, `Err` if the timer is shutdown.
+    pub fn push(&self, entry: &Arc<Entry>) -> Result<bool, Error> {
+        // First, set the queued bit on the entry
+        let queued = entry.queued.fetch_or(true, SeqCst).into();
+
+        if queued {
+            // Already queued, nothing more to do
+            return Ok(false);
+        }
+
+        let ptr = Arc::into_raw(entry.clone()) as *mut _;
+
+        let mut curr = self.head.load(SeqCst);
+
+        loop {
+            if curr == SHUTDOWN {
+                // Don't leak the entry node
+                let _ = unsafe { Arc::from_raw(ptr) };
+
+                return Err(Error::shutdown());
+            }
+
+            // Update the `next` pointer. This is safe because setting the queued
+            // bit is a "lock" on this field.
+            unsafe {
+                *(entry.next_atomic.get()) = curr;
+            }
+
+            let actual = self.head.compare_and_swap(curr, ptr, SeqCst);
+
+            if actual == curr {
+                break;
+            }
+
+            curr = actual;
+        }
+
+        Ok(true)
+    }
+
+    /// Take all entries from the stack
+    pub fn take(&self) -> AtomicStackEntries {
+        let ptr = self.head.swap(ptr::null_mut(), SeqCst);
+        AtomicStackEntries { ptr }
+    }
+
+    /// Drain all remaining nodes in the stack and prevent any new nodes from
+    /// being pushed onto the stack.
+    pub fn shutdown(&self) {
+        // Shutdown the processing queue
+        let ptr = self.head.swap(SHUTDOWN, SeqCst);
+
+        // Let the drop fn of `AtomicStackEntries` handle draining the stack
+        drop(AtomicStackEntries { ptr });
+    }
+}
+
+// ===== impl AtomicStackEntries =====
+
+impl Iterator for AtomicStackEntries {
+    type Item = Arc<Entry>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.ptr.is_null() {
+            return None;
+        }
+
+        // Convert the pointer to an `Arc<Entry>`
+        let entry = unsafe { Arc::from_raw(self.ptr) };
+
+        // Update `self.ptr` to point to the next element of the stack
+        self.ptr = unsafe { (*entry.next_atomic.get()) };
+
+        // Unset the queued flag
+        let res = entry.queued.fetch_and(false, SeqCst);
+        debug_assert!(res);
+
+        // Return the entry
+        Some(entry)
+    }
+}
+
+impl Drop for AtomicStackEntries {
+    fn drop(&mut self) {
+        while let Some(entry) = self.next() {
+            // Flag the entry as errored
+            entry.error();
+        }
+    }
+}