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/wheel/level.rs

@@ -0,0 +1,190 @@
+use wheel::Stack;
+
+use std::fmt;
+
+/// Wheel for a single level in the timer. This wheel contains 64 slots.
+pub(crate) struct Level<T> {
+    level: usize,
+
+    /// Bit field tracking which slots currently contain entries.
+    ///
+    /// Using a bit field to track slots that contain entries allows avoiding a
+    /// scan to find entries. This field is updated when entries are added or
+    /// removed from a slot.
+    ///
+    /// The least-significant bit represents slot zero.
+    occupied: u64,
+
+    /// Slots
+    slot: [T; LEVEL_MULT],
+}
+
+/// Indicates when a slot must be processed next.
+#[derive(Debug)]
+pub(crate) struct Expiration {
+    /// The level containing the slot.
+    pub level: usize,
+
+    /// The slot index.
+    pub slot: usize,
+
+    /// The instant at which the slot needs to be processed.
+    pub deadline: u64,
+}
+
+/// Level multiplier.
+///
+/// Being a power of 2 is very important.
+const LEVEL_MULT: usize = 64;
+
+impl<T: Stack> Level<T> {
+    pub fn new(level: usize) -> Level<T> {
+        // Rust's derived implementations for arrays require that the value
+        // contained by the array be `Copy`. So, here we have to manually
+        // initialize every single slot.
+        macro_rules! s {
+            () => { T::default() };
+        };
+
+        Level {
+            level,
+            occupied: 0,
+            slot: [
+                // It does not look like the necessary traits are
+                // derived for [T; 64].
+                s!(), s!(), s!(), s!(), s!(), s!(), s!(), s!(),
+                s!(), s!(), s!(), s!(), s!(), s!(), s!(), s!(),
+                s!(), s!(), s!(), s!(), s!(), s!(), s!(), s!(),
+                s!(), s!(), s!(), s!(), s!(), s!(), s!(), s!(),
+                s!(), s!(), s!(), s!(), s!(), s!(), s!(), s!(),
+                s!(), s!(), s!(), s!(), s!(), s!(), s!(), s!(),
+                s!(), s!(), s!(), s!(), s!(), s!(), s!(), s!(),
+                s!(), s!(), s!(), s!(), s!(), s!(), s!(), s!(),
+            ],
+        }
+    }
+
+    /// Finds the slot that needs to be processed next and returns the slot and
+    /// `Instant` at which this slot must be processed.
+    pub fn next_expiration(&self, now: u64) -> Option<Expiration> {
+        // Use the `occupied` bit field to get the index of the next slot that
+        // needs to be processed.
+        let slot = match self.next_occupied_slot(now) {
+            Some(slot) => slot,
+            None => return None,
+        };
+
+        // From the slot index, calculate the `Instant` at which it needs to be
+        // processed. This value *must* be in the future with respect to `now`.
+
+        let level_range = level_range(self.level);
+        let slot_range = slot_range(self.level);
+
+        // TODO: This can probably be simplified w/ power of 2 math
+        let level_start = now - (now % level_range);
+        let deadline = level_start + slot as u64 * slot_range;
+
+        debug_assert!(deadline >= now, "deadline={}; now={}; level={}; slot={}; occupied={:b}",
+                      deadline, now, self.level, slot, self.occupied);
+
+        Some(Expiration {
+            level: self.level,
+            slot,
+            deadline,
+        })
+    }
+
+    fn next_occupied_slot(&self, now: u64) -> Option<usize> {
+        if self.occupied == 0 {
+            return None;
+        }
+
+        // Get the slot for now using Maths
+        let now_slot = (now / slot_range(self.level)) as usize;
+        let occupied = self.occupied.rotate_right(now_slot as u32);
+        let zeros = occupied.trailing_zeros() as usize;
+        let slot = (zeros + now_slot) % 64;
+
+        Some(slot)
+    }
+
+    pub fn add_entry(&mut self, when: u64, item: T::Owned, store: &mut T::Store) {
+        let slot = slot_for(when, self.level);
+
+        self.slot[slot].push(item, store);
+        self.occupied |= occupied_bit(slot);
+    }
+
+    pub fn remove_entry(&mut self, when: u64, item: &T::Borrowed, store: &mut T::Store) {
+        let slot = slot_for(when, self.level);
+
+        self.slot[slot].remove(item, store);
+
+        if self.slot[slot].is_empty() {
+            // The bit is currently set
+            debug_assert!(self.occupied & occupied_bit(slot) != 0);
+
+            // Unset the bit
+            self.occupied ^= occupied_bit(slot);
+        }
+    }
+
+    pub fn pop_entry_slot(&mut self, slot: usize, store: &mut T::Store) -> Option<T::Owned> {
+        let ret = self.slot[slot].pop(store);
+
+        if ret.is_some() && self.slot[slot].is_empty() {
+            // The bit is currently set
+            debug_assert!(self.occupied & occupied_bit(slot) != 0);
+
+            self.occupied ^= occupied_bit(slot);
+        }
+
+        ret
+    }
+}
+
+impl<T> fmt::Debug for Level<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Level")
+            .field("occupied", &self.occupied)
+            .finish()
+    }
+}
+
+fn occupied_bit(slot: usize) -> u64 {
+    (1 << slot)
+}
+
+fn slot_range(level: usize) -> u64 {
+    LEVEL_MULT.pow(level as u32) as u64
+}
+
+fn level_range(level: usize) -> u64 {
+    LEVEL_MULT as u64 * slot_range(level)
+}
+
+/// Convert a duration (milliseconds) and a level to a slot position
+fn slot_for(duration: u64, level: usize) -> usize {
+    ((duration >> (level * 6)) % LEVEL_MULT as u64) as usize
+}
+
+/*
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_slot_for() {
+        for pos in 1..64 {
+            assert_eq!(pos as usize, slot_for(pos, 0));
+        }
+
+        for level in 1..5 {
+            for pos in level..64 {
+                let a = pos * 64_usize.pow(level as u32);
+                assert_eq!(pos as usize, slot_for(a as u64, level));
+            }
+        }
+    }
+}
+*/

tokio-timer/src/wheel/mod.rs

@@ -0,0 +1,289 @@
+mod level;
+mod stack;
+
+pub(crate) use self::stack::Stack;
+pub(crate) use self::level::Expiration;
+use self::level::Level;
+
+use std::borrow::Borrow;
+use std::usize;
+
+/// Timing wheel implementation.
+///
+/// This type provides the hashed timing wheel implementation that backs `Timer`
+/// and `DelayQueue`.
+///
+/// The structure is generic over `T: Stack`. This allows handling timeout data
+/// being stored on the heap or in a slab. In order to support the latter case,
+/// the slab must be passed into each function allowing the implementation to
+/// lookup timer entries.
+///
+/// See `Timer` documentation for some implementation notes.
+#[derive(Debug)]
+pub(crate) struct Wheel<T> {
+    /// The number of milliseconds elapsed since the wheel started.
+    elapsed: u64,
+
+    /// Timer wheel.
+    ///
+    /// Levels:
+    ///
+    /// * 1 ms slots / 64 ms range
+    /// * 64 ms slots / ~ 4 sec range
+    /// * ~ 4 sec slots / ~ 4 min range
+    /// * ~ 4 min slots / ~ 4 hr range
+    /// * ~ 4 hr slots / ~ 12 day range
+    /// * ~ 12 day slots / ~ 2 yr range
+    levels: Vec<Level<T>>,
+}
+
+/// Number of levels. Each level has 64 slots. By using 6 levels with 64 slots
+/// each, the timer is able to track time up to 2 years into the future with a
+/// precision of 1 millisecond.
+const NUM_LEVELS: usize = 6;
+
+/// The maximum duration of a delay
+const MAX_DURATION: u64 = 1 << (6 * NUM_LEVELS);
+
+#[derive(Debug)]
+pub(crate) enum InsertError {
+    Elapsed,
+    Invalid,
+}
+
+/// Poll expirations from the wheel
+#[derive(Debug, Default)]
+pub(crate) struct Poll {
+    now: u64,
+    expiration: Option<Expiration>,
+}
+
+impl<T> Wheel<T>
+where
+    T: Stack,
+{
+    /// Create a new timing wheel
+    pub fn new() -> Wheel<T> {
+        let levels = (0..NUM_LEVELS)
+            .map(Level::new)
+            .collect();
+
+        Wheel {
+            elapsed: 0,
+            levels,
+        }
+    }
+
+    /// Return the number of milliseconds that have elapsed since the timing
+    /// wheele's creation.
+    pub fn elapsed(&self) -> u64 {
+        self.elapsed
+    }
+
+    /// Insert an entry into the timing wheel.
+    ///
+    /// # Arguments
+    ///
+    /// * `when`: is the instant at which the the entry should be fired. It is
+    ///           represented as the number of milliseconds since the creation
+    ///           of the timing wheel.
+    ///
+    /// * `item`: The item to insert into the wheel.
+    ///
+    /// * `store`: The slab or `()` when using heap storage.
+    ///
+    /// # Return
+    ///
+    /// Returns `Ok` when the item is successfully inserted, `Err` otherwise.
+    ///
+    /// `Err(Elapsed)` indicates that `when` represents an instant that has
+    /// already passed. In this case, the caller should fire the timeout
+    /// immediateely.
+    ///
+    /// `Err(Invalid)` indicates an invalid `when` argumeent as been supplied.
+    pub fn insert(&mut self, when: u64, item: T::Owned, store: &mut T::Store)
+        -> Result<(), (T::Owned, InsertError)>
+    {
+        if when <= self.elapsed {
+            return Err((item, InsertError::Elapsed));
+        } else if when - self.elapsed > MAX_DURATION {
+            return Err((item, InsertError::Invalid));
+        }
+
+        // Get the level at which the entry should be stored
+        let level = self.level_for(when);
+
+        self.levels[level].add_entry(when, item, store);
+
+        debug_assert!({
+            self.levels[level].next_expiration(self.elapsed)
+                .map(|e| e.deadline >= self.elapsed)
+                .unwrap_or(true)
+        });
+
+        Ok(())
+    }
+
+    /// Remove `item` from thee timing wheel.
+    pub fn remove(&mut self, item: &T::Borrowed, store: &mut T::Store) {
+        let when = T::when(item, store);
+        let level = self.level_for(when);
+
+        self.levels[level].remove_entry(when, item, store);
+    }
+
+    /// Instant at which to poll
+    pub fn poll_at(&self) -> Option<u64> {
+        self.next_expiration()
+            .map(|expiration| expiration.deadline)
+    }
+
+    pub fn poll(&mut self, poll: &mut Poll, store: &mut T::Store)
+        -> Option<T::Owned>
+    {
+        loop {
+            if poll.expiration.is_none() {
+                poll.expiration = self.next_expiration()
+                    .and_then(|expiration| {
+                        if expiration.deadline > poll.now {
+                            None
+                        } else {
+                            Some(expiration)
+                        }
+                    });
+            }
+
+            match poll.expiration {
+                Some(ref expiration) => {
+                    if let Some(item) = self.poll_expiration(expiration, store) {
+                        return Some(item);
+                    }
+
+                    self.set_elapsed(expiration.deadline);
+                }
+                None => {
+                    self.set_elapsed(poll.now);
+                    return None;
+                }
+            }
+
+            poll.expiration = None;
+        }
+    }
+
+    /// Returns the instant at which the next timeout expires.
+    fn next_expiration(&self) -> Option<Expiration> {
+        // Check all levels
+        for level in 0..NUM_LEVELS {
+            if let Some(expiration) = self.levels[level].next_expiration(self.elapsed) {
+                // There cannot be any expirations at a higher level that happen
+                // before this one.
+                debug_assert!({
+                    let mut res = true;
+
+                    for l2 in (level+1)..NUM_LEVELS {
+                        if let Some(e2) = self.levels[l2].next_expiration(self.elapsed) {
+                            if e2.deadline < expiration.deadline {
+                                res = false;
+                            }
+                        }
+                    }
+
+                    res
+                });
+
+                return Some(expiration);
+            }
+        }
+
+        None
+    }
+
+    pub fn poll_expiration(&mut self, expiration: &Expiration, store: &mut T::Store)
+        -> Option<T::Owned>
+    {
+        while let Some(item) = self.pop_entry(expiration, store) {
+            if expiration.level == 0 {
+                debug_assert_eq!(T::when(item.borrow(), store), expiration.deadline);
+
+                return Some(item);
+            } else {
+                let when = T::when(item.borrow(), store);
+
+                let next_level = expiration.level - 1;
+
+                self.levels[next_level]
+                    .add_entry(when, item, store);
+            }
+        }
+
+        None
+    }
+
+    fn set_elapsed(&mut self, when: u64) {
+        assert!(self.elapsed <= when, "elapsed={:?}; when={:?}", self.elapsed, when);
+
+        if when > self.elapsed {
+            self.elapsed = when;
+        }
+    }
+
+    fn pop_entry(&mut self, expiration: &Expiration, store: &mut T::Store) -> Option<T::Owned> {
+        self.levels[expiration.level].pop_entry_slot(expiration.slot, store)
+    }
+
+    fn level_for(&self, when: u64) -> usize {
+        level_for(self.elapsed, when)
+    }
+}
+
+fn level_for(elapsed: u64, when: u64) -> usize {
+    let masked = elapsed ^ when;
+
+    assert!(masked != 0, "elapsed={}; when={}", elapsed, when);
+
+    let leading_zeros = masked.leading_zeros() as usize;
+    let significant = 63 - leading_zeros;
+    significant / 6
+}
+
+impl Poll {
+    pub fn new(now: u64) -> Poll {
+        Poll {
+            now,
+            expiration: None,
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_level_for() {
+        for pos in 1..64 {
+            assert_eq!(0, level_for(0, pos), "level_for({}) -- binary = {:b}", pos, pos);
+        }
+
+        for level in 1..5 {
+            for pos in level..64 {
+                let a = pos * 64_usize.pow(level as u32);
+                assert_eq!(level, level_for(0, a as u64),
+                           "level_for({}) -- binary = {:b}", a, a);
+
+                if pos > level {
+                    let a = a - 1;
+                    assert_eq!(level, level_for(0, a as u64),
+                               "level_for({}) -- binary = {:b}", a, a);
+                }
+
+                if pos < 64 {
+                    let a = a + 1;
+                    assert_eq!(level, level_for(0, a as u64),
+                               "level_for({}) -- binary = {:b}", a, a);
+                }
+            }
+        }
+    }
+}

tokio-timer/src/wheel/stack.rs

@@ -0,0 +1,26 @@
+use std::borrow::Borrow;
+
+/// Abstracts the stack operations needed to track timeouts.
+pub(crate) trait Stack: Default {
+    /// Type of the item stored in the stack
+    type Owned: Borrow<Self::Borrowed>;
+
+    /// Borrowed item
+    type Borrowed;
+
+    /// Item storage, this allows a slab to be used instead of just the heap
+    type Store;
+
+    /// Returns `true` if the stack is empty
+    fn is_empty(&self) -> bool;
+
+    /// Push an item onto the stack
+    fn push(&mut self, item: Self::Owned, store: &mut Self::Store);
+
+    /// Pop an item from the stack
+    fn pop(&mut self, store: &mut Self::Store) -> Option<Self::Owned>;
+
+    fn remove(&mut self, item: &Self::Borrowed, store: &mut Self::Store);
+
+    fn when(item: &Self::Borrowed, store: &Self::Store) -> u64;
+}