time: clean time driver (#2905)

* remove unnecessary wheel::Poll

the timer wheel uses the `wheel::Poll` struct as input when
advancing the timer to the next time step.  the `Poll` struct
contains an instant representing the time step to advance to
and also contains an optional and mutable reference to an
`Expiration` struct.  from what I can tell, the latter field
is only used in the context of polling the wheel and does not
need to be exposed outside of that method.  without the
expiration field the `Poll` struct is nothing more than a
wrapper around the instant being polled.  this change removes
the `Poll` struct and updates integration points accordingly.

* remove Stack trait in favor of concrete Stack implementation

* remove timer Registration struct

2 files changed, 23 insertions, 34 deletions

diff --git a/tokio-util/src/time/delay_queue.rs b/tokio-util/src/time/delay_queue.rs
index b23c24e6..92c922b8 100644
--- a/tokio-util/src/time/delay_queue.rs
+++ b/tokio-util/src/time/delay_queue.rs
@@ -141,7 +141,7 @@ pub struct DelayQueue<T> {
     delay: Option<Delay>,
 
     /// Wheel polling state
-    poll: wheel::Poll,
+    wheel_now: u64,
 
     /// Instant at which the timer starts
     start: Instant,
@@ -251,7 +251,7 @@ impl<T> DelayQueue<T> {
             slab: Slab::with_capacity(capacity),
             expired: Stack::default(),
             delay: None,
-            poll: wheel::Poll::new(0),
+            wheel_now: 0,
             start: Instant::now(),
         }
     }
@@ -733,11 +733,11 @@ impl<T> DelayQueue<T> {
 
                 let now = crate::time::ms(delay.deadline() - self.start, crate::time::Round::Down);
 
-                self.poll = wheel::Poll::new(now);
+                self.wheel_now = now;
             }
 
             // We poll the wheel to get the next value out before finding the next deadline.
-            let wheel_idx = self.wheel.poll(&mut self.poll, &mut self.slab);
+            let wheel_idx = self.wheel.poll(self.wheel_now, &mut self.slab);
 
             self.delay = self.next_deadline().map(sleep_until);
 
diff --git a/tokio-util/src/time/wheel/mod.rs b/tokio-util/src/time/wheel/mod.rs
index a2ef27fc..478037a3 100644
--- a/tokio-util/src/time/wheel/mod.rs
+++ b/tokio-util/src/time/wheel/mod.rs
@@ -51,13 +51,6 @@ pub(crate) enum InsertError {
     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,
@@ -136,19 +129,18 @@ where
         self.next_expiration().map(|expiration| expiration.deadline)
     }
 
-    pub(crate) fn poll(&mut self, poll: &mut Poll, store: &mut T::Store) -> Option<T::Owned> {
+    /// Advances the timer up to the instant represented by `now`.
+    pub(crate) fn poll(&mut self, now: u64, 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)
-                    }
-                });
-            }
+            let expiration = self.next_expiration().and_then(|expiration| {
+                if expiration.deadline > now {
+                    None
+                } else {
+                    Some(expiration)
+                }
+            });
 
-            match poll.expiration {
+            match expiration {
                 Some(ref expiration) => {
                     if let Some(item) = self.poll_expiration(expiration, store) {
                         return Some(item);
@@ -157,12 +149,14 @@ where
                     self.set_elapsed(expiration.deadline);
                 }
                 None => {
-                    self.set_elapsed(poll.now);
+                    // in this case the poll did not indicate an expiration
+                    // _and_ we were not able to find a next expiration in
+                    // the current list of timers.  advance to the poll's
+                    // current time and do nothing else.
+                    self.set_elapsed(now);
                     return None;
                 }
             }
-
-            poll.expiration = None;
         }
     }
 
@@ -197,6 +191,10 @@ where
         res
     }
 
+    /// iteratively find entries that are between the wheel's current
+    /// time and the expiration time.  for each in that population either
+    /// return it for notification (in the case of the last level) or tier
+    /// it down to the next level (in all other cases).
     pub(crate) fn poll_expiration(
         &mut self,
         expiration: &Expiration,
@@ -251,15 +249,6 @@ fn level_for(elapsed: u64, when: u64) -> usize {
     significant / 6
 }
 
-impl Poll {
-    pub(crate) fn new(now: u64) -> Poll {
-        Poll {
-            now,
-            expiration: None,
-        }
-    }
-}
-
 #[cfg(all(test, not(loom)))]
 mod test {
     use super::*;