time: use intrusive lists for timer tracking (#3080)

More-or-less a half-rewrite of the current time driver, supporting the
use of intrusive futures for timer registration.

Fixes: #3028, #3069

tokio-util/src/time/delay_queue.rs

@@ -14,7 +14,7 @@ use std::cmp;
 use std::future::Future;
 use std::marker::PhantomData;
 use std::pin::Pin;
-use std::task::{self, Poll};
+use std::task::{self, Poll, Waker};
 
 /// A queue of delayed elements.
 ///
@@ -145,6 +145,11 @@ pub struct DelayQueue<T> {
 
     /// Instant at which the timer starts
     start: Instant,
+
+    /// Waker that is invoked when we potentially need to reset the timer.
+    /// Because we lazily create the timer when the first entry is created, we
+    /// need to awaken any poller that polled us before that point.
+    waker: Option<Waker>,
 }
 
 /// An entry in `DelayQueue` that has expired and removed.
@@ -253,6 +258,7 @@ impl<T> DelayQueue<T> {
             delay: None,
             wheel_now: 0,
             start: Instant::now(),
+            waker: None,
         }
     }
 
@@ -330,6 +336,10 @@ impl<T> DelayQueue<T> {
         };
 
         if should_set_delay {
+            if let Some(waker) = self.waker.take() {
+                waker.wake();
+            }
+
             let delay_time = self.start + Duration::from_millis(when);
             if let Some(ref mut delay) = &mut self.delay {
                 delay.reset(delay_time);
@@ -348,6 +358,15 @@ impl<T> DelayQueue<T> {
         &mut self,
         cx: &mut task::Context<'_>,
     ) -> Poll<Option<Result<Expired<T>, Error>>> {
+        if !self
+            .waker
+            .as_ref()
+            .map(|w| w.will_wake(cx.waker()))
+            .unwrap_or(false)
+        {
+            self.waker = Some(cx.waker().clone());
+        }
+
         let item = ready!(self.poll_idx(cx));
         Poll::Ready(item.map(|result| {
             result.map(|idx| {
@@ -533,6 +552,7 @@ impl<T> DelayQueue<T> {
 
         let next_deadline = self.next_deadline();
         if let (Some(ref mut delay), Some(deadline)) = (&mut self.delay, next_deadline) {
+            // This should awaken us if necessary (ie, if already expired)
             delay.reset(deadline);
         }
     }