time: Eagerly bind delays to timer (#1800)

## Motivation

Similar to #1666, it is no longer necessary to lazily register delays with the
executions default timer. All delays are expected to be created from within a
runtime, and should panic if not done so.

## Solution

`tokio::time` now assumes there to be a `CURRENT_TIMER` set when creating a
delay; this can be assumed if called within a tokio runtime. If there is no
current timer, the application will panic with a "no current timer" message.

## Follow-up

Similar to #1666, `HandlePriv` can probably be removed, but this mainly prepares
for 0.2 API changes. Because it is not in the public API, this can be done in a
following change.

Signed-off-by: Kevin Leimkuhler <kleimkuhler@icloud.com>

8 files changed, 145 insertions, 178 deletions

diff --git a/tokio-test/tests/block_on.rs b/tokio-test/tests/block_on.rs
index b50f9ae1..7aec82cc 100644
--- a/tokio-test/tests/block_on.rs
+++ b/tokio-test/tests/block_on.rs
@@ -20,5 +20,10 @@ fn async_fn() {
 #[test]
 fn test_delay() {
     let deadline = Instant::now() + Duration::from_millis(100);
-    assert_eq!((), block_on(delay_until(deadline)));
+    assert_eq!(
+        (),
+        block_on(async {
+            delay_until(deadline).await;
+        })
+    );
 }
diff --git a/tokio/src/time/delay.rs b/tokio/src/time/delay.rs
index e3b605e7..bae3d9c8 100644
--- a/tokio/src/time/delay.rs
+++ b/tokio/src/time/delay.rs
@@ -42,7 +42,7 @@ pub fn delay_for(duration: Duration) -> Delay {
 #[derive(Debug)]
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct Delay {
-    /// The link between the `Delay` instance at the timer that drives it.
+    /// The link between the `Delay` instance and the timer that drives it.
     ///
     /// This also stores the `deadline` value.
     registration: Registration,
@@ -76,21 +76,12 @@ impl Delay {
     pub fn reset(&mut self, deadline: Instant) {
         self.registration.reset(deadline);
     }
-
-    /// Register the delay with the timer instance for the current execution
-    /// context.
-    fn register(&mut self) {
-        self.registration.register();
-    }
 }
 
 impl Future for Delay {
     type Output = ();
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
-        // Ensure the `Delay` instance is associated with a timer.
-        self.register();
-
+    fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
         // `poll_elapsed` can return an error in two cases:
         //
         // - AtCapacity: this is a pathlogical case where far too many
diff --git a/tokio/src/time/delay_queue.rs b/tokio/src/time/delay_queue.rs
index 6a7cc6b3..1fd70625 100644
--- a/tokio/src/time/delay_queue.rs
+++ b/tokio/src/time/delay_queue.rs
@@ -228,15 +228,19 @@ impl<T> DelayQueue<T> {
     /// ```rust
     /// # use tokio::time::DelayQueue;
     /// # use std::time::Duration;
-    /// let mut delay_queue = DelayQueue::with_capacity(10);
     ///
-    /// // These insertions are done without further allocation
-    /// for i in 0..10 {
-    ///     delay_queue.insert(i, Duration::from_secs(i));
-    /// }
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::with_capacity(10);
     ///
-    /// // This will make the queue allocate additional storage
-    /// delay_queue.insert(11, Duration::from_secs(11));
+    ///     // These insertions are done without further allocation
+    ///     for i in 0..10 {
+    ///         delay_queue.insert(i, Duration::from_secs(i));
+    ///     }
+    ///
+    ///     // This will make the queue allocate additional storage
+    ///     delay_queue.insert(11, Duration::from_secs(11));
+    /// # }
     /// ```
     pub fn with_capacity(capacity: usize) -> DelayQueue<T> {
         DelayQueue {
@@ -279,13 +283,16 @@ impl<T> DelayQueue<T> {
     /// ```rust
     /// use tokio::time::{DelayQueue, Duration, Instant};
     ///
-    /// let mut delay_queue = DelayQueue::new();
-    /// let key = delay_queue.insert_at(
-    ///     "foo", Instant::now() + Duration::from_secs(5));
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert_at(
+    ///         "foo", Instant::now() + Duration::from_secs(5));
     ///
-    /// // Remove the entry
-    /// let item = delay_queue.remove(&key);
-    /// assert_eq!(*item.get_ref(), "foo");
+    ///     // Remove the entry
+    ///     let item = delay_queue.remove(&key);
+    ///     assert_eq!(*item.get_ref(), "foo");
+    /// # }
     /// ```
     ///
     /// [`poll`]: #method.poll
@@ -380,12 +387,15 @@ impl<T> DelayQueue<T> {
     /// use tokio::time::DelayQueue;
     /// use std::time::Duration;
     ///
-    /// let mut delay_queue = DelayQueue::new();
-    /// let key = delay_queue.insert("foo", Duration::from_secs(5));
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
     ///
-    /// // Remove the entry
-    /// let item = delay_queue.remove(&key);
-    /// assert_eq!(*item.get_ref(), "foo");
+    ///     // Remove the entry
+    ///     let item = delay_queue.remove(&key);
+    ///     assert_eq!(*item.get_ref(), "foo");
+    /// # }
     /// ```
     ///
     /// [`poll`]: #method.poll
@@ -430,12 +440,15 @@ impl<T> DelayQueue<T> {
     /// use tokio::time::DelayQueue;
     /// use std::time::Duration;
     ///
-    /// let mut delay_queue = DelayQueue::new();
-    /// let key = delay_queue.insert("foo", Duration::from_secs(5));
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
     ///
-    /// // Remove the entry
-    /// let item = delay_queue.remove(&key);
-    /// assert_eq!(*item.get_ref(), "foo");
+    ///     // Remove the entry
+    ///     let item = delay_queue.remove(&key);
+    ///     assert_eq!(*item.get_ref(), "foo");
+    /// # }
     /// ```
     pub fn remove(&mut self, key: &Key) -> Expired<T> {
         use crate::time::wheel::Stack;
@@ -477,14 +490,17 @@ impl<T> DelayQueue<T> {
     /// ```rust
     /// use tokio::time::{DelayQueue, Duration, Instant};
     ///
-    /// let mut delay_queue = DelayQueue::new();
-    /// let key = delay_queue.insert("foo", Duration::from_secs(5));
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
     ///
-    /// // "foo" is scheduled to be returned in 5 seconds
+    ///     // "foo" is scheduled to be returned in 5 seconds
     ///
-    /// delay_queue.reset_at(&key, Instant::now() + Duration::from_secs(10));
+    ///     delay_queue.reset_at(&key, Instant::now() + Duration::from_secs(10));
     ///
-    /// // "foo"is now scheduled to be returned in 10 seconds
+    ///     // "foo"is now scheduled to be returned in 10 seconds
+    /// # }
     /// ```
     pub fn reset_at(&mut self, key: &Key, when: Instant) {
         self.wheel.remove(&key.index, &mut self.slab);
@@ -531,14 +547,17 @@ impl<T> DelayQueue<T> {
     /// use tokio::time::DelayQueue;
     /// use std::time::Duration;
     ///
-    /// let mut delay_queue = DelayQueue::new();
-    /// let key = delay_queue.insert("foo", Duration::from_secs(5));
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
     ///
-    /// // "foo" is scheduled to be returned in 5 seconds
+    ///     // "foo" is scheduled to be returned in 5 seconds
     ///
-    /// delay_queue.reset(&key, Duration::from_secs(10));
+    ///     delay_queue.reset(&key, Duration::from_secs(10));
     ///
-    /// // "foo"is now scheduled to be returned in 10 seconds
+    ///     // "foo"is now scheduled to be returned in 10 seconds
+    /// # }
     /// ```
     pub fn reset(&mut self, key: &Key, timeout: Duration) {
         self.reset_at(key, Instant::now() + timeout);
@@ -558,15 +577,18 @@ impl<T> DelayQueue<T> {
     /// use tokio::time::DelayQueue;
     /// use std::time::Duration;
     ///
-    /// let mut delay_queue = DelayQueue::new();
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
     ///
-    /// delay_queue.insert("foo", Duration::from_secs(5));
+    ///     delay_queue.insert("foo", Duration::from_secs(5));
     ///
-    /// assert!(!delay_queue.is_empty());
+    ///     assert!(!delay_queue.is_empty());
     ///
-    /// delay_queue.clear();
+    ///     delay_queue.clear();
     ///
-    /// assert!(delay_queue.is_empty());
+    ///     assert!(delay_queue.is_empty());
+    /// # }
     /// ```
     pub fn clear(&mut self) {
         self.slab.clear();
@@ -612,12 +634,15 @@ impl<T> DelayQueue<T> {
     /// use tokio::time::DelayQueue;
     /// use std::time::Duration;
     ///
-    /// let mut delay_queue = DelayQueue::new();
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
     ///
-    /// delay_queue.insert("hello", Duration::from_secs(10));
-    /// delay_queue.reserve(10);
+    ///     delay_queue.insert("hello", Duration::from_secs(10));
+    ///     delay_queue.reserve(10);
     ///
-    /// assert!(delay_queue.capacity() >= 11);
+    ///     assert!(delay_queue.capacity() >= 11);
+    /// # }
     /// ```
     pub fn reserve(&mut self, additional: usize) {
         self.slab.reserve(additional);
@@ -634,11 +659,14 @@ impl<T> DelayQueue<T> {
     /// use tokio::time::DelayQueue;
     /// use std::time::Duration;
     ///
-    /// let mut delay_queue = DelayQueue::new();
-    /// assert!(delay_queue.is_empty());
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     assert!(delay_queue.is_empty());
     ///
-    /// delay_queue.insert("hello", Duration::from_secs(5));
-    /// assert!(!delay_queue.is_empty());
+    ///     delay_queue.insert("hello", Duration::from_secs(5));
+    ///     assert!(!delay_queue.is_empty());
+    /// # }
     /// ```
     pub fn is_empty(&self) -> bool {
         self.slab.is_empty()
diff --git a/tokio/src/time/driver/entry.rs b/tokio/src/time/driver/entry.rs
index 97ce34de..708b6cfb 100644
--- a/tokio/src/time/driver/entry.rs
+++ b/tokio/src/time/driver/entry.rs
@@ -6,7 +6,7 @@ use crate::time::{Duration, Error, Instant};
 use std::cell::UnsafeCell;
 use std::ptr;
 use std::sync::atomic::AtomicBool;
-use std::sync::atomic::Ordering::{Relaxed, SeqCst};
+use std::sync::atomic::Ordering::SeqCst;
 use std::sync::{Arc, Weak};
 use std::task::{self, Poll};
 use std::u64;
@@ -31,7 +31,7 @@ pub(crate) struct Entry {
     /// without all `Delay` instances having completed.
     ///
     /// When `None`, the entry has not yet been linked with a timer instance.
-    inner: Option<Weak<Inner>>,
+    inner: Weak<Inner>,
 
     /// Tracks the entry state. This value contains the following information:
     ///
@@ -104,18 +104,41 @@ const ERROR: u64 = u64::MAX;
 // ===== impl Entry =====
 
 impl Entry {
-    pub(crate) fn new(deadline: Instant, duration: Duration) -> Entry {
-        Entry {
+    pub(crate) fn new(deadline: Instant, duration: Duration) -> Arc<Entry> {
+        let handle_priv = HandlePriv::current();
+        let handle = handle_priv.inner().unwrap();
+
+        // Increment the number of active timeouts
+        if handle.increment().is_err() {
+            // TODO(kleimkuhler): Transition to error state instead of
+            // panicking?
+            panic!("failed to add entry; timer at capacity");
+        };
+
+        let when = handle.normalize_deadline(deadline);
+        let state = if when <= handle.elapsed() {
+            ELAPSED
+        } else {
+            when
+        };
+
+        let entry = Arc::new(Entry {
             time: CachePadded(UnsafeCell::new(Time { deadline, duration })),
-            inner: None,
+            inner: handle_priv.into_inner(),
             waker: AtomicWaker::new(),
-            state: AtomicU64::new(0),
+            state: AtomicU64::new(state),
             queued: AtomicBool::new(false),
             next_atomic: UnsafeCell::new(ptr::null_mut()),
             when: UnsafeCell::new(None),
             next_stack: UnsafeCell::new(None),
             prev_stack: UnsafeCell::new(ptr::null_mut()),
+        });
+
+        if handle.queue(&entry).is_err() {
+            entry.error();
         }
+
+        entry
     }
 
     /// Only called by `Registration`
@@ -129,77 +152,6 @@ impl Entry {
         &mut *self.time.0.get()
     }
 
-    /// Returns `true` if the `Entry` is currently associated with a timer
-    /// instance.
-    pub(crate) fn is_registered(&self) -> bool {
-        self.inner.is_some()
-    }
-
-    /// Only called by `Registration`
-    pub(crate) fn register(me: &mut Arc<Self>) {
-        let handle = match HandlePriv::try_current() {
-            Ok(handle) => handle,
-            Err(_) => {
-                // Could not associate the entry with a timer, transition the
-                // state to error
-                Arc::get_mut(me).unwrap().transition_to_error();
-
-                return;
-            }
-        };
-
-        Entry::register_with(me, handle)
-    }
-
-    /// Only called by `Registration`
-    pub(crate) fn register_with(me: &mut Arc<Self>, handle: HandlePriv) {
-        assert!(!me.is_registered(), "only register an entry once");
-
-        let deadline = me.time_ref().deadline;
-
-        let inner = match handle.inner() {
-            Some(inner) => inner,
-            None => {
-                // Could not associate the entry with a timer, transition the
-                // state to error
-                Arc::get_mut(me).unwrap().transition_to_error();
-
-                return;
-            }
-        };
-
-        // Increment the number of active timeouts
-        if inner.increment().is_err() {
-            Arc::get_mut(me).unwrap().transition_to_error();
-
-            return;
-        }
-
-        // Associate the entry with the timer
-        Arc::get_mut(me).unwrap().inner = Some(handle.into_inner());
-
-        let when = inner.normalize_deadline(deadline);
-
-        // Relaxed OK: At this point, there are no other threads that have
-        // access to this entry.
-        if when <= inner.elapsed() {
-            me.state.store(ELAPSED, Relaxed);
-            return;
-        } else {
-            me.state.store(when, Relaxed);
-        }
-
-        if inner.queue(me).is_err() {
-            // The timer has shutdown, transition the entry to the error state.
-            me.error();
-        }
-    }
-
-    fn transition_to_error(&mut self) {
-        self.inner = Some(Weak::new());
-        self.state = AtomicU64::new(ERROR);
-    }
-
     /// The current entry state as known by the timer. This is not the value of
     /// `state`, but lets the timer know how to converge its state to `state`.
     pub(crate) fn when_internal(&self) -> Option<u64> {
@@ -317,10 +269,6 @@ impl Entry {
 
     /// Only called by `Registration`
     pub(crate) fn reset(entry: &mut Arc<Entry>) {
-        if !entry.is_registered() {
-            return;
-        }
-
         let inner = match entry.upgrade_inner() {
             Some(inner) => inner,
             None => return,
@@ -367,7 +315,7 @@ impl Entry {
     }
 
     fn upgrade_inner(&self) -> Option<Arc<Inner>> {
-        self.inner.as_ref().and_then(|inner| inner.upgrade())
+        self.inner.upgrade()
     }
 }
 
diff --git a/tokio/src/time/driver/handle.rs b/tokio/src/time/driver/handle.rs
index 5d2e6b9b..86ff904e 100644
--- a/tokio/src/time/driver/handle.rs
+++ b/tokio/src/time/driver/handle.rs
@@ -1,6 +1,4 @@
 use crate::time::driver::Inner;
-use crate::time::Error;
-
 use std::cell::RefCell;
 use std::fmt;
 use std::marker::PhantomData;
@@ -24,7 +22,7 @@ thread_local! {
 }
 
 #[derive(Debug)]
-///Unsets default timer handler on drop.
+/// Guard that unsets the current default timer on drop.
 pub(crate) struct DefaultGuard<'a> {
     prev: Option<HandlePriv>,
     _lifetime: PhantomData<&'a u8>,
@@ -39,7 +37,7 @@ impl Drop for DefaultGuard<'_> {
     }
 }
 
-///Sets handle to default timer, returning guard that unsets it on drop.
+/// Sets handle to default timer, returning guard that unsets it on drop.
 ///
 /// # Panics
 ///
@@ -82,11 +80,13 @@ impl Default for Handle {
 impl HandlePriv {
     /// Try to get a handle to the current timer.
     ///
-    /// Returns `Err` if no handle is found.
-    pub(crate) fn try_current() -> Result<HandlePriv, Error> {
+    /// # Panics
+    ///
+    /// This function panics if there is no current timer set.
+    pub(crate) fn current() -> Self {
         CURRENT_TIMER.with(|current| match *current.borrow() {
-            Some(ref handle) => Ok(handle.clone()),
-            None => Err(Error::shutdown()),
+            Some(ref handle) => handle.clone(),
+            None => panic!("no current timer"),
         })
     }
 
diff --git a/tokio/src/time/driver/registration.rs b/tokio/src/time/driver/registration.rs
index 27b4c1cc..728d2993 100644
--- a/tokio/src/time/driver/registration.rs
+++ b/tokio/src/time/driver/registration.rs
@@ -15,11 +15,8 @@ pub(crate) struct Registration {
 
 impl Registration {
     pub(crate) fn new(deadline: Instant, duration: Duration) -> Registration {
-        fn is_send<T: Send + Sync>() {}
-        is_send::<Registration>();
-
         Registration {
-            entry: Arc::new(Entry::new(deadline, duration)),
+            entry: Entry::new(deadline, duration),
         }
     }
 
@@ -27,12 +24,6 @@ impl Registration {
         self.entry.time_ref().deadline
     }
 
-    pub(crate) fn register(&mut self) {
-        if !self.entry.is_registered() {
-            Entry::register(&mut self.entry)
-        }
-    }
-
     pub(crate) fn reset(&mut self, deadline: Instant) {
         unsafe {
             self.entry.time_mut().deadline = deadline;
diff --git a/tokio/src/time/tests/mod.rs b/tokio/src/time/tests/mod.rs
index 0aead136..6a9f25da 100644
--- a/tokio/src/time/tests/mod.rs
+++ b/tokio/src/time/tests/mod.rs
@@ -1,4 +1,24 @@
 mod mock_clock;
-
 mod test_delay;
 mod test_queue;
+
+use crate::time::{self, Instant};
+use std::time::Duration;
+
+fn assert_send<T: Send>() {}
+fn assert_sync<T: Sync>() {}
+
+#[test]
+fn registration_is_send_and_sync() {
+    use crate::time::driver::Registration;
+
+    assert_send::<Registration>();
+    assert_sync::<Registration>();
+}
+
+#[test]
+#[should_panic]
+fn delay_is_eager() {
+    let when = Instant::now() + Duration::from_millis(100);
+    let _ = time::delay_until(when);
+}
diff --git a/tokio/src/time/tests/test_delay.rs b/tokio/src/time/tests/test_delay.rs
index 8b52e0a3..43bfe379 100644
--- a/tokio/src/time/tests/test_delay.rs
+++ b/tokio/src/time/tests/test_delay.rs
@@ -181,29 +181,13 @@ fn delayed_delay_level_1() {
 }
 
 #[test]
+#[should_panic]
 fn creating_delay_outside_of_context() {
     let now = Instant::now();
 
     // This creates a delay outside of the context of a mock timer. This tests
-    // that it will still expire.
-    let mut fut = task::spawn(delay_until(now + ms(500)));
-
-    mock(|clock| {
-        // This registers the delay with the timer
-        assert_pending!(fut.poll());
-
-        // Wait some time... the timer is cascading
-        clock.turn_for(ms(1000));
-        assert_eq!(clock.advanced(), ms(448));
-
-        assert_pending!(fut.poll());
-
-        clock.turn_for(ms(1000));
-        assert_eq!(clock.advanced(), ms(500));
-
-        // The delay has elapsed
-        assert_ready!(fut.poll());
-    });
+    // that it will panic.
+    let _fut = task::spawn(delay_until(now + ms(500)));
 }
 
 #[test]