time: Remove `HandlePriv` (#1896)

## Motivation

#1800 removed the lazy binding of `Delay`s to timers. With the removal of the
logic required for that, `HandlePriv` is no longer needed. This PR removes the
use of `HandlePriv`.

A `TODO` was also removed that would panic if when registering a new `Delay`
the current timer handle was full. That has been fixed to now immediately
transition that `Delay` to an error state that can be handled in a similar way
to other error states.

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

3 files changed, 38 insertions, 63 deletions

diff --git a/tokio/src/time/driver/entry.rs b/tokio/src/time/driver/entry.rs
index 708b6cfb..d5fab897 100644
--- a/tokio/src/time/driver/entry.rs
+++ b/tokio/src/time/driver/entry.rs
@@ -1,6 +1,6 @@
 use crate::loom::sync::atomic::AtomicU64;
 use crate::sync::AtomicWaker;
-use crate::time::driver::{HandlePriv, Inner};
+use crate::time::driver::{Handle, Inner};
 use crate::time::{Duration, Error, Instant};
 
 use std::cell::UnsafeCell;
@@ -105,36 +105,24 @@ const ERROR: u64 = u64::MAX;
 
 impl Entry {
     pub(crate) fn new(deadline: Instant, duration: Duration) -> Arc<Entry> {
-        let handle_priv = HandlePriv::current();
-        let handle = handle_priv.inner().unwrap();
+        let inner = Handle::current().inner().unwrap();
+        let entry: Entry;
 
         // 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
+        if inner.increment().is_err() {
+            entry = Entry::new2(deadline, duration, Weak::new(), ERROR)
         } else {
-            when
-        };
-
-        let entry = Arc::new(Entry {
-            time: CachePadded(UnsafeCell::new(Time { deadline, duration })),
-            inner: handle_priv.into_inner(),
-            waker: AtomicWaker::new(),
-            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()),
-        });
+            let when = inner.normalize_deadline(deadline);
+            let state = if when <= inner.elapsed() {
+                ELAPSED
+            } else {
+                when
+            };
+            entry = Entry::new2(deadline, duration, Arc::downgrade(&inner), state);
+        }
 
-        if handle.queue(&entry).is_err() {
+        let entry = Arc::new(entry);
+        if inner.queue(&entry).is_err() {
             entry.error();
         }
 
@@ -314,6 +302,20 @@ impl Entry {
         }
     }
 
+    fn new2(deadline: Instant, duration: Duration, inner: Weak<Inner>, state: u64) -> Self {
+        Self {
+            time: CachePadded(UnsafeCell::new(Time { deadline, duration })),
+            inner,
+            waker: AtomicWaker::new(),
+            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()),
+        }
+    }
+
     fn upgrade_inner(&self) -> Option<Arc<Inner>> {
         self.inner.upgrade()
     }
diff --git a/tokio/src/time/driver/handle.rs b/tokio/src/time/driver/handle.rs
index 86ff904e..7d21ad0d 100644
--- a/tokio/src/time/driver/handle.rs
+++ b/tokio/src/time/driver/handle.rs
@@ -5,26 +5,20 @@ use std::marker::PhantomData;
 use std::sync::{Arc, Weak};
 
 /// Handle to time driver instance.
-#[derive(Debug, Clone)]
-pub(crate) struct Handle {
-    inner: Option<HandlePriv>,
-}
-
-/// Like `Handle` but never `None`.
 #[derive(Clone)]
-pub(crate) struct HandlePriv {
+pub(crate) struct Handle {
     inner: Weak<Inner>,
 }
 
 thread_local! {
     /// Tracks the timer for the current execution context.
-    static CURRENT_TIMER: RefCell<Option<HandlePriv>> = RefCell::new(None)
+    static CURRENT_TIMER: RefCell<Option<Handle>> = RefCell::new(None)
 }
 
 #[derive(Debug)]
 /// Guard that unsets the current default timer on drop.
 pub(crate) struct DefaultGuard<'a> {
-    prev: Option<HandlePriv>,
+    prev: Option<Handle>,
     _lifetime: PhantomData<&'a u8>,
 }
 
@@ -47,10 +41,6 @@ pub(crate) fn set_default(handle: &Handle) -> DefaultGuard<'_> {
         let mut current = current.borrow_mut();
         let prev = current.take();
 
-        let handle = handle
-            .as_priv()
-            .unwrap_or_else(|| panic!("`handle` does not reference a timer"));
-
         *current = Some(handle.clone());
 
         DefaultGuard {
@@ -61,23 +51,11 @@ pub(crate) fn set_default(handle: &Handle) -> DefaultGuard<'_> {
 }
 
 impl Handle {
-    pub(crate) fn new(inner: Weak<Inner>) -> Handle {
-        let inner = HandlePriv { inner };
-        Handle { inner: Some(inner) }
+    /// Create a new timer `Handle` from a shared `Inner` timer state.
+    pub(crate) fn new(inner: Weak<Inner>) -> Self {
+        Handle { inner }
     }
 
-    fn as_priv(&self) -> Option<&HandlePriv> {
-        self.inner.as_ref()
-    }
-}
-
-impl Default for Handle {
-    fn default() -> Handle {
-        Handle { inner: None }
-    }
-}
-
-impl HandlePriv {
     /// Try to get a handle to the current timer.
     ///
     /// # Panics
@@ -94,15 +72,10 @@ impl HandlePriv {
     pub(crate) fn inner(&self) -> Option<Arc<Inner>> {
         self.inner.upgrade()
     }
-
-    /// Consume the handle, returning the weak Inner ref.
-    pub(crate) fn into_inner(self) -> Weak<Inner> {
-        self.inner
-    }
 }
 
-impl fmt::Debug for HandlePriv {
+impl fmt::Debug for Handle {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "HandlePriv")
+        write!(f, "Handle")
     }
 }
diff --git a/tokio/src/time/driver/mod.rs b/tokio/src/time/driver/mod.rs
index e3b45101..4a1ba4fe 100644
--- a/tokio/src/time/driver/mod.rs
+++ b/tokio/src/time/driver/mod.rs
@@ -7,7 +7,7 @@ mod entry;
 use self::entry::Entry;
 
 mod handle;
-pub(crate) use self::handle::{set_default, Handle, HandlePriv};
+pub(crate) use self::handle::{set_default, Handle};
 
 mod registration;
 pub(crate) use self::registration::Registration;