time: notify when resetting a Delay to a time in the past (#2290)

If a Delay has been polled, then the task that polled it may be waiting
for a notification.  If the delay gets reset to a time in the past, then
it immediately becomes elapsed, so it should notify the relevant task.

1 files changed, 12 insertions, 12 deletions

diff --git a/tokio/src/time/driver/entry.rs b/tokio/src/time/driver/entry.rs
index 20cc8240..8e1e6b2f 100644
--- a/tokio/src/time/driver/entry.rs
+++ b/tokio/src/time/driver/entry.rs
@@ -266,8 +266,9 @@ impl Entry {
         let when = inner.normalize_deadline(deadline);
         let elapsed = inner.elapsed();
 
+        let next = if when <= elapsed { ELAPSED } else { when };
+
         let mut curr = entry.state.load(SeqCst);
-        let mut notify;
 
         loop {
             // In these two cases, there is no work to do when resetting the
@@ -278,16 +279,6 @@ impl Entry {
                 return;
             }
 
-            let next;
-
-            if when <= elapsed {
-                next = ELAPSED;
-                notify = !is_elapsed(curr);
-            } else {
-                next = when;
-                notify = true;
-            }
-
             let actual = entry.state.compare_and_swap(curr, next, SeqCst);
 
             if curr == actual {
@@ -297,7 +288,16 @@ impl Entry {
             curr = actual;
         }
 
-        if notify {
+        // If the state has transitioned to 'elapsed' then wake the task as
+        // this entry is ready to be polled.
+        if !is_elapsed(curr) && is_elapsed(next) {
+            entry.waker.wake();
+        }
+
+        // The driver tracks all non-elapsed entries; notify the driver that it
+        // should update its state for this entry unless the entry had already
+        // elapsed and remains elapsed.
+        if !is_elapsed(curr) || !is_elapsed(next) {
             let _ = inner.queue(entry);
         }
     }