time: do not panic on Interval::poll_tick if the sum of the timeout and period overflow

tokio/src/time/interval.rs

@@ -480,7 +480,9 @@ impl Interval {
             self.missed_tick_behavior
                 .next_timeout(timeout, now, self.period)
         } else {
-            timeout + self.period
+            timeout
+                .checked_add(self.period)
+                .unwrap_or_else(Instant::far_future)
         };
 
         // When we arrive here, the internal delay returned `Poll::Ready`.

tokio/tests/time_interval.rs

@@ -6,7 +6,7 @@ use std::task::{Context, Poll};
 
 use futures::{Stream, StreamExt};
 use tokio::time::{self, Duration, Instant, Interval, MissedTickBehavior};
-use tokio_test::{assert_pending, assert_ready_eq, task};
+use tokio_test::{assert_pending, assert_ready, assert_ready_eq, task};
 
 // Takes the `Interval` task, `start` variable, and optional time deltas
 // For each time delta, it polls the `Interval` and asserts that the result is
@@ -469,3 +469,9 @@ async fn stream_with_interval_poll_tick_no_waking() {
     // task when returning `Poll::Ready`.
     assert_eq!(items, vec![]);
 }
+
+#[tokio::test(start_paused = true)]
+async fn interval_doesnt_panic_max_duration_when_polling() {
+    let mut timer = task::spawn(time::interval(Duration::MAX));
+    assert_ready!(timer.enter(|cx, mut timer| timer.poll_tick(cx)));
+}