time: add Ticker.Reset

This CL implements Ticker.Reset method in time package.

Benchmark:
name                 time/op
TickerReset-12       6.41µs ±10%
TickerResetNaive-12  95.7µs ±12%

Fixes golang#33184

Change-Id: I12c651f81e452541bcbbc748b45f038aae1f8dae
Reviewed-on: https://go-review.googlesource.com/c/go/+/217362
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

api/next.txt

@@ -1 +1,2 @@
 pkg testing, method (*T) Deadline() (time.Time, bool)
+pkg time, method (*Ticker) Reset(Duration)

doc/go1.15.html

@@ -80,3 +80,13 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 <p>
 TODO
 </p>
+
+<dl id="time"><dt><a href="/pkg/time/">time</a></dt>
+  <dd>
+    <p><!-- golang.org/issue/33184 -->
+       The new method
+       <a href="/pkg/time#Ticker.Reset"><code>Ticker.Reset</code></a>
+       supports changing the duration of a ticker.
+    </p>
+  </dd>
+</dl><!-- time -->

src/runtime/time.go

@@ -233,6 +233,12 @@ func resetTimer(t *timer, when int64) {
 	resettimer(t, when)
 }
 
+// modTimer modifies an existing timer.
+//go:linkname modTimer time.modTimer
+func modTimer(t *timer, when, period int64, f func(interface{}, uintptr), arg interface{}, seq uintptr) {
+	modtimer(t, when, period, f, arg, seq)
+}
+
 // Go runtime.
 
 // Ready the goroutine arg.
@@ -402,7 +408,7 @@ func dodeltimer0(pp *p) bool {
 }
 
 // modtimer modifies an existing timer.
-// This is called by the netpoll code.
+// This is called by the netpoll code or time.Ticker.Reset.
 func modtimer(t *timer, when, period int64, f func(interface{}, uintptr), arg interface{}, seq uintptr) {
 	if when < 0 {
 		when = maxWhen

src/time/sleep.go

@@ -39,6 +39,7 @@ func when(d Duration) int64 {
 func startTimer(*runtimeTimer)
 func stopTimer(*runtimeTimer) bool
 func resetTimer(*runtimeTimer, int64)
+func modTimer(t *runtimeTimer, when, period int64, f func(interface{}, uintptr), arg interface{}, seq uintptr)
 
 // The Timer type represents a single event.
 // When the Timer expires, the current time will be sent on C,

src/time/tick.go

@@ -46,6 +46,15 @@ func (t *Ticker) Stop() {
 	stopTimer(&t.r)
 }
 
+// Reset stops a ticker and resets its period to the specified duration.
+// The next tick will arrive after the new period elapses.
+func (t *Ticker) Reset(d Duration) {
+	if t.r.f == nil {
+		panic("time: Reset called on uninitialized Ticker")
+	}
+	modTimer(&t.r, when(d), int64(d), t.r.f, t.r.arg, t.r.seq)
+}
+
 // Tick is a convenience wrapper for NewTicker providing access to the ticking
 // channel only. While Tick is useful for clients that have no need to shut down
 // the Ticker, be aware that without a way to shut it down the underlying

src/time/tick_test.go

@@ -36,13 +36,17 @@ func TestTicker(t *testing.T) {
 	for i := 0; i < 5; i++ {
 		ticker := NewTicker(delta)
 		t0 := Now()
-		for i := 0; i < count; i++ {
+		for i := 0; i < count/2; i++ {
+			<-ticker.C
+		}
+		ticker.Reset(delta * 2)
+		for i := count / 2; i < count; i++ {
 			<-ticker.C
 		}
 		ticker.Stop()
 		t1 := Now()
 		dt := t1.Sub(t0)
-		target := delta * Duration(count)
+		target := 3 * delta * Duration(count/2)
 		slop := target * 2 / 10
 		if dt < target-slop || dt > target+slop {
 			errs = append(errs, fmt.Sprintf("%d %s ticks took %s, expected [%s,%s]", count, delta, dt, target-slop, target+slop))
@@ -118,3 +122,24 @@ func BenchmarkTicker(b *testing.B) {
 		ticker.Stop()
 	})
 }
+
+func BenchmarkTickerReset(b *testing.B) {
+	benchmark(b, func(n int) {
+		ticker := NewTicker(Nanosecond)
+		for i := 0; i < n; i++ {
+			ticker.Reset(Nanosecond * 2)
+		}
+		ticker.Stop()
+	})
+}
+
+func BenchmarkTickerResetNaive(b *testing.B) {
+	benchmark(b, func(n int) {
+		ticker := NewTicker(Nanosecond)
+		for i := 0; i < n; i++ {
+			ticker.Stop()
+			ticker = NewTicker(Nanosecond * 2)
+		}
+		ticker.Stop()
+	})
+}