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 #33184

Change-Id: I4cbd31796efa012b2a297bb342158f11a4a31fef
Reviewed-on: https://go-review.googlesource.com/c/go/+/220424
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

4 files changed, 50 insertions, 4 deletions

diff --git a/src/runtime/time.go b/src/runtime/time.go
index af5db4cc58..9e1129537a 100644
--- a/src/runtime/time.go
+++ b/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
diff --git a/src/time/sleep.go b/src/time/sleep.go
index 37de846b11..bd0ed9aaba 100644
--- a/src/time/sleep.go
+++ b/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,
diff --git a/src/time/tick.go b/src/time/tick.go
index e4cd43aa82..152d5a706b 100644
--- a/src/time/tick.go
+++ b/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
diff --git a/src/time/tick_test.go b/src/time/tick_test.go
index 71ea3672b8..c0c6e76b53 100644
--- a/src/time/tick_test.go
+++ b/src/time/tick_test.go
@@ -22,7 +22,12 @@ func TestTicker(t *testing.T) {
 
 	// On Darwin ARM64 the tick frequency seems limited. Issue 35692.
 	if runtime.GOOS == "darwin" && runtime.GOARCH == "arm64" {
-		count = 5
+		// The following test will run ticker count/2 times then reset
+		// the ticker to double the duration for the rest of count/2.
+		// Since tick frequency is limited on Darwin ARM64, use even
+		// number to give the ticks more time to let the test pass.
+		// See CL 220638.
+		count = 6
 		delta = 100 * Millisecond
 	}
 
@@ -36,13 +41,17 @@ 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 +127,24 @@ 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()
+	})
+}