time: add Timer.Reset

Fixes #4412.

R=adg, rsc, rogpeppe, andrewdg, bradfitz
CC=golang-dev
https://golang.org/cl/7086050

src/pkg/time/sleep.go

@@ -35,10 +35,10 @@ type Timer struct {
 
 // Stop prevents the Timer from firing.
 // It returns true if the call stops the timer, false if the timer has already
-// expired or stopped.
+// expired or been stopped.
 // Stop does not close the channel, to prevent a read from the channel succeeding
 // incorrectly.
-func (t *Timer) Stop() (ok bool) {
+func (t *Timer) Stop() bool {
 	return stopTimer(&t.r)
 }
 
@@ -58,6 +58,17 @@ func NewTimer(d Duration) *Timer {
 	return t
 }
 
+// Reset changes the timer to expire after duration d.
+// It returns true if the timer had been active, false if the timer had
+// expired or been stopped.
+func (t *Timer) Reset(d Duration) bool {
+	when := nano() + int64(d)
+	active := stopTimer(&t.r)
+	t.r.when = when
+	startTimer(&t.r)
+	return active
+}
+
 func sendTime(now int64, c interface{}) {
 	// Non-blocking send of time on c.
 	// Used in NewTimer, it cannot block anyway (buffer).

src/pkg/time/sleep_test.go

@@ -245,3 +245,27 @@ func TestSleepZeroDeadlock(t *testing.T) {
 	}
 	<-c
 }
+
+func TestReset(t *testing.T) {
+	t0 := NewTimer(100 * Millisecond)
+	Sleep(50 * Millisecond)
+	if t0.Reset(150*Millisecond) != true {
+		t.Fatalf("resetting unfired timer returned false")
+	}
+	Sleep(100 * Millisecond)
+	select {
+	case <-t0.C:
+		t.Fatalf("timer fired early")
+	default:
+	}
+	Sleep(100 * Millisecond)
+	select {
+	case <-t0.C:
+	default:
+		t.Fatalf("reset timer did not fire")
+	}
+
+	if t0.Reset(50*Millisecond) != false {
+		t.Fatalf("resetting expired timer returned true")
+	}
+}