Add Context cancellation to clock.

common/clock/systemclock.go

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package clock
 
 import (
         "time"
+
+        "golang.org/x/net/context"
 )
 
 // Implementation of Clock that uses Go's standard library.
 type systemClock struct{}
 
 // System clock instance.
 var systemClockInstance systemClock
 
 var _ Clock = systemClock{}
 
 // GetSystemClock returns an instance of a Clock whose method calls directly use
 // Go's "time" library.
 func GetSystemClock() Clock {
         return systemClockInstance
 }
 
 func (systemClock) Now() time.Time {
         return time.Now()
 }
 
-func (systemClock) Sleep(d time.Duration) {
-        time.Sleep(d)
+func (sc systemClock) Sleep(c context.Context, d time.Duration) error {
+        ar := <-sc.After(c, d)
+        return ar.Err
 }
 
-func (systemClock) NewTimer() Timer {
-        return new(systemTimer)
+func (systemClock) NewTimer(ctx context.Context) Timer {
+        return newSystemTimer(ctx)
 }
 
-func (systemClock) After(d time.Duration) <-chan time.Time {
-        return time.After(d)
+func (sc systemClock) After(ctx context.Context, d time.Duration) <-chan TimerResult {
+        t := sc.NewTimer(ctx)
+        t.Reset(d)
+        return t.GetC()
 }