Add Context cancellation to clock.

common/clock/clock.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"
 )
 
 // Clock is an interface to system time.
 //
 // The standard clock is SystemClock, which falls through to the system time
 // library. Another clock, FakeClock, is available to simulate time facilities
 // for testing.
 type Clock interface {
         // Returns the current time (see time.Now).
         Now() time.Time
-        // Sleeps the current goroutine (see time.Sleep)
-        Sleep(time.Duration)
+
+        // Sleeps the current goroutine (see time.Sleep).
+        //
+        // If the supplied Context is canceled prior to the specified duration,
+        // CancelSleep will return the Context's error. If the sleep completes
+        // naturally, it will return nil.
+        Sleep(context.Context, time.Duration) error
+
         // Creates a new Timer instance, bound to this Clock.
-        NewTimer() Timer
+        //
+        // If the supplied Context is canceled, the timer will expire immediately.
+        NewTimer(c context.Context) Timer
+
         // Waits a duration, then sends the current time over the returned channel.
-        After(time.Duration) <-chan time.Time
+        //
+        // If the supplied Context is canceled, the timer will expire immediately.
+        After(context.Context, time.Duration) <-chan TimerResult
 }