Add Context cancellation to clock.

common/clock/clockcontext.go

 // Copyright 2015 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 (
         "sync"
         "time"
 
@@ skipping 53 matching lines @@
         d := deadline.Sub(Now(c))
         if d <= 0 {
                 // Deadline has already passed.
                 c.setError(context.DeadlineExceeded)
                 cancelFunc()
                 return c, cancelFunc
         }
 
         // Invoke our cancelFunc after the specified time.
         go func() {
-                select {
-                case <-c.Done():
-                        break
-
-                case <-After(c, d):
+                if ar := <-After(c, d); ar.Err == nil {
+                        // Timer expired naturally.
                         c.setError(context.DeadlineExceeded)
                         cancelFunc()
                 }
         }()
         return c, cancelFunc
 }
 
 // WithTimeout is a clock library implementation of context.WithTimeout that
 // uses the clock library's time features instead of the Go time library.
 //
 // For more information, see context.WithTimeout.
 func WithTimeout(parent context.Context, timeout time.Duration) (context.Context, context.CancelFunc) {
         return WithDeadline(parent, Now(parent).Add(timeout))
 }