Add Context cancellation to clock.

common/clock/timer.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 (
         "time"
 )
 
 // Timer is a wrapper around the time.Timer structure.
 //
 // A Timer is instantiated from a Clock instance and started via its Reset()
 // method.
 type Timer interface {
-        // GetC returns the underlying timer's channel, or nil if the timer is no
-        // running.
-        GetC() <-chan time.Time
+        // GetC returns the underlying timer's channel.
+        //
+        // If the Timer is interrupted via Stop, its channel will block indefinitely.
+        GetC() <-chan TimerResult
 
         // Reset configures the timer to expire after a specified duration.
         //
         // If the timer is already running, its previous state will be cleared and
-        // this method will return true.
+        // this method will return true. The channel returned by GetC() will not
+        // change due to Reset.
         Reset(d time.Duration) bool
 
         // Stop clears any timer tasking, rendering it inactive.
         //
         // Stop may be called on an inactive timer, in which case nothing will happen
-        // If the timer is active, it will be stopped and this method will return true.
+        // If the timer is active, it will be stopped and this method will return
+        // true.
+        //
+        // If a timer is stopped, its GetC channel will block indefinitely to avoid
+        // erroneously unblocking goroutines that are waiting on it. This is
+        // consistent with time.Timer.
         Stop() bool
 }
+
+// TimerResult is the result for a timer operation.
+//
+// Time will be set to the time when the result was generated. If the source of
+// the result was prematurely terminated (e.g., due to Context cancellation)
+// Err will be non-nil.
+type TimerResult struct {
+        time.Time
+
+        // Err, if not nil, indicates that After did not finish naturally and contains
+        // the reason why.
+        Err error
+}