Add Context cancellation to clock.

common/clock/timer.go

Index: common/clock/timer.go
diff --git a/common/clock/timer.go b/common/clock/timer.go
index aa03c383dda4f710b8c1745d62fca5d645a9f1c8..6b0a47b5443efa84a80b9511ca8869c04bdd8841 100644
--- a/common/clock/timer.go
+++ b/common/clock/timer.go
@@ -13,19 +13,39 @@ import (
 // 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
+}