Add Context cancellation to clock.

common/clock/systemtimer_test.go

Index: common/clock/systemtimer_test.go
diff --git a/common/clock/systemtimer_test.go b/common/clock/systemtimer_test.go
index 8b9cbc54174ea2dfe1f4d9fb861fe771ad73ecb0..39d0a731b054a1d967864118fa303dcbd13e63a6 100644
--- a/common/clock/systemtimer_test.go
+++ b/common/clock/systemtimer_test.go
@@ -8,6 +8,8 @@ import (
 	"testing"
 	"time"
 
+	"golang.org/x/net/context"
+
 	. "github.com/smartystreets/goconvey/convey"
 )
 
@@ -15,16 +17,24 @@ func TestSystemTimer(t *testing.T) {
 	t.Parallel()
 
 	Convey(`A systemTimer instance`, t, func() {
-		t := new(systemTimer)
+		ctx, cancelFunc := context.WithCancel(context.Background())
+		t := GetSystemClock().NewTimer(ctx)
+		defer t.Stop()
 
-		Convey(`Should start with a nil channel.`, func() {
-			So(t.GetC(), ShouldBeNil)
+		Convey(`Should start with a non-nil channel.`, func() {
+			So(t.GetC(), ShouldNotBeNil)
 		})
 
 		Convey(`When stopped, should return inactive.`, func() {
 			So(t.Stop(), ShouldBeFalse)
 		})
 
+		Convey(`Will return prematurely if canceled.`, func() {
+			t.Reset(time.Hour)
+			cancelFunc()
+			So((<-t.GetC()).Err, ShouldEqual, context.Canceled)
+		})
+
 		Convey(`When reset`, func() {
 			active := t.Reset(1 * time.Hour)
 			So(active, ShouldBeFalse)
@@ -58,10 +68,6 @@ func TestSystemTimer(t *testing.T) {
 			Convey(`Should have a non-nil channel.`, func() {
 				So(t.GetC(), ShouldNotBeNil)
 			})
-
-			Reset(func() {
-				t.Stop()
-			})
 		})
 	})
 }