context: test WithCancel with canceled parent

Change-Id: I32079cc12cfffb8520f0073a8b5119705dc0cd1b
Reviewed-on: https://go-review.googlesource.com/27401Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/context/context_test.go

@@ -584,6 +584,21 @@ func TestCancelRemoves(t *testing.T) {
 	checkChildren("after cancelling WithTimeout child", ctx, 0)
 }
 
+func TestWithCancelCanceledParent(t *testing.T) {
+	parent, pcancel := WithCancel(Background())
+	pcancel()
+
+	c, _ := WithCancel(parent)
+	select {
+	case <-c.Done():
+	case <-time.After(5 * time.Second):
+		t.Fatal("timeout waiting for Done")
+	}
+	if got, want := c.Err(), Canceled; got != want {
+		t.Errorf("child not cancelled; got = %v, want = %v", got, want)
+	}
+}
+
 func TestWithValueChecksKey(t *testing.T) {
 	panicVal := recoveredValue(func() { WithValue(Background(), []byte("foo"), "bar") })
 	if panicVal == nil {