Merge branch 'fix-double' into 'master'

Avoid panic on double-cancellation

See merge request grab-x/async!5

task.go

@@ -114,7 +114,12 @@ func (t *task) Cancel() {
 
 	// Attempt to cancel the task if it's in the running state
 	if t.cancel != nil {
-		close(t.cancel)
+		select {
+		case <-t.cancel:
+			return
+		default:
+			close(t.cancel)
+		}
 	}
 }
 

task_test.go

@@ -116,3 +116,19 @@ func TestTaskCancelRunning(t *testing.T) {
 	_, err := task.Outcome()
 	assert.Equal(t, errCancelled, err)
 }
+
+func TestTaskCancelTwice(t *testing.T) {
+	task := Invoke(context.Background(), func(context.Context) (interface{}, error) {
+		time.Sleep(500 * time.Millisecond)
+		return 1, nil
+	})
+
+	assert.NotPanics(t, func() {
+		for i := 0; i < 100; i++ {
+			task.Cancel()
+		}
+	})
+
+	_, err := task.Outcome()
+	assert.Equal(t, errCancelled, err)
+}