Initial QSfera import

Server/vendor/github.com/open-policy-agent/opa/v1/topdown/cancel.go

@@ -0,0 +1,33 @@
+// Copyright 2017 The OPA Authors.  All rights reserved.
+// Use of this source code is governed by an Apache2
+// license that can be found in the LICENSE file.
+
+package topdown
+
+import (
+	"sync/atomic"
+)
+
+// Cancel defines the interface for cancelling topdown queries. Cancel
+// operations are thread-safe and idempotent.
+type Cancel interface {
+	Cancel()
+	Cancelled() bool
+}
+
+type cancel struct {
+	flag int32
+}
+
+// NewCancel returns a new Cancel object.
+func NewCancel() Cancel {
+	return &cancel{}
+}
+
+func (c *cancel) Cancel() {
+	atomic.StoreInt32(&c.flag, 1)
+}
+
+func (c *cancel) Cancelled() bool {
+	return atomic.LoadInt32(&c.flag) != 0
+}