clientv3: Add KeyExists and KeyNotExists Cmp helpers

This is quite useful for transactions.

clientv3/clientv3util/util.go

@@ -0,0 +1,32 @@
+// Copyright 2017 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package clientv3util
+
+import (
+	"github.com/coreos/etcd/clientv3"
+)
+
+// KeyExists returns a comparison operation that evaluates to true iff the given
+// key exists. It does this by checking if the key `Version` is greater than 0.
+// It is a useful guard in transaction delete operations.
+func KeyExists(key string) clientv3.Cmp {
+	return clientv3.Compare(clientv3.Version(key), ">", 0)
+}
+
+// KeyMissing returns a comparison operation that evaluates to true iff the
+// given key does not exist.
+func KeyMissing(key string) clientv3.Cmp {
+	return clientv3.Compare(clientv3.Version(key), "=", 0)
+}