clientv3: Move to client/v3 (just file move)

Mechanical:
% git mv clientv3 client/v3
% git mv client/mock client/v3/

client/v3/clientv3util/example_key_test.go

@@ -0,0 +1,66 @@
+// 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_test
+
+import (
+	"context"
+	"log"
+
+	"go.etcd.io/etcd/v3/clientv3"
+	"go.etcd.io/etcd/v3/clientv3/clientv3util"
+)
+
+func ExampleKeyMissing() {
+	cli, err := clientv3.New(clientv3.Config{
+		Endpoints: []string{"127.0.0.1:2379"},
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer cli.Close()
+	kvc := clientv3.NewKV(cli)
+
+	// perform a put only if key is missing
+	// It is useful to do the check atomically to avoid overwriting
+	// the existing key which would generate potentially unwanted events,
+	// unless of course you wanted to do an overwrite no matter what.
+	_, err = kvc.Txn(context.Background()).
+		If(clientv3util.KeyMissing("purpleidea")).
+		Then(clientv3.OpPut("purpleidea", "hello world")).
+		Commit()
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleKeyExists() {
+	cli, err := clientv3.New(clientv3.Config{
+		Endpoints: []string{"127.0.0.1:2379"},
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer cli.Close()
+	kvc := clientv3.NewKV(cli)
+
+	// perform a delete only if key already exists
+	_, err = kvc.Txn(context.Background()).
+		If(clientv3util.KeyExists("purpleidea")).
+		Then(clientv3.OpDelete("purpleidea")).
+		Commit()
+	if err != nil {
+		log.Fatal(err)
+	}
+}