ordering: use x/net/context and add doc.go

Compilation troubles when using the "context" package.

clientv3/ordering/doc.go

@@ -0,0 +1,42 @@
+// 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 ordering is a clientv3 wrapper that caches response header revisions
+// to detect ordering violations from stale responses. Users may define a
+// policy on how to handle the ordering violation, but typically the client
+// should connect to another endpoint and reissue the request.
+//
+// The most common situation where an ordering violation happens is a client
+// reconnects to a partitioned member and issues a serializable read. Since the
+// partitioned member is likely behind the last member, it may return a Get
+// response based on a store revision older than the store revision used to
+// service a prior Get on the former endpoint.
+//
+// First, create a client:
+//
+//	cli, err := clientv3.New(clientv3.Config{Endpoints: []string{"localhost:2379"}})
+//	if err != nil {
+//		// handle error!
+//	}
+//
+// Next, override the client interface with the ordering wrapper:
+//
+//	vf := func(op clientv3.Op, resp clientv3.OpResponse, prevRev int64) error {
+//		return fmt.Errorf("ordering: issued %+v, got %+v, expected rev=%v", op, resp, prevRev)
+//	}
+//	cli.KV = ordering.NewKV(cli.KV, vf)
+//
+// Now calls using 'cli' will reject order violations with an error.
+//
+package ordering

clientv3/ordering/kv.go

@@ -15,10 +15,10 @@
 package ordering
 
 import (
-	"context"
 	"sync"
 
 	"github.com/coreos/etcd/clientv3"
+	"golang.org/x/net/context"
 )
 
 // kvOrdering ensures that serialized requests do not return