concurrency: set keyPrefix when calling ResumeElection()

This change allows users to use Observe() after resuming an election as
Observe() requires a valid keyPrefix to function properly.

clientv3/concurrency/election.go

@@ -48,6 +48,7 @@ func NewElection(s *Session, pfx string) *Election {
 // ResumeElection initializes an election with a known leader.
 func ResumeElection(s *Session, pfx string, leaderKey string, leaderRev int64) *Election {
 	return &Election{
+		keyPrefix:     pfx,
 		session:       s,
 		leaderKey:     leaderKey,
 		leaderRev:     leaderRev,

clientv3/concurrency/election_test.go

@@ -0,0 +1,116 @@
+// Copyright 2018 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 concurrency_test
+
+import (
+	"context"
+	"log"
+	"testing"
+
+	"time"
+
+	"strings"
+
+	"go.etcd.io/etcd/clientv3"
+	"go.etcd.io/etcd/clientv3/concurrency"
+)
+
+func TestResumeElection(t *testing.T) {
+	const prefix = "/resume-election/"
+
+	cli, err := clientv3.New(clientv3.Config{Endpoints: endpoints})
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer cli.Close()
+
+	s, err := concurrency.NewSession(cli)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer s.Close()
+
+	e := concurrency.NewElection(s, prefix)
+
+	// Entire test should never take more than 10 seconds
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*10)
+	defer cancel()
+
+	// Become leader
+	if err := e.Campaign(ctx, "candidate1"); err != nil {
+		t.Fatalf("Campaign() returned non nil err: %s", err)
+	}
+
+	// Get the leadership details of the current election
+	leader, err := e.Leader(ctx)
+	if err != nil {
+		t.Fatalf("Leader() returned non nil err: %s", err)
+	}
+
+	// Recreate the election
+	e = concurrency.ResumeElection(s, prefix,
+		string(leader.Kvs[0].Key), leader.Kvs[0].CreateRevision)
+
+	respChan := make(chan *clientv3.GetResponse)
+	go func() {
+		o := e.Observe(ctx)
+		respChan <- nil
+		for {
+			select {
+			case resp, ok := <-o:
+				if !ok {
+					t.Fatal("Observe() channel closed prematurely")
+				}
+				// Ignore any observations that candidate1 was elected
+				if string(resp.Kvs[0].Value) == "candidate1" {
+					continue
+				}
+				respChan <- &resp
+				return
+			}
+		}
+	}()
+
+	// Wait until observe goroutine is running
+	<-respChan
+
+	// Put some random data to generate a change event, this put should be
+	// ignored by Observe() because it is not under the election prefix.
+	_, err = cli.Put(ctx, "foo", "bar")
+	if err != nil {
+		t.Fatalf("Put('foo') returned non nil err: %s", err)
+	}
+
+	// Resign as leader
+	if err := e.Resign(ctx); err != nil {
+		t.Fatalf("Resign() returned non nil err: %s", err)
+	}
+
+	// Elect a different candidate
+	if err := e.Campaign(ctx, "candidate2"); err != nil {
+		t.Fatalf("Campaign() returned non nil err: %s", err)
+	}
+
+	// Wait for observed leader change
+	resp := <-respChan
+
+	kv := resp.Kvs[0]
+	if !strings.HasPrefix(string(kv.Key), prefix) {
+		t.Errorf("expected observed election to have prefix '%s' got '%s'", prefix, string(kv.Key))
+	}
+	if string(kv.Value) != "candidate2" {
+		t.Errorf("expected new leader to be 'candidate1' got '%s'", string(kv.Value))
+	}
+}