dm: fix leader did not retire after delete the key

dm/pkg/election/election.go

@@ -264,6 +264,12 @@ func (e *Election) campaignLoop(ctx context.Context, session *concurrency.Sessio
 			e.l.Debug("begin to campaign", zap.Stringer("current member", e.info))
 
 			err2 := elec.Campaign(ctx2, e.infoStr)
+			failpoint.Inject("mockCapaignSucceedButReturnErr", func() {
+				if err2 == nil {
+					err2 = errors.New("mock campaign succeed but return error")
+					time.Sleep(time.Second)
+				}
+			})
 			if err2 != nil {
 				// because inner commit may return undetermined error, we try to delete the election key manually
 				deleted, err3 := e.ClearSessionIfNeeded(ctx, e.ID())
@@ -282,6 +288,7 @@ func (e *Election) campaignLoop(ctx context.Context, session *concurrency.Sessio
 		var (
 			leaderKey  string
 			leaderInfo *CampaignerInfo
+			revision   int64
 		)
 		eleObserveCh := elec.Observe(ctx2)
 
@@ -300,6 +307,7 @@ func (e *Election) campaignLoop(ctx context.Context, session *concurrency.Sessio
 				e.l.Info("get response from election observe", zap.String("key", string(resp.Kvs[0].Key)), zap.String("value", string(resp.Kvs[0].Value)))
 				leaderKey = string(resp.Kvs[0].Key)
 				leaderInfo, err = getCampaignerInfo(resp.Kvs[0].Value)
+				revision = resp.Header.Revision
 				if err != nil {
 					// this should never happened
 					e.l.Error("fail to get leader information", zap.String("value", string(resp.Kvs[0].Value)), zap.Error(err))
@@ -330,7 +338,7 @@ func (e *Election) campaignLoop(ctx context.Context, session *concurrency.Sessio
 
 		e.l.Info("become leader", zap.Stringer("current member", e.info))
 		e.notifyLeader(ctx, leaderInfo) // become the leader now
-		e.watchLeader(ctx, session, leaderKey, elec)
+		e.watchLeader(ctx, session, leaderKey, elec, revision)
 		e.l.Info("retire from leader", zap.Stringer("current member", e.info))
 		e.notifyLeader(ctx, nil) // need to re-campaign
 		oldLeaderID = ""
@@ -359,8 +367,8 @@ func (e *Election) notifyLeader(ctx context.Context, leaderInfo *CampaignerInfo)
 	}
 }
 
-func (e *Election) watchLeader(ctx context.Context, session *concurrency.Session, key string, elec *concurrency.Election) {
-	e.l.Debug("watch leader key", zap.String("key", key))
+func (e *Election) watchLeader(ctx context.Context, session *concurrency.Session, key string, elec *concurrency.Election, revision int64) {
+	e.l.Debug("watch leader key", zap.String("key", key), zap.Int64("revision", revision), zap.Stringer("current member", e.info))
 
 	e.campaignMu.Lock()
 	e.resignCh = make(chan struct{})
@@ -374,7 +382,7 @@ func (e *Election) watchLeader(ctx context.Context, session *concurrency.Session
 
 	wCtx, cancel := context.WithCancel(ctx)
 	defer cancel()
-	wch := e.cli.Watch(wCtx, key)
+	wch := e.cli.Watch(wCtx, key, clientv3.WithRev(revision+1))
 
 	for {
 		if e.evictLeader.Load() {

dm/pkg/election/election_test.go

@@ -412,3 +412,88 @@ func (t *testElectionSuite) TestElectionDeleteKey(c *C) {
 	c.Assert(err, IsNil)
 	wg.Wait()
 }
+
+func (t *testElectionSuite) TestElectionSucceedButReturnError(c *C) {
+	var (
+		timeout    = 5 * time.Second
+		sessionTTL = 60
+		key        = "unit-test/election-succeed-but-return-error"
+		ID1        = "member1"
+		ID2        = "member2"
+		addr1      = "127.0.0.1:1"
+		addr2      = "127.0.0.1:2"
+	)
+	cli, err := etcdutil.CreateClient([]string{t.endPoint}, nil)
+	c.Assert(err, IsNil)
+	defer cli.Close()
+
+	ctx1, cancel1 := context.WithCancel(context.Background())
+	defer cancel1()
+
+	e1, err := NewElection(ctx1, cli, sessionTTL, key, ID1, addr1, t.notifyBlockTime)
+	c.Assert(err, IsNil)
+	defer e1.Close()
+
+	// e1 should become the leader
+	select {
+	case leader := <-e1.LeaderNotify():
+		c.Assert(leader.ID, Equals, ID1)
+	case <-time.After(timeout):
+		c.Fatal("leader campaign timeout")
+	}
+	c.Assert(e1.IsLeader(), IsTrue)
+	_, leaderID, leaderAddr, err := e1.LeaderInfo(ctx1)
+	c.Assert(err, IsNil)
+	c.Assert(leaderID, Equals, e1.ID())
+	c.Assert(leaderAddr, Equals, addr1)
+
+	c.Assert(failpoint.Enable("github.com/pingcap/tiflow/dm/pkg/election/mockCapaignSucceedButReturnErr", `return()`), IsNil)
+	//nolint:errcheck
+	defer failpoint.Disable("github.com/pingcap/tiflow/dm/pkg/election/mockCapaignSucceedButReturnErr")
+
+	// start e2
+	ctx2, cancel2 := context.WithCancel(context.Background())
+	defer cancel2()
+	e2, err := NewElection(ctx2, cli, sessionTTL, key, ID2, addr2, t.notifyBlockTime)
+	c.Assert(err, IsNil)
+	defer e2.Close()
+	select {
+	case leader := <-e2.leaderCh:
+		c.Assert(leader.ID, Equals, ID1)
+	case <-time.After(timeout):
+		c.Fatal("leader campaign timeout")
+	}
+	// but the leader should still be e1
+	_, leaderID, leaderAddr, err = e2.LeaderInfo(ctx2)
+	c.Assert(err, IsNil)
+	c.Assert(leaderID, Equals, e1.ID())
+	c.Assert(leaderAddr, Equals, addr1)
+	c.Assert(e2.IsLeader(), IsFalse)
+
+	e1.Close() // stop the campaign for e1
+	c.Assert(e1.IsLeader(), IsFalse)
+
+	ctx3, cancel3 := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel3()
+	deleted, err := e2.ClearSessionIfNeeded(ctx3, ID1)
+	c.Assert(err, IsNil)
+	c.Assert(deleted, IsFalse)
+
+	// e2 should become the leader
+	select {
+	case leader := <-e2.LeaderNotify():
+		c.Assert(leader.ID, Equals, ID2)
+	case <-time.After(timeout):
+		c.Fatal("leader campaign timeout")
+	}
+
+	// the leader retired after deleted the key
+	select {
+	case err2 := <-e2.ErrorNotify():
+		c.Fatalf("delete the leader key should not get an error, %v", err2)
+	case leader := <-e2.LeaderNotify():
+		c.Assert(leader, IsNil)
+	case <-time.After(timeout):
+		c.Fatal("leader retire timeout")
+	}
+}