[reparentutil] ERS should not attempt to WaitForRelayLogsToApply on primary tablets that were not running replication

go/vt/vtctl/reparentutil/emergency_reparenter.go

@@ -351,17 +351,44 @@ func (erp *EmergencyReparenter) waitForAllRelayLogsToApply(
 	groupCtx, groupCancel := context.WithTimeout(ctx, opts.WaitReplicasTimeout)
 	defer groupCancel()
 
+	waiterCount := 0
+
 	for candidate := range validCandidates {
+		// When we called StopReplicationAndBuildStatusMaps, we got back two
+		// maps: (1) the StopReplicationStatus of any replicas that actually
+		// stopped replication; and (2) the MasterStatus of anything that
+		// returned ErrNotReplica, which is a tablet that is either the current
+		// primary or is stuck thinking it is a MASTER but is not in actuality.
+		//
+		// If we have a tablet in the validCandidates map that does not appear
+		// in the statusMap, then we have either (a) the current primary, which
+		// is not replicating, so it is not applying relay logs; or (b) a tablet
+		// that is stuck thinking it is MASTER but is not in actuality. In that
+		// second case - (b) - we will most likely find that the stuck MASTER
+		// does not have a winning position, and fail the ERS. If, on the other
+		// hand, it does have a winning position, we are trusting the operator
+		// to know what they are doing by emergency-reparenting onto that
+		// tablet. In either case, it does not make sense to wait for relay logs
+		// to apply on a tablet that was never applying relay logs in the first
+		// place, so we skip it, and log that we did.
+		status, ok := statusMap[candidate]
+		if !ok {
+			erp.logger.Infof("EmergencyReparent candidate %v not in replica status map; this means it was not running replication (because it was formerly MASTER), so skipping WaitForRelayLogsToApply step for this candidate", candidate)
+			continue
+		}
+
 		go func(alias string) {
 			var err error
 			defer func() { errCh <- err }()
-			err = WaitForRelayLogsToApply(groupCtx, erp.tmc, tabletMap[alias], statusMap[alias])
+			err = WaitForRelayLogsToApply(groupCtx, erp.tmc, tabletMap[alias], status)
 		}(candidate)
+
+		waiterCount++
 	}
 
 	errgroup := concurrency.ErrorGroup{
-		NumGoroutines:        len(validCandidates),
-		NumRequiredSuccesses: len(validCandidates),
+		NumGoroutines:        waiterCount,
+		NumRequiredSuccesses: waiterCount,
 		NumAllowedErrors:     0,
 	}
 	rec := errgroup.Wait(groupCancel, errCh)

go/vt/vtctl/reparentutil/emergency_reparenter_test.go

@@ -338,6 +338,111 @@ func TestEmergencyReparenter_reparentShardLocked(t *testing.T) {
 			},
 			shouldErr: false,
 		},
+		{
+			name: "success with existing primary",
+			ts:   memorytopo.NewServer("zone1"),
+			tmc: &emergencyReparenterTestTMClient{
+				DemoteMasterResults: map[string]struct {
+					Status *replicationdatapb.MasterStatus
+					Error  error
+				}{
+					"zone1-0000000100": {
+						Status: &replicationdatapb.MasterStatus{
+							Position: "MySQL56/3E11FA47-71CA-11E1-9E33-C80AA9429562:1-21",
+						},
+					},
+				},
+				PopulateReparentJournalResults: map[string]error{
+					"zone1-0000000102": nil,
+				},
+				PromoteReplicaResults: map[string]struct {
+					Result string
+					Error  error
+				}{
+					"zone1-0000000102": {
+						Result: "ok",
+						Error:  nil,
+					},
+				},
+				SetMasterResults: map[string]error{
+					"zone1-0000000100": nil,
+					"zone1-0000000101": nil,
+				},
+				StopReplicationAndGetStatusResults: map[string]struct {
+					Status     *replicationdatapb.Status
+					StopStatus *replicationdatapb.StopReplicationStatus
+					Error      error
+				}{
+					"zone1-0000000100": { // This tablet claims MASTER, so is not running replication.
+						Error: mysql.ErrNotReplica,
+					},
+					"zone1-0000000101": {
+						StopStatus: &replicationdatapb.StopReplicationStatus{
+							Before: &replicationdatapb.Status{},
+							After: &replicationdatapb.Status{
+								MasterUuid:       "3E11FA47-71CA-11E1-9E33-C80AA9429562",
+								RelayLogPosition: "MySQL56/3E11FA47-71CA-11E1-9E33-C80AA9429562:1-21",
+							},
+						},
+					},
+					"zone1-0000000102": {
+						StopStatus: &replicationdatapb.StopReplicationStatus{
+							Before: &replicationdatapb.Status{},
+							After: &replicationdatapb.Status{
+								MasterUuid:       "3E11FA47-71CA-11E1-9E33-C80AA9429562",
+								RelayLogPosition: "MySQL56/3E11FA47-71CA-11E1-9E33-C80AA9429562:1-26",
+							},
+						},
+					},
+				},
+				WaitForPositionResults: map[string]map[string]error{
+					"zone1-0000000101": {
+						"MySQL56/3E11FA47-71CA-11E1-9E33-C80AA9429562:1-21": nil,
+					},
+					"zone1-0000000102": {
+						"MySQL56/3E11FA47-71CA-11E1-9E33-C80AA9429562:1-26": nil,
+					},
+				},
+			},
+			shards: []*vtctldatapb.Shard{
+				{
+					Keyspace: "testkeyspace",
+					Name:     "-",
+				},
+			},
+			tablets: []*topodatapb.Tablet{
+				{
+					Alias: &topodatapb.TabletAlias{
+						Cell: "zone1",
+						Uid:  100,
+					},
+					Keyspace: "testkeyspace",
+					Shard:    "-",
+					Type:     topodatapb.TabletType_MASTER,
+				},
+				{
+					Alias: &topodatapb.TabletAlias{
+						Cell: "zone1",
+						Uid:  101,
+					},
+					Keyspace: "testkeyspace",
+					Shard:    "-",
+				},
+				{
+					Alias: &topodatapb.TabletAlias{
+						Cell: "zone1",
+						Uid:  102,
+					},
+					Keyspace: "testkeyspace",
+					Shard:    "-",
+					Hostname: "most up-to-date position, wins election",
+				},
+			},
+			keyspace:  "testkeyspace",
+			shard:     "-",
+			opts:      EmergencyReparentOptions{},
+			shouldErr: false,
+		},
 		{
 			name:       "shard not found",
 			ts:         memorytopo.NewServer("zone1"),