datamanager: implement maintenance mode

when datamanager is started in maintenance mode no goroutines are scheduled and
no initial etcd initialization is done

internal/datamanager/datamanager.go

@@ -87,6 +87,7 @@ type DataManagerConfig struct {
 	// MinCheckpointWalsNum is the minimum number of wals required before doing a checkpoint
 	MinCheckpointWalsNum int
 	MaxDataFileSize      int64
+	MaintenanceMode      bool
 }
 
 type DataManager struct {
@@ -100,6 +101,7 @@ type DataManager struct {
 	checkpointInterval   time.Duration
 	minCheckpointWalsNum int
 	maxDataFileSize      int64
+	maintenanceMode      bool
 }
 
 func NewDataManager(ctx context.Context, logger *zap.Logger, conf *DataManagerConfig) (*DataManager, error) {
@@ -133,6 +135,7 @@ func NewDataManager(ctx context.Context, logger *zap.Logger, conf *DataManagerCo
 		checkpointInterval:   conf.CheckpointInterval,
 		minCheckpointWalsNum: conf.MinCheckpointWalsNum,
 		maxDataFileSize:      conf.MaxDataFileSize,
+		maintenanceMode:      conf.MaintenanceMode,
 	}
 
 	// add trailing slash the basepath
@@ -171,6 +174,12 @@ func etcdWalKey(walSeq string) string {
 	return path.Join(etcdWalsDir, walSeq)
 }
 
+// SetMaintenanceMode sets the datamanager in maintenance mode. This method must
+// be called before invoking the Run method
+func (d *DataManager) SetMaintenanceMode(maintenanceMode bool) {
+	d.maintenanceMode = maintenanceMode
+}
+
 // deleteEtcd deletes all etcd data excluding keys used for locking
 func (d *DataManager) deleteEtcd(ctx context.Context) error {
 	prefixes := []string{
@@ -192,26 +201,32 @@ func (d *DataManager) deleteEtcd(ctx context.Context) error {
 }
 
 func (d *DataManager) Run(ctx context.Context, readyCh chan struct{}) error {
-	for {
-		err := d.InitEtcd(ctx, nil)
-		if err == nil {
-			break
+	if !d.maintenanceMode {
+		for {
+			err := d.InitEtcd(ctx, nil)
+			if err == nil {
+				break
+			}
+			d.log.Errorf("failed to initialize etcd: %+v", err)
+			time.Sleep(1 * time.Second)
 		}
-		d.log.Errorf("failed to initialize etcd: %+v", err)
-		time.Sleep(1 * time.Second)
+
+		readyCh <- struct{}{}
+
+		go d.watcherLoop(ctx)
+		go d.syncLoop(ctx)
+		go d.checkpointLoop(ctx)
+		go d.walCleanerLoop(ctx)
+		go d.compactChangeGroupsLoop(ctx)
+		go d.etcdPingerLoop(ctx)
+
+	} else {
+		d.log.Infof("datamanager starting in maintenance mode")
+		readyCh <- struct{}{}
 	}
 
-	readyCh <- struct{}{}
-
-	go d.watcherLoop(ctx)
-	go d.syncLoop(ctx)
-	go d.checkpointLoop(ctx)
-	go d.walCleanerLoop(ctx)
-	go d.compactChangeGroupsLoop(ctx)
-	go d.etcdPingerLoop(ctx)
-
 	<-ctx.Done()
-	d.log.Infof("walmanager exiting")
+	d.log.Infof("datamanager exiting")
 
 	return nil
 }