datamanager: create a new wal and checkpoint in initEtcd

When doing an initEtcd (new instance or etcd reset) create a new wal (that will
have a new sequence epoch) and do a checkpoint.

In this way:

* readdb will detect that an epoch change and do a full resync
* we always have a data file (also if empty) that provides the last checkpointed
wal. This information could be used by readdb to resync

internal/datamanager/wal.go

@@ -590,7 +590,7 @@ func (d *DataManager) sync(ctx context.Context) error {
 		switch walData.WalStatus {
 		case WalStatusCommitted:
 			walFilePath := d.storageWalStatusFile(walData.WalSequence)
-			d.log.Debugf("syncing committed wal to storage")
+			d.log.Debugf("syncing committed wal %q to storage", walData.WalSequence)
 			header := &WalHeader{
 				WalDataFileID:       walData.WalDataFileID,
 				PreviousWalSequence: walData.PreviousWalSequence,
@@ -924,7 +924,7 @@ func (d *DataManager) InitEtcd(ctx context.Context) error {
 		walData := &WalData{
 			WalSequence:   wal.WalSequence,
 			WalDataFileID: header.WalDataFileID,
-			WalStatus:     WalStatusCommitted,
+			WalStatus:     WalStatusCommittedStorage,
 		}
 		if wal.Checkpointed {
 			walData.WalStatus = WalStatusCheckpointed
@@ -1063,9 +1063,53 @@ func (d *DataManager) InitEtcd(ctx context.Context) error {
 		}
 	}
 
+	//  insert an empty wal and make it already committedstorage
+	walSequence, err := sequence.IncSequence(ctx, d.e, etcdWalSeqKey)
+	if err != nil {
+		return err
+	}
+
+	walDataFileID := uuid.NewV4().String()
+	walDataFilePath := d.storageWalDataFile(walDataFileID)
+	walKey := etcdWalKey(walSequence.String())
+
+	if err := d.ost.WriteObject(walDataFilePath, bytes.NewReader([]byte{}), 0, true); err != nil {
+		return err
+	}
+	d.log.Debugf("wrote wal file: %s", walDataFilePath)
+
+	walFilePath := d.storageWalStatusFile(walSequence.String())
+	d.log.Infof("syncing committed wal %q to storage", walSequence.String())
+	header := &WalHeader{
+		WalDataFileID:       walDataFileID,
+		PreviousWalSequence: lastCommittedStorageWalSequence,
+	}
+	headerj, err := json.Marshal(header)
+	if err != nil {
+		return err
+	}
+	walFileCommittedPath := walFilePath + ".committed"
+	if err := d.ost.WriteObject(walFileCommittedPath, bytes.NewReader(headerj), int64(len(headerj)), true); err != nil {
+		return err
+	}
+
+	lastCommittedStorageWalSequence = walSequence.String()
+
+	walData := &WalData{
+		WalSequence:   walSequence.String(),
+		WalDataFileID: walDataFileID,
+		WalStatus:     WalStatusCommittedStorage,
+	}
+
 	walsData := &WalsData{
 		LastCommittedWalSequence: lastCommittedStorageWalSequence,
 	}
+
+	walDataj, err := json.Marshal(walData)
+	if err != nil {
+		return err
+	}
+
 	walsDataj, err := json.Marshal(walsData)
 	if err != nil {
 		return err
@@ -1081,6 +1125,7 @@ func (d *DataManager) InitEtcd(ctx context.Context) error {
 	cmp = append(cmp, etcdclientv3.Compare(etcdclientv3.CreateRevision(etcdWalsDataKey), "=", 0))
 	then = append(then, etcdclientv3.OpPut(etcdWalsDataKey, string(walsDataj)))
 	then = append(then, etcdclientv3.OpPut(etcdLastCommittedStorageWalSeqKey, lastCommittedStorageWalSequence))
+	then = append(then, etcdclientv3.OpPut(walKey, string(walDataj)))
 	txn = d.e.Client().Txn(ctx).If(cmp...).Then(then...)
 	tresp, err := txn.Commit()
 	if err != nil {
@@ -1090,5 +1135,10 @@ func (d *DataManager) InitEtcd(ctx context.Context) error {
 		return errors.Errorf("failed to sync etcd: walsdata already written")
 	}
 
+	// force a checkpoint
+	if err := d.checkpoint(ctx, true); err != nil {
+		return err
+	}
+
 	return nil
 }