[#428] engine: Add low_mem config parameter

Concurrent initialization in case of the metabase resync leads to
high memory consumption and potential OOM.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/engine/control.go

@@ -12,6 +12,7 @@ import (
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/shard"
 	"go.uber.org/zap"
+	"golang.org/x/sync/errgroup"
 )
 
 type shardInitError struct {
@@ -74,23 +75,30 @@ func (e *StorageEngine) Init(ctx context.Context) error {
 	e.mtx.Lock()
 	defer e.mtx.Unlock()
 
-	var wg sync.WaitGroup
 	var errCh = make(chan shardInitError, len(e.shards))
+	var eg errgroup.Group
+	if e.cfg.lowMem && e.anyShardRequiresRefill() {
+		eg.SetLimit(1)
+	}
 
 	for id, sh := range e.shards {
-		wg.Add(1)
-		go func(id string, sh *shard.Shard) {
-			defer wg.Done()
+		id := id
+		sh := sh
+		eg.Go(func() error {
 			if err := sh.Init(ctx); err != nil {
 				errCh <- shardInitError{
 					err: err,
 					id:  id,
 				}
 			}
-		}(id, sh.Shard)
+			return nil
+		})
 	}
-	wg.Wait()
+	err := eg.Wait()
 	close(errCh)
+	if err != nil {
+		return fmt.Errorf("failed to initialize shards: %w", err)
+	}
 
 	for res := range errCh {
 		if res.err != nil {
@@ -125,6 +133,15 @@ func (e *StorageEngine) Init(ctx context.Context) error {
 	return nil
 }
 
+func (e *StorageEngine) anyShardRequiresRefill() bool {
+	for _, sh := range e.shards {
+		if sh.NeedRefillMetabase() {
+			return true
+		}
+	}
+	return false
+}
+
 var errClosed = errors.New("storage engine is closed")
 
 // Close releases all StorageEngine's components. Waits for all data-related operations to complete.