peapod: Optimize read-only mode

It makes no sense to repeatedly begin writeable BoltDB transactions and
start flushing routine when underlying BoltDB instance is read-only.

Require root bucket to be presented in the BoltDB instance in read-only
configuration.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

pkg/local_object_storage/peapod/peapod.go

@@ -30,6 +30,8 @@ type batch struct {
 // Peapod is a single low-level key/value database optimized to work with big
 // number of stored units.
 type Peapod struct {
+	readOnly bool
+
 	bolt *bbolt.DB
 
 	currentBatchMtx sync.RWMutex
@@ -52,14 +54,29 @@ func New(path string, perm fs.FileMode, readOnly bool) (*Peapod, error) {
 		return nil, fmt.Errorf("open BoltDB instance: %w", err)
 	}
 
-	if !readOnly {
-		err = db.Update(func(tx *bbolt.Tx) error {
-			_, err = tx.CreateBucketIfNotExists(rootBucket)
-			return err
+	if readOnly {
+		err = db.View(func(tx *bbolt.Tx) error {
+			if tx.Bucket(rootBucket) == nil {
+				return errMissingRootBucket
+			}
+			return nil
 		})
 		if err != nil {
-			return nil, fmt.Errorf("create root bucket in BoltDB instance: %w", err)
+			return nil, fmt.Errorf("check root bucket presence in BoltDB instance: %w", err)
 		}
+
+		return &Peapod{
+			readOnly: true,
+			bolt:     db,
+		}, nil
+	}
+
+	err = db.Update(func(tx *bbolt.Tx) error {
+		_, err = tx.CreateBucketIfNotExists(rootBucket)
+		return err
+	})
+	if err != nil {
+		return nil, fmt.Errorf("create root bucket in BoltDB instance: %w", err)
 	}
 
 	res := &Peapod{
@@ -77,8 +94,10 @@ func New(path string, perm fs.FileMode, readOnly bool) (*Peapod, error) {
 
 // Close syncs data and closes the database.
 func (x *Peapod) Close() error {
-	close(x.chClose)
-	<-x.chFlushDone
+	if !x.readOnly {
+		close(x.chClose)
+		<-x.chFlushDone
+	}
 	return x.bolt.Close()
 }
 

pkg/local_object_storage/peapod/write.go

@@ -38,6 +38,10 @@ func (x *Peapod) Delete(ctx context.Context, addr oid.Address) error {
 }
 
 func (x *Peapod) batch(ctx context.Context, fBktRoot func(bktRoot *bbolt.Bucket) error) error {
+	if x.readOnly {
+		return bbolt.ErrDatabaseReadOnly
+	}
+
 	x.currentBatchMtx.RLock()
 
 	currentBatch := x.currentBatch