[#791] blobovnicza: Support read-only mode

There is a need to open Blobovnicza instances in read-only mode in some
cases.

Add `ReadOnly` option. Do not create dir path in RO. Open underlying BoltDB
instance with ReadOnly flag. Document thal all writing operations should not
be called in ro (otherwise BoltDB txs fail).

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/blobovnicza/blobovnicza.go

@@ -106,3 +106,10 @@ func WithLogger(l *logger.Logger) Option {
 		c.log = l.With(zap.String("component", "Blobovnicza"))
 	}
 }
+
+// ReadOnly returns option to open Blobovnicza in read-only mode.
+func ReadOnly() Option {
+	return func(c *cfg) {
+		c.boltOptions.ReadOnly = true
+	}
+}

pkg/local_object_storage/blobovnicza/control.go

@@ -16,17 +16,24 @@ import (
 func (b *Blobovnicza) Open() error {
 	b.log.Debug("creating directory for BoltDB",
 		zap.String("path", b.path),
+		zap.Bool("ro", b.boltOptions.ReadOnly),
 	)
 
-	err := util.MkdirAllX(path.Dir(b.path), b.perm)
+	var err error
-	if err == nil {
+
+	if !b.boltOptions.ReadOnly {
+		err = util.MkdirAllX(path.Dir(b.path), b.perm)
+		if err != nil {
+			return err
+		}
+	}
+
 	b.log.Debug("opening BoltDB",
 		zap.String("path", b.path),
 		zap.Stringer("permissions", b.perm),
 	)
 
 	b.boltDB, err = bbolt.Open(b.path, b.perm, b.boltOptions)
-	}
 
 	return err
 }
@@ -34,6 +41,8 @@ func (b *Blobovnicza) Open() error {
 // Init initializes internal database structure.
 //
 // If Blobovnicza is already initialized, then no action is taken.
+//
+// Should not be called in read-only configuration.
 func (b *Blobovnicza) Init() error {
 	b.log.Debug("initializing...",
 		zap.Uint64("object size limit", b.objSizeLimit),

pkg/local_object_storage/blobovnicza/delete.go

@@ -27,6 +27,8 @@ func (p *DeletePrm) SetAddress(addr *objectSDK.Address) {
 // did not allow to completely delete the object.
 //
 // Returns ErrNotFound if the object to be deleted is not in blobovnicza.
+//
+// Should not be called in read-only configuration.
 func (b *Blobovnicza) Delete(prm *DeletePrm) (*DeleteRes, error) {
 	addrKey := addressKey(prm.addr)
 

pkg/local_object_storage/blobovnicza/put.go

@@ -47,6 +47,8 @@ func (p *PutPrm) SetMarshaledObject(data []byte) {
 // did not allow to completely save the object.
 //
 // Returns ErrFull if blobovnicza is filled.
+//
+// Should not be called in read-only configuration.
 func (b *Blobovnicza) Put(prm *PutPrm) (*PutRes, error) {
 	addr := prm.addr
 	if addr == nil {