[#211] blobstor: Implement Exists method

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

pkg/local_object_storage/blobstor/delete_big.go

@@ -34,10 +34,9 @@ func (b *BlobStor) DeleteBig(prm *DeleteBigPrm) (*DeleteBigRes, error) {
 }
 
 func (t *fsTree) delete(addr *object.Address) error {
-	p := t.treePath(addr)
-
-	if _, err := os.Stat(p); os.IsNotExist(err) {
-		return errFileNotFound
+	p, err := t.exists(addr)
+	if err != nil {
+		return err
 	}
 
 	return os.Remove(p)

pkg/local_object_storage/blobstor/exists.go

@@ -1,5 +1,13 @@
 package blobstor
 
+import (
+	"os"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	objectSDK "github.com/nspcc-dev/neofs-api-go/pkg/object"
+	"github.com/pkg/errors"
+)
+
 // ExistsPrm groups the parameters of Exists operation.
 type ExistsPrm struct {
 	address
@@ -20,5 +28,47 @@ func (r ExistsRes) Exists() bool {
 // Returns any error encountered that did not allow
 // to completely check object existence.
 func (b *BlobStor) Exists(prm *ExistsPrm) (*ExistsRes, error) {
-	panic("implement me")
+	// check presence in shallow dir first (cheaper)
+	exists, err := b.existsBig(prm.addr)
+	if !exists {
+		// TODO: do smth if err != nil
+
+		// check presence in blobovnicza
+		exists, err = b.existsSmall(prm.addr)
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	return &ExistsRes{
+		exists: exists,
+	}, err
+}
+
+// checks if object is presented in shallow dir.
+func (b *BlobStor) existsBig(addr *object.Address) (bool, error) {
+	_, err := b.fsTree.exists(addr)
+	if errors.Is(err, errFileNotFound) {
+		return false, nil
+	}
+
+	return err == nil, err
+}
+
+// checks if object is presented in blobovnicza.
+func (b *BlobStor) existsSmall(addr *object.Address) (bool, error) {
+	// TODO: implement
+	return false, nil
+}
+
+func (t *fsTree) exists(addr *objectSDK.Address) (string, error) {
+	p := t.treePath(addr)
+
+	_, err := os.Stat(p)
+	if os.IsNotExist(err) {
+		err = errFileNotFound
+	}
+
+	return p, err
 }