[#231] engine: Add List method

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/local_object_storage/engine/select.go

@@ -65,6 +65,44 @@ func (e *StorageEngine) Select(prm *SelectPrm) (*SelectRes, error) {
 	}, nil
 }
 
+// List returns `limit` available physically storage object addresses in engine.
+// If limit is zero, then returns all available object addresses.
+func (e *StorageEngine) List(limit uint64) (*SelectRes, error) {
+	addrList := make([]*object.Address, limit)
+	uniqueMap := make(map[string]struct{})
+	ln := uint64(0)
+
+	// consider iterating over shuffled shards
+	e.iterateOverUnsortedShards(func(sh *shard.Shard) (stop bool) {
+		res, err := sh.List() // consider limit result of shard iterator
+		if err != nil {
+			// TODO: smth wrong with shard, need to be processed
+			e.log.Warn("could not select objects from shard",
+				zap.Stringer("shard", sh.ID()),
+				zap.String("error", err.Error()),
+			)
+		} else {
+			for _, addr := range res.AddressList() { // save only unique values
+				if _, ok := uniqueMap[addr.String()]; !ok {
+					uniqueMap[addr.String()] = struct{}{}
+					addrList = append(addrList, addr)
+
+					ln++
+					if limit > 0 && ln >= limit {
+						return true
+					}
+				}
+			}
+		}
+
+		return false
+	})
+
+	return &SelectRes{
+		addrList: addrList,
+	}, nil
+}
+
 // Select selects objects from local storage using provided filters.
 func Select(storage *StorageEngine, fs object.SearchFilters) ([]*object.Address, error) {
 	res, err := storage.Select(new(SelectPrm).
@@ -77,7 +115,13 @@ func Select(storage *StorageEngine, fs object.SearchFilters) ([]*object.Address,
 	return res.AddressList(), nil
 }
 
-// SelectAll selects all objects from local storage.
-func SelectAll(storage *StorageEngine) ([]*object.Address, error) {
-	return Select(storage, object.SearchFilters{})
+// List returns `limit` available physically storage object addresses in
+// engine. If limit is zero, then returns all available object addresses.
+func List(storage *StorageEngine, limit uint64) ([]*object.Address, error) {
+	res, err := storage.List(limit)
+	if err != nil {
+		return nil, err
+	}
+
+	return res.AddressList(), nil
 }