[#791] writecache: Implement iterator over small objects

There is a need to list addresses of the small objects stored in WriteCache
database.

Implement `IterateDB` function which accepts BoltDB instance and iterate
over all saved objects and passes their addresses to the hander.

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

pkg/local_object_storage/writecache/iterate.go

@@ -0,0 +1,42 @@
+package writecache
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	"go.etcd.io/bbolt"
+)
+
+// ErrNoDefaultBucket is returned by IterateDB when default bucket for objects is missing.
+var ErrNoDefaultBucket = errors.New("no default bucket")
+
+// IterateDB iterates over all objects stored in bbolt.DB instance and passes them to f until error return.
+// It is assumed that db is an underlying database of some WriteCache instance.
+//
+// Returns ErrNoDefaultBucket if there is no default bucket in db.
+//
+// DB must not be nil and should be opened.
+func IterateDB(db *bbolt.DB, f func(*object.Address) error) error {
+	return db.View(func(tx *bbolt.Tx) error {
+		b := tx.Bucket(defaultBucket)
+		if b == nil {
+			return ErrNoDefaultBucket
+		}
+
+		var addr *object.Address
+
+		return b.ForEach(func(k, v []byte) error {
+			if addr == nil {
+				addr = object.NewAddress()
+			}
+
+			err := addr.Parse(string(k))
+			if err != nil {
+				return fmt.Errorf("could not parse object address: %w", err)
+			}
+
+			return f(addr)
+		})
+	})
+}