Delete volume is empty (#4561)

* use onlyEmpty for deleteVolume
https://github.com/seaweedfs/seaweedfs/issues/4559

* fix IsEmpty

* fix test

---------

Co-authored-by: Konstantin Lebedev <9497591+kmlebedev@users.noreply.github.co>

weed/storage/store.go

@@ -507,8 +507,11 @@ func (s *Store) UnmountVolume(i needle.VolumeId) error {
 	return fmt.Errorf("volume %d not found on disk", i)
 }
 
-func (s *Store) DeleteVolume(i needle.VolumeId) error {
+func (s *Store) DeleteVolume(i needle.VolumeId, onlyEmpty bool) error {
 	v := s.findVolume(i)
+	if onlyEmpty && !v.IsEmpty() {
+		return fmt.Errorf("delete volume %d not empty", i)
+	}
 	if v == nil {
 		return fmt.Errorf("delete volume %d not found on disk", i)
 	}

weed/storage/volume.go

@@ -332,3 +332,6 @@ func (v *Volume) IsReadOnly() bool {
 	defer v.noWriteLock.RUnlock()
 	return v.noWriteOrDelete || v.noWriteCanDelete || v.location.isDiskSpaceLow
 }
+func (v *Volume) IsEmpty() bool {
+	return v.ContentSize() == 0
+}