Avoid repeated default role assignments

When using the metadata storage (the current default) the default role
assignments were recreated at every start of the settings service. Leading to
duplicated role assignments

Fixes: owncloud#3432

changelog/unreleased/fix-duplicated-demouser-roles.md

@@ -0,0 +1,6 @@
+Bugfix: Fix default role assignment for demo users
+
+The roles-assignments for demo users where duplicated with every
+restart of the settings service.
+
+https://github.com/owncloud/ocis/issues/3432

services/settings/pkg/store/metadata/store.go

@@ -131,11 +131,21 @@ func (s *Store) initMetadataClient(mdc MetadataClient) error {
 			return err
 		}
 
+		assIDs, err := mdc.ReadDir(ctx, accountPath(accountUUID))
+		if err != nil {
+			return err
+		}
+		if len(assIDs) > 0 {
+			// There is already a role assignment for this ID, skip to the next
+			continue
+		}
+
 		ass := &settingsmsg.UserRoleAssignment{
 			Id:          uuid.Must(uuid.NewV4()).String(),
 			AccountUuid: accountUUID,
 			RoleId:      roleID,
 		}
+
 		b, err := json.Marshal(ass)
 		if err != nil {
 			return err