lib-storage: Prevent renaming mailbox under itself.

This resulted earlier in a loop, which broke the mailbox index.
Note that IMAP already prevented this, so it could only be triggered by
other tools, like doveadm.

src/lib-storage/list/mailbox-list-index-backend.c

@@ -582,6 +582,7 @@ index_list_rename_mailbox(struct mailbox_list *_oldlist, const char *oldname,
 			  struct mailbox_list *_newlist, const char *newname)
 {
 	struct index_mailbox_list *list = (struct index_mailbox_list *)_oldlist;
+	const unsigned int oldname_len = strlen(oldname);
 	struct mailbox_list_index_sync_context *sync_ctx;
 	struct mailbox_list_index_record oldrec, newrec;
 	struct mailbox_list_index_node *oldnode, *newnode, *child;
@@ -595,6 +596,13 @@ index_list_rename_mailbox(struct mailbox_list *_oldlist, const char *oldname,
 		return -1;
 	}
 
+	if (strncmp(oldname, newname, oldname_len) == 0 &&
+	   newname[oldname_len] == mailbox_list_get_hierarchy_sep(_newlist)) {
+		mailbox_list_set_error(_oldlist, MAIL_ERROR_NOTPOSSIBLE,
+			"Can't rename mailbox under itself.");
+		return -1;
+	}
+
 	if (mailbox_list_index_sync_begin(&list->list, &sync_ctx) < 0)
 		return -1;