Purge of indexed virtual columns may cause hang on table-rebuilding DDL

When a table is renamed to an internal #sql2 or #sql-ib name during a table-rebuilding DDL operation such as OPTIMIZE TABLE or ALTER TABLE, and shortly after that a purge operation in an index on virtual columns is attempted, the operation could fail, but purge would fail to release the table reference:

diff --git a/storage/innobase/handler/ha_innodb.cc b/storage/innobase/handler/ha_innodb.cc

index c3a6bd663b3..35e091e0f11 100644

--- a/storage/innobase/handler/ha_innodb.cc

+++ b/storage/innobase/handler/ha_innodb.cc

@@ -21425,6 +21425,7 @@ static TABLE* innodb_acquire_mdl(THD* thd, dict_table_t* table)

 	if (!table_name_parse(table->name, db_buf, tbl_buf,

 			      db_buf_len, tbl_buf_len)) {

+		table->release();

 		return NULL;

 	}

This bug was found during the development of a backup-friendly TRUNCATE (RENAME, CREATE, DROP) in MDEV-13564. It would occasionally cause the TRUNCATE TABLE statement to hang during the DROP part in the test gcol.innodb_virtual_purge_debug.

A similar hang should be possible in a table-rebuilding ALTER TABLE of a table that had indexed virtual columns before the operation.