MDEV-17043 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.

innodb_acquire_mdl(): Release the reference if the table name is not
valid for acquiring a meta-data lock (MDL).

innodb_find_table_for_vc(): Add a debug assertion if the table name
is not valid. This code path is for DML execution. The table
should have a valid name for executing DML, and furthermore a MDL
will prevent the table from being renamed.

row_vers_build_clust_v_col(): Add a debug assertion that both indexes
must belong to the same table.

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;
 	}
 
@@ -21507,6 +21508,7 @@ static TABLE* innodb_find_table_for_vc(THD* thd, dict_table_t* table)
 
 	if (!table_name_parse(table->name, db_buf, tbl_buf,
 			      db_buf_len, tbl_buf_len)) {
+		ut_ad(!"invalid table name");
 		return NULL;
 	}
 

storage/innobase/row/row0vers.cc

@@ -456,6 +456,7 @@ row_vers_build_clust_v_col(
 	byte*		record= 0;
 
 	ut_ad(dict_index_has_virtual(index));
+	ut_ad(index->table == clust_index->table);
 
 	if (vcol_info != NULL) {
 		vcol_info->set_used();