Remove one of the major sources of race condiitons in mariadb-test.
Normally, mariadb_close() sends COM_QUIT to the server and immediately
disconnects. In mariadb-test it means the test can switch to another
connection and sends queries to the server before the server even
started parsing the COM_QUIT packet and these queries can see the
connection as fully active, as it didn't reach dispatch_command yet.
This is a major source of instability in tests and many - but not all,
still less than a half - tests employ workarounds. The correct one
is a pair count_sessions.inc/wait_until_count_sessions.inc.
Also very popular was wait_until_disconnected.inc, which was completely
useless, because it verifies that the connection is closed, and after
disconnect it always is, it didn't verify whether the server processed
COM_QUIT. Sadly the placebo was as widely used as the real thing.
Let's fix this by making mariadb-test `disconnect` command _to wait_ for
the server to confirm. This makes almost all workarounds redundant.
In some cases count_sessions.inc/wait_until_count_sessions.inc is still
needed, though, as only `disconnect` command is changed:
* after external tools, like `exec $MYSQL`
* after failed `connect` command
* replication, after `STOP SLAVE`
* Federated/CONNECT/SPIDER/etc after `DROP TABLE`
and also in some XA tests, because an XA transaction is dissociated from
the THD very late, after the server has closed the client connection.
Collateral cleanups: fix comments, remove some redundant statements:
* DROP IF EXISTS if nothing is known to exist
* DROP table/view before DROP DATABASE
* REVOKE privileges before DROP USER
etc
- InnoDB throws ASAN error while adding the index on virtual column
of system versioned table. InnoDB wrongly assumes that virtual
column collation type changes, creates new column with different
character set. This leads to failure while detaching the column
from indexes.
- Newly created InnoDB fulltext index does have only one column
and doesn't associate with primary key fields during DDL.
init_change_cols() has strict assertion that number of fields
should be greater than number of collation change columns.
Problem:
=======
ALTER TABLE in InnoDB fails to detect duplicate entries
for the unique index when the character set or collation of
an indexed column is changed in such a way that the character
encoding is compatible with the old table definition.
In this case, any secondary indexes on the changed columns
would be rebuilt (DROP INDEX, ADD INDEX).
Solution:
========
During ALTER TABLE, InnoDB keeps track of columns whose collation
changed, and will fill in the correct metadata when sorting the
index records, or applying changes from concurrent DML.
This metadata will be allocated in the dict_index_t::heap of
the being-created secondary indexes.
The fix was developed by Thirunarayanan Balathandayuthapani
and simplified by me.
ha_innobase::check_if_supported_inplace_alter(): Refuse to change the
collation of a column that would become or remain indexed as part of
the ALTER TABLE operation.
In MariaDB Server 10.6, we will allow this type of operation;
that fix depends on MDEV-15250.
