The test tends to fail if many parallel instances of it are executed:

```
mysqltest: At line 23: query 'ALTER TABLE t1 ADD PRIMARY KEY (f1)' failed:
1317: Query execution was interrupted
```

The `ALTER` fails because it is BF aborted due to an earlier `INSERT SELECT`
that is being applied:

```
INSERT INTO t1 (f1) SELECT ...

--connection node_2
SET GLOBAL wsrep_desync = TRUE;
SET SESSION wsrep_on = FALSE;

ALTER TABLE t1 ADD PRIMARY KEY (f1);

SET SESSION wsrep_on = TRUE;
SET GLOBAL wsrep_desync = FALSE;
```

And because the `ALTER` is executed with `wsrep_on = OFF`, it does not
run in total order isolation.
To avoid the problem it must be ensured that the `ALTER` only after the
large `INSERT SELECT` is done. To do so it is sufficient to issue
`SELECT COUNT(*) FROM t1;` from `node_2` before turning off wsrep.
The `SELECT` will trigger `wsrep_sync_wait` and proceed only after the
`INSERT SELECT` from node_1 is done.