MDEV-25226 Assertion when wsrep_on set OFF with SR transaction

This patch makes the following changes around variable wsrep_on:

1) Variable wsrep_on can no longer be updated from a session that has
an active transaction running. The original behavior allowed cases
like this:

     BEGIN;
     INSERT INTO t1 VALUES (1);
     SET SESSION wsrep_on = OFF;
     INSERT INTO t1 VALUES (2);
     COMMIT;

With regular transactions this would result in no replication
events (not even value 1). With streaming replication it would be
unnecessarily complex to achieve the same behavior. In the above
example, it would be possible for value 1 to be already replicated if
it happened to fill a separate fragment, while value 2 wouldn't.

2) Global variable wsrep_on no longer affects current sessions, only
subsequent ones. This is to avoid a similar case to the above, just
using just by using global wsrep_on instead session wsrep_on:

      --connection conn_1
      BEGIN;
      INSERT INTO t1 VALUES(1);

      --connection conn_2
      SET GLOBAL wsrep_on = OFF;

      --connection conn_1
      INSERT INTO t1 VALUES(2);
      COMMIT;

The above example results in the transaction to be replicated, as
global wsrep_on will only affect the session wsrep_on of new
connections.

Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/mysql-test/suite/galera_sr/r/MDEV-25226.result b/mysql-test/suite/galera_sr/r/MDEV-25226.result
new file mode 100644
index 00000000000..4699023562d
--- /dev/null
+++ b/mysql-test/suite/galera_sr/r/MDEV-25226.result
@@ -0,0 +1,24 @@
+connection node_2;
+connection node_1;
+CREATE TABLE t1 (f1 INTEGER PRIMARY KEY);
+SET SESSION wsrep_trx_fragment_size=1;
+START TRANSACTION;
+INSERT INTO t1 VALUES(1);
+SET SESSION wsrep_on=OFF;
+ERROR 25000: You are not allowed to execute this command in a transaction
+SET GLOBAL wsrep_on=OFF;
+ERROR 25000: You are not allowed to execute this command in a transaction
+INSERT INTO t1 VALUES(2);
+COMMIT;
+connection node_1;
+SELECT * FROM t1;
+f1
+1
+2
+connection node_2;
+SELECT * FROM t1;
+f1
+1
+2
+connection node_1;
+DROP TABLE t1;