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, 12 insertions, 0 deletions

diff --git a/sql/wsrep_mysqld.cc b/sql/wsrep_mysqld.cc
index c3c4e4a0bf4..0338d7ad054 100644
--- a/sql/wsrep_mysqld.cc
+++ b/sql/wsrep_mysqld.cc
@@ -132,6 +132,18 @@ uint  wsrep_ignore_apply_errors= 0;
  */
 
 /*
+ * Cached variables
+ */
+
+// Whether the Galera write-set replication provider is set
+// wsrep_provider && strcmp(wsrep_provider, WSREP_NONE)
+bool WSREP_PROVIDER_EXISTS_;
+
+// Whether the Galera write-set replication is enabled
+// global_system_variables.wsrep_on && WSREP_PROVIDER_EXISTS_
+bool WSREP_ON_;
+
+/*
  * Other wsrep global variables.
  */