MDEV-9519: Data corruption will happen on the Galera cluster size change

If we have a 2+ node cluster which is replicating from an async master
and the binlog_format is set to STATEMENT and multi-row inserts are executed
on a table with an auto_increment column such that values are automatically
generated by MySQL, then the server node generates wrong auto_increment
values, which are different from what was generated on the async master.

In the title of the MDEV-9519 it was proposed to ban start slave on a Galera
if master binlog_format = statement and wsrep_auto_increment_control = 1,
but the problem can be solved without such a restriction.

The causes and fixes:

1. We need to improve processing of changing the auto-increment values
after changing the cluster size.

2. If wsrep auto_increment_control switched on during operation of
the node, then we should immediately update the auto_increment_increment
and auto_increment_offset global variables, without waiting of the next
invocation of the wsrep_view_handler_cb() callback. In the current version
these variables retain its initial values if wsrep_auto_increment_control
is switched on during operation of the node, which leads to inconsistent
results on the different nodes in some scenarios.

3. If wsrep auto_increment_control switched off during operation of the node,
then we must return the original values of the auto_increment_increment and
auto_increment_offset global variables, as the user has set. To make this
possible, we need to add a "shadow copies" of these variables (which stores
the latest values set by the user).

https://jira.mariadb.org/browse/MDEV-9519

1 files changed, 14 insertions, 0 deletions

diff --git a/sql/mysqld.cc b/sql/mysqld.cc
index 5452f7164e6..813f0988b74 100644
--- a/sql/mysqld.cc
+++ b/sql/mysqld.cc
@@ -4563,6 +4563,20 @@ static int init_common_variables()
     return 1;
   }
 
+#ifdef WITH_WSREP
+  /*
+    We need to initialize auxiliary variables, that will be
+    further keep the original values of auto-increment options
+    as they set by the user. These variables used to restore
+    user-defined values of the auto-increment options after
+    setting of the wsrep_auto_increment_control to 'OFF'.
+  */
+  global_system_variables.saved_auto_increment_increment=
+    global_system_variables.auto_increment_increment;
+  global_system_variables.saved_auto_increment_offset=
+    global_system_variables.auto_increment_offset;
+#endif /* WITH_WSREP */
+
   return 0;
 }