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, 21 insertions, 0 deletions

diff --git a/sql/wsrep_thd.cc b/sql/wsrep_thd.cc
index eb26da61282..551e710cfeb 100644
--- a/sql/wsrep_thd.cc
+++ b/sql/wsrep_thd.cc
@@ -676,3 +676,24 @@ bool wsrep_thd_has_explicit_locks(THD *thd)
   assert(thd);
   return thd->mdl_context.has_explicit_locks();
 }
+
+/*
+  Get auto increment variables for THD. Use global settings for
+  applier threads.
+ */
+void wsrep_thd_auto_increment_variables(THD* thd,
+                                        unsigned long long* offset,
+                                        unsigned long long* increment)
+{
+  if (thd->wsrep_exec_mode == REPL_RECV &&
+      thd->wsrep_conflict_state != REPLAYING)
+  {
+    *offset= global_system_variables.auto_increment_offset;
+    *increment= global_system_variables.auto_increment_increment;
+  }
+  else
+  {
+    *offset= thd->variables.auto_increment_offset;
+    *increment= thd->variables.auto_increment_increment;
+  }
+}