Merged fix for MDEV-6646 from maria-5.5-galera.

mysql-test/suite/galera/galera_2nodes.cnf

@@ -7,6 +7,8 @@ wsrep_provider=@ENV.WSREP_PROVIDER
 wsrep_cluster_address='gcomm://'
 wsrep_provider_options='base_port=@mysqld.1.#galera_port'
 wsrep_sst_receive_address='127.0.0.1:@mysqld.1.#sst_port'
+# enforce read-committed characteristics across the cluster
+wsrep_causal_reads=ON
 
 [mysqld.2]
 binlog-format=row
@@ -14,6 +16,8 @@ wsrep_provider=@ENV.WSREP_PROVIDER
 wsrep_cluster_address='gcomm://127.0.0.1:@mysqld.1.#galera_port'
 wsrep_provider_options='base_port=@mysqld.2.#galera_port'
 wsrep_sst_receive_address='127.0.0.1:@mysqld.2.#sst_port'
+# enforce read-committed characteristics across the cluster
+wsrep_causal_reads=ON
 
 [ENV]
 NODE_MYPORT_1= @mysqld.1.port

sql/sql_class.cc

@@ -1614,6 +1614,14 @@ void THD::init(void)
 
   wsrep_TOI_pre_query     = NULL;
   wsrep_TOI_pre_query_len = 0;
+
+  /*
+    @@wsrep_causal_reads is now being handled via wsrep_sync_wait, update it
+    appropriately.
+  */
+  if (variables.wsrep_causal_reads)
+    variables.wsrep_sync_wait|= WSREP_SYNC_WAIT_BEFORE_READ;
+
 #endif
   if (variables.sql_log_bin)
     variables.option_bits|= OPTION_BIN_LOG;