MDEV-38073: Always use tx_read_only=false for Wsrep system threads

Apparently, there was a separate issue with applier thread variables:
wsrep_plugins_post_init() would overwrite thd->variables for every
applier thread and forget to restore proper read only context.
Then, upon every server transaction termination,
trans_reset_one_shot_statistics() would set thread's read only context
to the one stored in thd->variables, thus spoiling the read only
context value for appliers.

Author: mariadb-DenisProtivensky

mysql-test/suite/galera/r/MDEV-38073.result

@@ -0,0 +1,17 @@
+connection node_2;
+connection node_1;
+connection node_2;
+connection node_1;
+CREATE TABLE t1 (id INT PRIMARY KEY) ENGINE=InnoDB;
+SET SESSION wsrep_trx_fragment_size=1;
+START TRANSACTION;
+INSERT INTO t1 VALUES (1);
+INSERT INTO t1 VALUES (2);
+COMMIT;
+connection node_2;
+SELECT COUNT(*) = 2 FROM t1;
+COUNT(*) = 2
+1
+connection node_2;
+connection node_1;
+DROP TABLE t1;

mysql-test/suite/galera/t/MDEV-38073.test

@@ -0,0 +1,39 @@
+#
+# Ensure that the transaction_read_only option does not apply to Wsrep system
+# threads (schema and appliers) and that the cluster keeps functioning.
+#
+
+--source include/galera_cluster.inc
+--source include/have_innodb.inc
+--source include/big_test.inc
+
+# Restart the node 2 in transaction_read_only mode.
+--connection node_2
+--source include/shutdown_mysqld.inc
+--let $start_mysqld_params=--transaction_read_only=TRUE
+--source include/start_mysqld.inc
+
+--connection node_1
+CREATE TABLE t1 (id INT PRIMARY KEY) ENGINE=InnoDB;
+
+# Note: this is not obligatory, but we want to test
+# SR transactions as well.
+SET SESSION wsrep_trx_fragment_size=1;
+
+START TRANSACTION;
+INSERT INTO t1 VALUES (1);
+INSERT INTO t1 VALUES (2);
+COMMIT;
+
+--connection node_2
+SELECT COUNT(*) = 2 FROM t1;
+
+# Restart the node 2 in normal mode.
+--connection node_2
+--source include/shutdown_mysqld.inc
+--let $start_mysqld_params=
+--source include/start_mysqld.inc
+
+# Cleanup
+--connection node_1
+DROP TABLE t1;

sql/sql_plugin.cc

@@ -4543,6 +4543,8 @@ my_bool post_init_callback(THD *thd, void *)
     set_current_thd(thd);
     plugin_thdvar_init(thd);
 
+    // Restore proper transaction read-only context
+    thd->variables.tx_read_only= false;
     // Restore option_bits
     thd->variables.option_bits= option_bits_saved;
   }

sql/wsrep_high_priority_service.cc

@@ -168,6 +168,10 @@ Wsrep_high_priority_service::Wsrep_high_priority_service(THD* thd)
    */
   thd->variables.option_bits|= OPTION_BIN_LOG;
 
+  /* Allow applying in a transaction read-only context */
+  thd->tx_read_only= false;
+  thd->variables.tx_read_only= false;
+
   thd->net.vio= 0;
   thd->reset_db(&db_str);
   thd->clear_error();

sql/wsrep_schema.cc

@@ -719,6 +719,9 @@ static void wsrep_init_thd_variables(THD *thd)
 {
   /* No Galera replication */
   thd->variables.wsrep_on= 0;
+  /* Allow modifications in a transaction read-only context */
+  thd->tx_read_only= false;
+  thd->variables.tx_read_only= false;
   /* No binlogging */
   thd->variables.sql_log_bin= 0;
   thd->variables.option_bits&= ~OPTION_BIN_LOG;

sql/wsrep_storage_service.cc

@@ -55,6 +55,10 @@ Wsrep_storage_service::Wsrep_storage_service(THD* thd)
 
   /* No binlogging */
 
+  /* Allow modifications in a transaction read-only context */
+  thd->tx_read_only= false;
+  thd->variables.tx_read_only= false;
+
   /* No general log */
   thd->variables.option_bits |= OPTION_LOG_OFF;