MXS-2450: Don't discard history if it's disabled

If the session command history is not enabled, it shouldn't be discarded
when a COM_CHANGE_USER is executed.

maxscale-system-test/CMakeLists.txt

@@ -942,6 +942,9 @@ add_test_executable(mxs2326_hint_clone.cpp mxs2326_hint_clone mxs2326_hint_clone
 # MXS-2417: Ignore persisted configs with load_persisted_configs=false
 add_test_executable(mxs2417_ignore_persisted_cnf.cpp mxs2417_ignore_persisted_cnf mxs2417_ignore_persisted_cnf LABELS REPL_BACKEND)
 
+# MXS-2450: Crash on COM_CHANGE_USER with disable_sescmd_history=true
+add_test_executable(mxs2450_change_user_crash.cpp mxs2450_change_user_crash mxs2450_change_user_crash LABELS REPL_BACKEND)
+
 ############################################
 # BEGIN: binlogrouter and avrorouter tests #
 ############################################

maxscale-system-test/cnf/maxscale.cnf.template.mxs2450_change_user_crash

@@ -0,0 +1,49 @@
+[maxscale]
+threads=###threads###
+log_info=1
+
+[server1]
+type=server
+address=###node_server_IP_1###
+port=###node_server_port_1###
+protocol=MySQLBackend
+
+[server2]
+type=server
+address=###node_server_IP_2###
+port=###node_server_port_2###
+protocol=MySQLBackend
+
+[server3]
+type=server
+address=###node_server_IP_3###
+port=###node_server_port_3###
+protocol=MySQLBackend
+
+[server4]
+type=server
+address=###node_server_IP_4###
+port=###node_server_port_4###
+protocol=MySQLBackend
+
+[MySQL Monitor]
+type=monitor
+module=mysqlmon
+servers=server1,server2,server3,server4
+user=maxskysql
+password=skysql
+monitor_interval=1000
+
+[RW Split Router]
+type=service
+router=readwritesplit
+servers=server1,server2,server3,server4
+user=maxskysql
+password=skysql
+disable_sescmd_history=true
+
+[RW Split Listener]
+type=listener
+service=RW Split Router
+protocol=MySQLClient
+port=4006

maxscale-system-test/mxs2450_change_user_crash.cpp

@@ -0,0 +1,20 @@
+/**
+ * MXS-2450: Crash on COM_CHANGE_USER with disable_sescmd_history=true
+ * https://jira.mariadb.org/browse/MXS-2450
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char *argv[])
+{
+    TestConnections test(argc, argv);
+    Connection conn = test.maxscales->rwsplit();
+    test.expect(conn.connect(), "Connection failed: %s", conn.error());
+
+    for (int i = 0; i < 10; i++)
+    {
+        test.expect(conn.reset_connection(), "Connection reset failed: %s", conn.error());
+    }
+
+    return test.global_result;
+}

server/modules/routing/readwritesplit/rwsplit_session_cmd.cc

@@ -162,9 +162,10 @@ void RWSplitSession::process_sescmd_response(SRWBackend& backend, GWBUF** ppPack
             *ppPacket = NULL;
         }
 
-        if (m_expected_responses == 0
+        if (m_expected_responses == 0 && !m_config.disable_sescmd_history
             && (command == MXS_COM_CHANGE_USER || command == MXS_COM_RESET_CONNECTION))
         {
+            mxb_assert_message(!m_sescmd_list.empty(), "Must have stored session commands");
             mxb_assert_message(m_slave_responses.empty(), "All responses should've been processed");
             // This is the last session command to finish that resets the session state, reset the history
             MXS_INFO("Resetting session command history (length: %lu)", m_sescmd_list.size());