MXS-1751: Fix crash with available_when_donor=true

The `MYSQL_ROW row` variable was being overwritten by the extra query done
by the SST method detection code. Moving it into its own function prevents
this and makes the code significantly easier to comprehend.

Added a test case that reproduced the problem (MaxScale crashed) and
verifies that the patch fixes the problem.

maxscale-system-test/CMakeLists.txt

@@ -621,6 +621,10 @@ add_test_executable(mxs1713_lots_of_databases.cpp mxs1713_lots_of_databases mxs1
 # https://jira.mariadb.org/browse/MXS-1731
 add_test_executable(mxs1731_old_persisted_config.cpp mxs1731_old_persisted_config replication LABELS REPL_BACKEND)
 
+# MXS-1751: Maxscale crashes when certain config is in play (with nodes down)
+# https://jira.mariadb.org/browse/MXS-1751
+add_test_executable(mxs1751_available_when_donor_crash.cpp mxs1751_available_when_donor_crash mxs1751_available_when_donor_crash LABELS GALERA_BACKEND)
+
 # 'namedserverfilter' test
 add_test_executable(namedserverfilter.cpp namedserverfilter namedserverfilter LABELS namedserverfilter LIGHT REPL_BACKEND)
 

maxscale-system-test/cnf/maxscale.cnf.template.mxs1751_available_when_donor_crash

@@ -0,0 +1,53 @@
+[maxscale]
+threads=###threads###
+
+[Galera Monitor]
+type=monitor
+module=galeramon
+servers=server1,server2,server3
+user=maxskysql
+passwd=skysql
+monitor_interval=100
+available_when_donor=true
+
+[RW Split Router]
+type=service
+router=readwritesplit
+servers=server1,server2,server3
+user=maxskysql
+passwd=skysql
+master_accept_reads=true
+
+[RW Split Listener]
+type=listener
+service=RW Split Router
+protocol=MySQLClient
+port=4006
+
+[CLI]
+type=service
+router=cli
+
+[CLI Listener]
+type=listener
+service=CLI
+protocol=maxscaled
+socket=default
+
+[server1]
+type=server
+address=###galera_server_IP_1###
+port=###galera_server_port_1###
+protocol=MySQLBackend
+
+[server2]
+type=server
+address=###galera_server_IP_2###
+port=###galera_server_port_2###
+protocol=MySQLBackend
+
+[server3]
+type=server
+address=###galera_server_IP_3###
+port=###galera_server_port_3###
+protocol=MySQLBackend

maxscale-system-test/mxs1751_available_when_donor_crash.cpp

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char* argv[])
+{
+    TestConnections test(argc, argv);
+
+    for (int i = 0; i < 2; i++)
+    {
+        test.galera->stop_node(0);
+        test.galera->stop_node(1);
+        test.galera->start_node(1);
+        test.galera->start_node(0);
+        test.galera->fix_replication();
+    }
+
+    return test.global_result;
+}

server/modules/monitor/galeramon/galeramon.c

@@ -260,6 +260,41 @@ static json_t* diagnostics_json(const MXS_MONITOR *mon)
     return rval;
 }
 
+static bool using_xtrabackup(MXS_MONITORED_SERVER *database, const char* server_string)
+{
+    bool rval = false;
+    MYSQL_RES* result;
+
+    if (mxs_mysql_query(database->con, "SHOW VARIABLES LIKE 'wsrep_sst_method'") == 0
+        && (result = mysql_store_result(database->con)) != NULL)
+    {
+        if (mysql_field_count(database->con) < 2)
+        {
+            mysql_free_result(result);
+            MXS_ERROR("Unexpected result for \"SHOW VARIABLES LIKE "
+                      "'wsrep_sst_method'\". Expected 2 columns."
+                      " MySQL Version: %s", server_string);
+        }
+
+        MYSQL_ROW row;
+
+        while ((row = mysql_fetch_row(result)))
+        {
+            if (row[1] && strncmp(row[1], "xtrabackup", 10) == 0)
+            {
+                rval = true;
+            }
+        }
+        mysql_free_result(result);
+    }
+    else
+    {
+        mon_report_query_error(database);
+    }
+
+    return rval;
+}
+
 /**
  * Monitor an individual server. Does not deal with the setting of master or
  * slave bits, except for clearing them when a server is not joined to the
@@ -273,8 +308,7 @@ monitorDatabase(MXS_MONITOR *mon, MXS_MONITORED_SERVER *database)
 {
     GALERA_MONITOR* handle = (GALERA_MONITOR*) mon->handle;
     MYSQL_ROW row;
-    MYSQL_RES *result, *result2;
-    int isjoined = 0;
+    MYSQL_RES *result;
     char *server_string;
 
     /* Don't even probe server flagged as in maintenance */
@@ -364,40 +398,19 @@ monitorDatabase(MXS_MONITOR *mon, MXS_MONITORED_SERVER *database)
                 info.local_index = local_index;
             }
 
+            ss_dassert(row[0] && row[1]);
+
             if (strcmp(row[0], "wsrep_local_state") == 0)
             {
                 if (strcmp(row[1], "4") == 0)
                 {
                     info.joined = 1;
                 }
                 /* Check if the node is a donor and is using xtrabackup, in this case it can stay alive */
-                else if (strcmp(row[1], "2") == 0 && handle->availableWhenDonor == 1)
+                else if (strcmp(row[1], "2") == 0 && handle->availableWhenDonor == 1 &&
+                         using_xtrabackup(database, server_string))
                 {
-                    if (mxs_mysql_query(database->con, "SHOW VARIABLES LIKE 'wsrep_sst_method'") == 0
-                        && (result2 = mysql_store_result(database->con)) != NULL)
-                    {
-                        if (mysql_field_count(database->con) < 2)
-                        {
-                            mysql_free_result(result);
-                            mysql_free_result(result2);
-                            MXS_ERROR("Unexpected result for \"SHOW VARIABLES LIKE "
-                                      "'wsrep_sst_method'\". Expected 2 columns."
-                                      " MySQL Version: %s", server_string);
-                            return;
-                        }
-                        while ((row = mysql_fetch_row(result2)))
-                        {
-                            if (strncmp(row[1], "xtrabackup", 10) == 0)
-                            {
-                                info.joined = 1;
-                            }
-                        }
-                        mysql_free_result(result2);
-                    }
-                    else
-                    {
-                        mon_report_query_error(database);
-                    }
+                    info.joined = 1;
                 }
                 else
                 {