From 4cb6d9227e1e95fa3991b11159a9b1ed0e0ac145 Mon Sep 17 00:00:00 2001
From: Markus Makela <markus.makela@mariadb.com>
Date: Mon, 17 Oct 2016 07:20:35 +0300
Subject: [PATCH] MXS-778: Set read-only masters into Slave status

When a master server is set into read-only mode, it can be treated as an
always up-to-date slave. This gives us a somewhat graceful way to prevent
writes to a master.

Usually setting the master into read-only mode is done before a change in
the replication topology to prevent stray writes arriving on the
master. MaxScale should respect the read-only mode and not send any writes
to a server that's in read-only mode.
---
 server/modules/monitor/mysqlmon/mysql_mon.c | 30 ++++++++++++++++-----
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/server/modules/monitor/mysqlmon/mysql_mon.c b/server/modules/monitor/mysqlmon/mysql_mon.c
index 901238f44..86967962d 100644
--- a/server/modules/monitor/mysqlmon/mysql_mon.c
+++ b/server/modules/monitor/mysqlmon/mysql_mon.c
@@ -49,6 +49,7 @@
 #include <maxscale/dcb.h>
 #include <maxscale/modutil.h>
 #include <maxscale/alloc.h>
+#include <maxscale/debug.h>
 
 /** Column positions for SHOW SLAVE STATUS */
 #define MYSQL55_STATUS_BINLOG_POS 5
@@ -1032,8 +1033,17 @@ void find_graph_cycles(MYSQL_MONITOR *handle, MONITOR_SERVERS *database, int nse
              * slave in this case can be either a normal slave or another
              * master.
              */
-            monitor_set_pending_status(graph[i].db, SERVER_MASTER | SERVER_STALE_STATUS);
-            monitor_clear_pending_status(graph[i].db, SERVER_SLAVE);
+            if (graph[i].info->read_only)
+            {
+                /** The master is in read-only mode, set it into Slave state */
+                monitor_set_pending_status(graph[i].db, SERVER_SLAVE);
+                monitor_clear_pending_status(graph[i].db, SERVER_MASTER | SERVER_STALE_STATUS);
+            }
+            else
+            {
+                monitor_set_pending_status(graph[i].db, SERVER_MASTER | SERVER_STALE_STATUS);
+                monitor_clear_pending_status(graph[i].db, SERVER_SLAVE);
+            }
         }
     }
 }
@@ -1315,10 +1325,11 @@ monitorMain(void *arg)
         ptr = mon->databases;
         while (ptr)
         {
-            MYSQL_SERVER_INFO *serv_info = hashtable_fetch(handle->server_info, ptr->server->unique_name);
-            ss_dassert(serv_info);
             if (!SERVER_IN_MAINT(ptr->server))
             {
+                MYSQL_SERVER_INFO *serv_info = hashtable_fetch(handle->server_info, ptr->server->unique_name);
+                ss_dassert(serv_info);
+
                 /** If "detect_stale_master" option is On, let's use the previous master.
                  *
                  * Multi-master mode detects the stale masters in find_graph_cycles().
@@ -1327,7 +1338,8 @@ monitorMain(void *arg)
                     (strcmp(ptr->server->name, root_master->server->name) == 0 &&
                      ptr->server->port == root_master->server->port) &&
                     (ptr->server->status & SERVER_MASTER) &&
-                    !(ptr->pending_status & SERVER_MASTER))
+                    !(ptr->pending_status & SERVER_MASTER) &&
+                    !serv_info->read_only)
                 {
                     /**
                      * In this case server->status will not be updated from pending_status
@@ -1877,7 +1889,13 @@ static MONITOR_SERVERS *get_replication_tree(MONITOR *mon, int num_servers)
                         monitor_clear_pending_status(handle->master, SERVER_MASTER);
                     }
 
-                    monitor_set_pending_status(master, SERVER_MASTER);
+                    MYSQL_SERVER_INFO* info = hashtable_fetch(handle->server_info,
+                                                              master->server->unique_name);
+                    ss_dassert(info);
+
+                    /** Only set the Master status if read_only is disabled */
+                    monitor_set_pending_status(master, info->read_only ? SERVER_SLAVE : SERVER_MASTER);
+
                     handle->master = master;
                 }
                 else