Fix low disk space maintenance

The setting didn't work because the code updated a status flag which
would be overwritten before being read. Also, promotion code now checks
that the server is not in maintenance.

server/modules/monitor/mariadbmon/cluster_discovery.cc

@@ -875,3 +875,18 @@ void MariaDBMonitor::update_topology()
         }
     }
 }
+
+void MariaDBMonitor::set_low_disk_slaves_maintenance()
+{
+    // Only set pure slave and standalone servers to maintenance.
+    for (MariaDBServer* server : m_servers)
+    {
+        if (server->is_low_on_disk_space() && server->is_usable()
+            && !server->is_master() && !server->is_relay_master())
+        {
+            // TODO: Handle relays somehow, e.g. switch with a slave
+            MXS_WARNING("Setting %s to maintenance because it is low on disk space.", server->name());
+            server->set_status(SERVER_MAINT);
+        }
+    }
+}

server/modules/monitor/mariadbmon/cluster_manipulation.cc

@@ -1676,20 +1676,6 @@ void MariaDBMonitor::enforce_read_only_on_slaves()
     }
 }
 
-void MariaDBMonitor::set_low_disk_slaves_maintenance()
-{
-    // Only set pure slave and standalone servers to maintenance.
-    for (MariaDBServer* server : m_servers)
-    {
-        if (server->has_status(SERVER_DISK_SPACE_EXHAUSTED) && server->is_usable()
-            && !server->is_master() && !server->is_relay_master())
-        {
-            server->set_status(SERVER_MAINT);
-            m_cluster_modified = true;
-        }
-    }
-}
-
 void MariaDBMonitor::handle_low_disk_space_master()
 {
     if (m_master && m_master->is_master() && m_master->is_low_on_disk_space())

server/modules/monitor/mariadbmon/mariadbmon.cc

@@ -460,6 +460,13 @@ void MariaDBMonitor::tick()
         update_external_master();
     }
 
+    /* Set low disk space slaves to maintenance. This needs to happen after roles have been assigned.
+     * Is not a real cluster operation, since nothing on the actual backends is changed. */
+    if (m_maintenance_on_low_disk_space)
+    {
+        set_low_disk_slaves_maintenance();
+    }
+
     // Sanity check. Master may not be both slave and master.
     mxb_assert(m_master == NULL || !m_master->has_status(SERVER_SLAVE | SERVER_MASTER));
 
@@ -532,13 +539,6 @@ void MariaDBMonitor::process_state_changes()
             enforce_read_only_on_slaves();
         }
 
-        /* Set low disk space slaves to maintenance.
-         */
-        if (m_maintenance_on_low_disk_space && !m_cluster_modified)
-        {
-            set_low_disk_slaves_maintenance();
-        }
-
         /* Check if the master server is on low disk space and act on it. */
         if (m_switchover_on_low_disk_space && !m_cluster_modified)
         {

server/modules/monitor/mariadbmon/mariadbserver.cc

@@ -1121,9 +1121,8 @@ bool MariaDBServer::can_be_demoted_failover(string* reason_out)
     return demotable;
 }
 
-bool MariaDBServer::can_be_promoted(OperationType op,
+bool MariaDBServer::can_be_promoted(OperationType op, const MariaDBServer* demotion_target,
-                                    const MariaDBServer* demotion_target,
+                                    string* reason_out)
-                                    std::string* reason_out)
 {
     bool promotable = false;
     string reason;
@@ -1134,6 +1133,10 @@ bool MariaDBServer::can_be_promoted(OperationType op,
     {
         reason = "it is already the master.";
     }
+    else if (!is_usable())
+    {
+        reason = "it is down or in maintenance.";
+    }
     else if (sstatus == NULL)
     {
         reason = string_printf("it is not replicating from '%s'.", demotion_target->name());