MXS-2314 Monitor decides whether servers are added to services

When the servers of a service are defined by a monitor, then
at startup all servers of the monitor should be added to relevant
services. Likewise, when a server is added to or removed from a
monitor at runtime, those changes should affect services as well.

However, whether that should happen or not depends upon the monitor.
In the case of the Clustrix monitor this should not happen as it
adds and removes servers depending on the runtime state of the
Clustrix cluster.

include/maxscale/monitor.hh

@@ -337,6 +337,26 @@ protected:
      */
     void detect_handle_state_changes();
 
+    /**
+     * @brief Called when a server has been added to the monitor.
+     *
+     * The default implementation will add the server to associated
+     * services.
+     *
+     * @param server  A server.
+     */
+    virtual void server_added(SERVER* server);
+
+    /**
+     * @brief Called when a server has been removed from the monitor.
+     *
+     * The default implementation will remove the server from associated
+     * services.
+     *
+     * @param server  A server.
+     */
+    virtual void server_removed(SERVER* server);
+
     /**
      * Contains monitor base class settings. Since monitors are stopped before a setting change,
      * the items cannot be modified while a monitor is running. No locking required.
@@ -363,6 +383,11 @@ protected:
 
     Settings m_settings;
 
+private:
+    void add_server(SERVER* server);
+
+    void remove_server(SERVER* server);
+
 private:
     friend class MonitorManager;
 
@@ -395,7 +420,34 @@ private:
     int launch_command(MXS_MONITORED_SERVER* ptr, EXTERNCMD* cmd);
 
     /**
-     * Populate services with the servers of the monitor.
+     * @brief Add a server to a monitor.
+     *
+     * Add a server to a monitor, provided the server is not currently
+     * being monitored by any monitor. Before adding the server to the
+     * monitor, the monitor is stopped if it is running and after the
+     * addition it is restarted if it was running.
+     *
+     * @param monitor  A monitor.
+     * @param server   A server.
+     *
+     * @return True, if the monitor was added, false otherwise.
+     */
+    static bool add_server(Monitor* mon, SERVER* server);
+
+    /**
+     * @brief Remove a server from a monitor.
+     *
+     * If the server is being monitored by the server, remove it.
+     * Before removing, the monitor is stopped if it is running and after
+     * the removal it is restarted if it was running.
+     *
+     * @param monitor  A monitor.
+     * @param server   A server.
+     */
+    static void remove_server(Monitor* mon, SERVER* server);
+
+    /**
+     * @brief The monitor should populate associated services.
      */
     virtual void populate_services();