MXS-2314 Prevent removal of servers from clustered services

If the servers of a service are defined by a monitor, then it must
not be possible to dynamically add or remove servers from the
service.

server/core/config_runtime.cc

@@ -140,6 +140,8 @@ bool runtime_link_server(Server* server, const char* target)
     Monitor* monitor = service ? NULL : monitor_find(target);
 
     if (service)
+    {
+        if (!service->uses_cluster())
         {
             if (serviceAddBackend(service, server))
             {
@@ -153,6 +155,14 @@ bool runtime_link_server(Server* server, const char* target)
                                      server->name());
             }
         }
+        else
+        {
+            config_runtime_error("The servers of the service '%s' are defined by the monitor '%s'. "
+                                 "Servers cannot explicitly be added to the service.",
+                                 service->name,
+                                 service->m_monitor->m_name);
+        }
+    }
     else if (monitor)
     {
         if (monitor_add_server(monitor, server))
@@ -185,22 +195,35 @@ bool runtime_unlink_server(Server* server, const char* target)
 
     if (service || monitor)
     {
-        rval = true;
-
         if (service)
+        {
+            if (!service->uses_cluster())
             {
                 serviceRemoveBackend(service, server);
                 service_serialize(service);
+                rval = true;
+            }
+            else
+            {
+                config_runtime_error("The servers of the service '%s' are defined by the monitor '%s'. "
+                                     "Servers cannot explicitly be removed from the service.",
+                                     service->name,
+                                     service->m_monitor->m_name);
+            }
         }
         else if (monitor)
         {
             monitor_remove_server(monitor, server);
             monitor_serialize(monitor);
+            rval = true;
         }
 
+        if (rval)
+        {
             const char* type = service ? "service" : "monitor";
             MXS_NOTICE("Removed server '%s' from %s '%s'", server->name(), type, target);
         }
+    }
 
     return rval;
 }

server/core/internal/service.hh

@@ -108,6 +108,11 @@ public:
     // TODO: Make this private.
     Monitor*    m_monitor { nullptr }; /**< A possibly associated monitor */
 
+    bool uses_cluster() const
+    {
+        return m_monitor != nullptr;
+    }
+
 private:
     FilterList  m_filters;          /**< Ordered list of filters */
     std::string m_name;             /**< Name of the service */