Add monitor reconfiguration helper

The MonitorManager function reconfigured a monitor and rolls back to the
old configuration if the new one doesn't work.

server/core/config_runtime.cc

@@ -745,23 +745,16 @@ bool do_alter_monitor(Monitor* monitor, const char* key, const char* value)
         return false;
     }
 
-    // Backup monitor parameters in case configure fails.
-    MXS_CONFIG_PARAMETER originals = monitor->parameters;
     MXS_CONFIG_PARAMETER modified = monitor->parameters;
-
     modified.set(key, value);
-    bool success = monitor->configure(&modified);
+
+    bool success = MonitorManager::reconfigure_monitor(monitor, modified);
+
     if (success)
     {
         MXS_NOTICE("Updated monitor '%s': %s=%s", monitor->m_name, key, value);
     }
-    else
+
-    {
-        // Configure failed, restore original configs. This should not fail.
-        // TODO: add a flag to monitor which prevents startup if config is wrong.
-        MXB_AT_DEBUG(bool check = ) monitor->configure(&originals);
-        mxb_assert(check);
-    }
     return success;
 }
 
@@ -2535,17 +2528,11 @@ bool runtime_alter_monitor_from_json(Monitor* monitor, json_t* new_json)
         MonitorManager::stop_monitor(monitor);
         auto old_params = monitor->parameters;
 
-        if (monitor->configure(&params))
+        if (MonitorManager::reconfigure_monitor(monitor, params))
         {
             MonitorManager::monitor_serialize(monitor);
             success = true;
         }
-        else
-        {
-            // Something went wrong, restore the old parameters
-            MXB_AT_DEBUG(bool check = ) monitor->configure(&old_params);
-            mxb_assert(check);
-        }
 
         if (restart)
         {

server/core/internal/monitor.hh

@@ -149,6 +149,18 @@ public:
      */
     static bool monitor_serialize(const mxs::Monitor* monitor);
 
+    /**
+     * Attempt to reconfigure a monitor
+     *
+     * If the configuration fails, the old parameters are restored.
+     *
+     * @param monitor    Monitor to reconfigure
+     * @param parameters New parameters to apply
+     *
+     * @return True if reconfiguration was successful
+     */
+    static bool reconfigure_monitor(mxs::Monitor* monitor, const MXS_CONFIG_PARAMETER& parameters);
+
     /**
      * @brief Convert monitor to JSON
      *

server/core/monitor.cc

@@ -792,6 +792,24 @@ bool MonitorManager::monitor_serialize(const Monitor* monitor)
     return rval;
 }
 
+// static
+bool MonitorManager::reconfigure_monitor(mxs::Monitor* monitor, const MXS_CONFIG_PARAMETER& parameters)
+{
+    // Backup monitor parameters in case configure fails.
+    auto orig = monitor->parameters;
+    monitor->parameters.clear();
+
+    bool success = monitor->configure(&parameters);
+
+    if (!success)
+    {
+        MXB_AT_DEBUG(bool check = ) monitor->configure(&orig);
+        mxb_assert(check);
+    }
+
+    return success;
+}
+
 json_t* MonitorManager::monitor_to_json(const Monitor* monitor, const char* host)
 {
     string self = MXS_JSON_API_MONITORS;