Add RAII class for stopping monitors

This way the state is encapsulated in the object and the required changes
are done in one place. This makes the code reusable across all functions
making it easier to implement better monitor alteration code.

server/core/config_runtime.cc

@@ -760,21 +760,14 @@ bool do_alter_monitor(Monitor* monitor, const char* key, const char* value)
 
 bool runtime_alter_monitor(Monitor* monitor, const char* key, const char* value)
 {
-    // If the monitor is already stopped, don't stop/start it.
+    MonitorStop stop(monitor);
-    bool was_running = (monitor->state() == MONITOR_STATE_RUNNING);
-    if (was_running)
-    {
-        MonitorManager::stop_monitor(monitor);
-    }
     bool success = do_alter_monitor(monitor, key, value);
+
     if (success)
     {
         MonitorManager::monitor_serialize(monitor);
     }
-    if (was_running)
+
-    {
-        MonitorManager::start_monitor(monitor);
-    }
     return success;
 }
 

server/core/internal/monitor.hh

@@ -178,3 +178,28 @@ public:
      */
     static void debug_wait_one_tick();
 };
+
+// RAII helper class for temprarily stopping monitors
+class MonitorStop
+{
+public:
+    MonitorStop(mxs::Monitor* monitor)
+        : m_monitor(monitor->state() == MONITOR_STATE_RUNNING ? monitor : nullptr)
+    {
+        if (m_monitor)
+        {
+            MonitorManager::stop_monitor(m_monitor);
+        }
+    }
+
+    ~MonitorStop()
+    {
+        if (m_monitor)
+        {
+            MonitorManager::start_monitor(m_monitor);
+        }
+    }
+
+private:
+    mxs::Monitor* m_monitor;
+};