Destroy monitor only if it's not used

If a service uses a monitor as the source of its servers, it must not be
destroyed before the monitor is removed from all services that use it.

server/core/config_runtime.cc

@@ -1457,6 +1457,14 @@ bool runtime_destroy_service(Service* service)
 bool runtime_destroy_monitor(Monitor* monitor)
 {
     bool rval = false;
+
+    if (Service* s = service_uses_monitor(monitor))
+    {
+        config_runtime_error("Monitor '%s' cannot be destroyed as it is used by service '%s'",
+                             monitor->m_name, s->name());
+    }
+    else
+    {
         char filename[PATH_MAX];
         snprintf(filename, sizeof(filename), "%s/%s.cnf", get_config_persistdir(), monitor->m_name);
 
@@ -1473,6 +1481,7 @@ bool runtime_destroy_monitor(Monitor* monitor)
         {
             rval = true;
         }
+    }
 
     if (rval)
     {

server/core/internal/service.hh

@@ -347,6 +347,15 @@ bool service_port_is_used(int port);
  */
 bool service_has_named_listener(Service* service, const char* name);
 
+/**
+ * See if a monitor is used by any service
+ *
+ * @param monitor Monitor to look for
+ *
+ * @return The first service that uses the monitor or nullptr if no service uses it
+ */
+Service* service_uses_monitor(mxs::Monitor* monitor);
+
 // Required by MaxAdmin
 int service_enable_root(Service* service, int action);
 

server/core/service.cc

@@ -840,6 +840,21 @@ Service* service_internal_find(const char* name)
     return nullptr;
 }
 
+Service* service_uses_monitor(mxs::Monitor* monitor)
+{
+    LockGuard guard(this_unit.lock);
+
+    for (Service* s : this_unit.services)
+    {
+        if (s->m_monitor == monitor)
+        {
+            return s;
+        }
+    }
+
+    return nullptr;
+}
+
 /**
  * Return a named service
  *