MXS-2149 Add REST-API watchdog

This will simply cause a task to be posted to each worker.
If the workers are running normally, the task will reach the
workers and the associated semaphore posted, and the REST-API
call will return. If any worker is not running normally, the
task will not be processed and the REST-API call will hang.

include/maxscale/routingworker.hh

@@ -609,3 +609,11 @@ json_t* mxs_rworker_to_json(const char* host, int id);
  * @see mxs_json_resource()
  */
 json_t* mxs_rworker_list_to_json(const char* host);
+
+/**
+ * @brief MaxScale worker watchdog
+ *
+ * If this function returns, then MaxScale is alive. If not,
+ * then some thread is dead.
+ */
+void mxs_rworker_watchdog();

server/core/resource.cc

@@ -900,6 +900,7 @@ HttpResponse cb_modulecmd(const HttpRequest& request)
 
 HttpResponse cb_send_ok(const HttpRequest& request)
 {
+    mxs_rworker_watchdog();
     return HttpResponse(MHD_HTTP_OK);
 }
 

server/core/routingworker.cc

@@ -1167,3 +1167,28 @@ json_t* mxs_rworker_list_to_json(const char* host)
     RoutingWorker::execute_concurrently(task);
     return task.resource();
 }
+
+namespace
+{
+
+class WatchdogTask : public Worker::Task
+{
+public:
+    WatchdogTask()
+    {
+    }
+
+    void execute(Worker& worker)
+    {
+        // Success if this is called.
+    }
+};
+
+}
+
+void mxs_rworker_watchdog()
+{
+    MXS_INFO("MaxScale watchdog called.");
+    WatchdogTask task;
+    RoutingWorker::execute_concurrently(task);
+}