Define event consumption interface for the application side event service

implementation.
Implement event pass-through to the applicaiton.

Refs codership/wsrep-lib#174

include/wsrep/event_service.hpp

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2020 Codership Oy <info@codership.com>
+ *
+ * This file is part of wsrep-lib.
+ *
+ * Wsrep-lib is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * Wsrep-lib is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with wsrep-lib.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+
+/** @file event_service.hpp
+ *
+ * Service interface for providing events to DBMS.
+ */
+
+#ifndef WSREP_EVENT_SERVICE_HPP
+#define WSREP_EVENT_SERVICE_HPP
+
+#include <string>
+
+namespace wsrep
+{
+
+    /** @class event_service
+     *
+     * Event service interface. This provides an interface corresponding
+     * to wsrep-API event service. For details see
+     * wsrep-API/wsrep_event_service.h
+     */
+    class event_service
+    {
+    public:
+        virtual ~event_service() { }
+
+        /**
+         * Process event with name name and value value.
+         */
+        virtual void process_event(const std::string& name,
+                                   const std::string& value) = 0;
+    };
+}
+
+#endif // WSREP_EVENT_SERVICE_HPP

include/wsrep/provider.hpp

@@ -47,6 +47,7 @@ namespace wsrep
     class thread_service;
     class tls_service;
     class allowlist_service;
+    class event_service;
 
     class stid
     {
@@ -424,10 +425,29 @@ namespace wsrep
             wsrep::thread_service* thread_service;
             wsrep::tls_service* tls_service;
             wsrep::allowlist_service* allowlist_service;
+            wsrep::event_service* event_service;
+
+            // some GCC and clang versions don't support C++11 default
+            // initializers fully, so we need to use explicit constructors
+            // instead:
+            // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88165
+            // https://bugs.llvm.org/show_bug.cgi?id=36684
             services()
                 : thread_service()
                 , tls_service()
                 , allowlist_service()
+                , event_service()
+            {
+            }
+
+            services(wsrep::thread_service* thr,
+                     wsrep::tls_service*    tls,
+                     wsrep::allowlist_service* all,
+                     wsrep::event_service*  event)
+                : thread_service(thr)
+                , tls_service(tls)
+                , allowlist_service(all)
+                , event_service(event)
             {
             }
         };