MXS-2794: Log changes in loaded users

By checking whether the users have changed whenever they are reloaded, we
improve the visibility of the user reloading process. Using a checksum
allows us to easily compress the information with acceptable loss of
accuracy. Using a CAS loop prevents duplicate messages without losing any
updates even if multiple user reloads result in different outcomes.

server/modules/authenticator/MariaDBAuth/dbusers.cc

@@ -1143,6 +1143,42 @@ bool query_and_process_users(const char* query, MYSQL* con, SERVICE* service, in
     return rval;
 }
 
+void log_loaded_users(MYSQL_AUTH* instance, SERVICE* service, Listener* port, SERVER* srv,
+                      const std::vector<User>& userlist, const std::vector<std::string>& dblist)
+{
+    uint64_t c = crc32(0, 0, 0);
+
+    for (const auto& user : userlist)
+    {
+        c = crc32(c, (uint8_t*)user.user.c_str(), user.user.length());
+        c = crc32(c, (uint8_t*)user.host.c_str(), user.host.length());
+        c = crc32(c, (uint8_t*)user.db.c_str(), user.db.length());
+        uint8_t anydb = user.anydb;
+        c = crc32(c, &anydb, sizeof(anydb));
+        c = crc32(c, (uint8_t*)user.pw.c_str(), user.pw.length());
+    }
+
+    for (const auto& db : dblist)
+    {
+        c = crc32(c, (uint8_t*)db.c_str(), db.length());
+    }
+
+    uint64_t old_c = instance->checksum;
+
+    while (old_c != c)
+    {
+        if (mxb::atomic::compare_exchange(&instance->checksum, &old_c, c,
+                                          mxb::atomic::RELAXED, mxb::atomic::RELAXED))
+        {
+            MXS_NOTICE("[%s] Loaded %lu MySQL users for listener '%s' from server '%s' with checksum 0x%0lx.",
+                       service->name(), userlist.size(), port->name(), srv->name(), c);
+            break;
+        }
+
+        old_c = instance->checksum;
+    }
+}
+
 int get_users_from_server(MYSQL* con, SERVER* server, SERVICE* service, Listener* listener)
 {
     auto server_version = server->version();
@@ -1216,6 +1252,8 @@ int get_users_from_server(MYSQL* con, SERVER* server, SERVICE* service, Listener
 
     if (rv)
     {
+        log_loaded_users(instance, service, listener, server, userlist, dblist);
+
         auto func = [instance, userlist, dblist]() {
                 sqlite3* handle = get_handle(instance);
 

server/modules/authenticator/MariaDBAuth/mysql_auth.cc

@@ -192,6 +192,7 @@ static void* mysql_auth_init(char** options)
         instance->skip_auth = false;
         instance->check_permissions = true;
         instance->lower_case_table_names = false;
+        instance->checksum = 0;
 
         for (int i = 0; options[i]; i++)
         {
@@ -820,12 +821,6 @@ static int mysql_auth_load_users(Listener* port)
                     " will probably fail as a result.",
                     service->name());
     }
-    else if (loaded > 0 && first_load)
-    {
-        mxb_assert(srv);
-        MXS_NOTICE("[%s] Loaded %d MySQL users for listener %s from server %s.",
-                   service->name(), loaded, port->name(), srv->name());
-    }
 
     return rc;
 }

server/modules/authenticator/MariaDBAuth/mysql_auth.hh

@@ -112,6 +112,7 @@ typedef struct mysql_auth
     bool      skip_auth;            /**< Authentication will always be successful */
     bool      check_permissions;
     bool      lower_case_table_names;   /**< Disable database case-sensitivity */
+    uint64_t  checksum;
 } MYSQL_AUTH;
 
 /**