unix_socket bypasses make_if_fail by not doing any network reads

mysql-test/r/failed_auth_unixsocket.result

+update mysql.user set plugin='unix_socket';
+flush privileges;
+connect(localhost,USER,,test,MASTER_PORT,MASTER_SOCKET);
+ERROR HY000: Plugin 'unix_socket' is not loaded
+ERROR HY000: Plugin 'unix_socket' is not loaded
+install plugin unix_socket soname 'auth_socket.so';
+connect(localhost,USER,,test,MASTER_PORT,MASTER_SOCKET);
+ERROR 28000: Access denied for user 'USER'@'localhost' (using password: NO)
+ERROR 28000: Access denied for user 'USER'@'localhost' (using password: NO)
+update mysql.user set plugin='';
+flush privileges;
+uninstall plugin unix_socket;

mysql-test/t/failed_auth_unixsocket.test

+--source include/have_unix_socket.inc
+
+#
+# MDEV-3909 remote user enumeration
+# unix_socket tests
+#
+update mysql.user set plugin='unix_socket';
+flush privileges;
+
+--replace_result $MASTER_MYSOCK MASTER_SOCKET $MASTER_MYPORT MASTER_PORT $USER USER
+--error ER_PLUGIN_IS_NOT_LOADED
+connect (fail,localhost,$USER);
+
+--error ER_PLUGIN_IS_NOT_LOADED
+change_user $USER;
+
+eval install plugin unix_socket soname '$AUTH_SOCKET_SO';
+
+--replace_result $MASTER_MYSOCK MASTER_SOCKET $MASTER_MYPORT MASTER_PORT $USER USER
+--error ER_ACCESS_DENIED_ERROR
+connect (fail,localhost,$USER);
+
+--error ER_ACCESS_DENIED_ERROR
+change_user $USER;
+
+update mysql.user set plugin='';
+flush privileges;
+uninstall plugin unix_socket;
+

sql/sql_acl.cc

@@ -8192,6 +8192,11 @@ bool acl_authenticate(THD *thd, uint connect_errors,
     auth_plugin_name= &mpvio.acl_user->plugin;
     res= do_auth_once(thd, auth_plugin_name, &mpvio);
   }
+  if (mpvio.make_it_fail)
+  {
+    mpvio.status= MPVIO_EXT::FAILURE;
+    res= CR_ERROR;
+  }
  
   Security_context *sctx= thd->security_ctx;
   ACL_USER *acl_user= mpvio.acl_user;