BUG#13427949: CHANGE MASTER TO USER='' (EMPTY USER) CAUSES ERRORS ON VALGRING

  
When passing an empty user to the connect function will cause
valgrind warnings. Seems that the client code is not prepared 
to handle empty users. On 5.6 this can even be triggered by 
START SLAVE PASSWORD='...'; i.e., without setting USER='...' on
the START SLAVE command (see WL#4143 for details on the new
additional START SLAVE commands).
  
To fix this, we disallow empty users when configuring the slave
connection parameters (this decision might be revisited if the 
client code accepts empty users in the future).

sql/slave.cc:
  We throw an error if an empty user is supplied to the connection 
  function.

mysql-test/suite/rpl/r/rpl_connection.result

+include/master-slave.inc
+[connection master]
+call mtr.add_suppression(".*Invalid .* username when attempting to connect to the master server.*");
+include/stop_slave.inc
+CHANGE MASTER TO MASTER_USER= '', MASTER_PASSWORD= '';
+START SLAVE;
+include/wait_for_slave_io_error.inc [errno=1045, 1593]
+include/stop_slave.inc
+CHANGE MASTER TO MASTER_USER= 'root', MASTER_PASSWORD= '';
+START SLAVE;
+include/rpl_end.inc

mysql-test/suite/rpl/t/rpl_connection.test

+--source include/not_embedded.inc
+--source include/master-slave.inc
+--source include/have_binlog_format_mixed.inc
+
+#
+# BUG#13427949: CHANGE MASTER TO USER='' (EMPTY USER) CAUSES ERRORS ON VALGRING 
+#
+
+--connection slave
+call mtr.add_suppression(".*Invalid .* username when attempting to connect to the master server.*");
+
+
+# Assert that we disallow empty users and that no problem
+--source include/stop_slave.inc
+CHANGE MASTER TO MASTER_USER= '', MASTER_PASSWORD= '';
+START SLAVE;
+--let $slave_io_errno= 1045, 1593
+--source include/wait_for_slave_io_error.inc
+--source include/stop_slave.inc
+
+CHANGE MASTER TO MASTER_USER= 'root', MASTER_PASSWORD= '';
+START SLAVE;
+
+--source include/rpl_end.inc 

sql/slave.cc

@@ -4208,6 +4208,16 @@ static int connect_to_master(THD* thd, MYSQL* mysql, Master_info* mi,
   if (opt_plugin_dir_ptr && *opt_plugin_dir_ptr)
     mysql_options(mysql, MYSQL_PLUGIN_DIR, opt_plugin_dir_ptr);
 
+  /* we disallow empty users */
+  if (mi->user == NULL || mi->user[0] == 0)
+  {
+    mi->report(ERROR_LEVEL, ER_SLAVE_FATAL_ERROR,
+               ER(ER_SLAVE_FATAL_ERROR),
+               "Invalid (empty) username when attempting to "
+               "connect to the master server. Connection attempt "
+               "terminated.");
+    DBUG_RETURN(1);
+  }
   while (!(slave_was_killed = io_slave_killed(thd,mi)) &&
          (reconnect ? mysql_reconnect(mysql) != 0 :
           mysql_real_connect(mysql, mi->host, mi->user, mi->password, 0,
@@ -4336,7 +4346,9 @@ MYSQL *rpl_connect_master(MYSQL *mysql)
   /* This one is not strictly needed but we have it here for completeness */
   mysql_options(mysql, MYSQL_SET_CHARSET_DIR, (char *) charsets_dir);
 
-  if (io_slave_killed(thd, mi)
+  if (mi->user == NULL
+      || mi->user[0] == 0
+      || io_slave_killed(thd, mi)
       || !mysql_real_connect(mysql, mi->host, mi->user, mi->password, 0,
                              mi->port, 0, 0))
   {