Bug#6123 - GRANT USAGE creates useless mysql.db row

  Prevent creation of a row which grants no rights
  Test included

mysql-test/r/grant.result

@@ -150,3 +150,9 @@ GRANT SELECT ON `ab%`.* TO 'test11'@'localhost'
 GRANT SELECT ON `a%`.* TO 'test11'@'localhost'
 delete from mysql.user where user='test11';
 delete from mysql.db where user='test11';
+create database db6123;
+grant usage on db6123.* to test6123 identified by 'magic123';
+select host,db,user,select_priv,insert_priv from mysql.db where db="db6123";
+host	db	user	select_priv	insert_priv
+delete from mysql.user where user='test6123';
+drop database db6123;

mysql-test/t/grant.test

@@ -105,3 +105,12 @@ flush privileges;
 show grants for test11@localhost;
 delete from mysql.user where user='test11';
 delete from mysql.db where user='test11';
+
+#
+# Bug#6123: GRANT USAGE inserts useless Db row
+#
+create database db6123;
+grant usage on db6123.* to test6123 identified by 'magic123';
+select host,db,user,select_priv,insert_priv from mysql.db where db="db6123";
+delete from mysql.user where user='test6123';
+drop database db6123;

sql/sql_acl.cc

@@ -1587,7 +1587,7 @@ static int replace_db_table(TABLE *table, const char *db,
 	goto table_error;			/* purecov: deadcode */
     }
   }
-  else if ((error=table->file->write_row(table->record[0])))
+  else if (rights && (error=table->file->write_row(table->record[0])))
   {
     if (error && error != HA_ERR_FOUND_DUPP_KEY) /* purecov: inspected */
       goto table_error; /* purecov: deadcode */
@@ -1597,6 +1597,7 @@ static int replace_db_table(TABLE *table, const char *db,
   if (old_row_exists)
     acl_update_db(combo.user.str,combo.host.str,db,rights);
   else
+  if (rights)
     acl_insert_db(combo.user.str,combo.host.str,db,rights);
   table->file->index_end();
   DBUG_RETURN(0);