handler+alter table

mysql-test/r/handler.result:
  alter table test
mysql-test/t/handler.test:
  alter table test
sql/sql_handler.cc:
  bugfix

mysql-test/r/handler.result

@@ -129,6 +129,7 @@ a	b
 handler t2 read next;
 a	b
 18	eee
+alter table t1 type=MyISAM;
 handler t2 read next;
 a	b
 19	fff

mysql-test/t/handler.test

@@ -58,8 +58,10 @@ handler t2 read a=(19) where b="yyy";
 
 handler t2 read first;
 handler t2 read next;
+alter table t1 type=MyISAM;
 handler t2 read next;
 !$1064 handler t2 read last;
 
 handler t2 close;
 drop table if exists t1;
+

sql/sql_handler.cc

@@ -130,7 +130,11 @@ int mysql_ha_read(THD *thd, TABLE_LIST *tables,
   select_limit+=offset_limit;
   send_fields(thd,list,1);
 
+  HANDLER_TABLES_HACK(thd);
   MYSQL_LOCK *lock=mysql_lock_tables(thd,&tables->table,1);
+  HANDLER_TABLES_HACK(thd);
+  if (!lock)
+     goto err0; // mysql_lock_tables() printed error message already
 
   for (uint num_rows=0; num_rows < select_limit; )
   {
@@ -238,6 +242,7 @@ ok:
   return 0;
 err:
   mysql_unlock_tables(thd,lock);
+err0:
   return -1;
 }