myisammrg/myrg_open.c:
  never mark merge table as readonly even if all underlying myisam tables are readonly,
  otherwise ALTER TABLE merge UNION=(t1,t2,t3,...) will not work
sql/handler.cc:
  map EACCES to ER_OPEN_AS_READONLY.
  can only happen on modifying merge table that has (some) read-only myisam tables