bug #702 - ALTER TABLE tm UNION=(t1,t2) doesn't work if t1.MYD is compressed

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

myisammrg/myrg_open.c

@@ -33,7 +33,7 @@
 MYRG_INFO *myrg_open(const char *name, int mode, int handle_locking)
 {
   int save_errno,i,errpos;
-  uint files,dir_length,length,options, key_parts;
+  uint files,dir_length,length,key_parts;
   ulonglong file_offset;
   char name_buff[FN_REFLEN*2],buff[FN_REFLEN],*end;
   MYRG_INFO info,*m_info;
@@ -110,13 +110,11 @@ MYRG_INFO *myrg_open(const char *name, int mode, int handle_locking)
   }
   errpos=2;
 
-  options= (uint) ~0;
   for (i=files ; i-- > 0 ; )
   {
     uint j;
     m_info->open_tables[i].table=isam;
     m_info->options|=isam->s->options;
-    options&=isam->s->options;
     m_info->records+=isam->state->records;
     m_info->del+=isam->state->del;
     m_info->data_file_length+=isam->state->data_file_length;
@@ -125,8 +123,7 @@ MYRG_INFO *myrg_open(const char *name, int mode, int handle_locking)
     if (i)
       isam=(MI_INFO*) (isam->open_list.next->data);
   }
-  /* Don't force readonly if not all tables are readonly */
-  if (! (options & (HA_OPTION_COMPRESS_RECORD | HA_OPTION_READ_ONLY_DATA)))
+  /* Don't mark table readonly, for ALTER TABLE ... UNION=(...) to work */
   m_info->options&= ~(HA_OPTION_COMPRESS_RECORD | HA_OPTION_READ_ONLY_DATA);
 
   /* Fix fileinfo for easyer debugging (actually set by rrnd) */

sql/handler.cc

@@ -801,6 +801,9 @@ void handler::print_error(int error, myf errflag)
 
   int textno=ER_GET_ERRNO;
   switch (error) {
+  case EACCES:
+    textno=ER_OPEN_AS_READONLY;
+    break;
   case EAGAIN:
     textno=ER_FILE_USED;
     break;