cleanup: ha_myisam::data_file_name and index_file_name

don't allocate them on THD::mem_root on every init(HA_STATUS_CONST) call,
do it once in open() (because they don't change) on TABLE::mem_root
(so they stay valid until the table is closed)

storage/myisam/ha_myisam.cc

@@ -729,6 +729,7 @@ int ha_myisam::open(const char *name, int mode, uint test_if_locked)
 {
   MI_KEYDEF *keyinfo;
   MI_COLUMNDEF *recinfo= 0;
+  char readlink_buf[FN_REFLEN], name_buff[FN_REFLEN];
   uint recs;
   uint i;
 
@@ -783,6 +784,30 @@ int ha_myisam::open(const char *name, int mode, uint test_if_locked)
     (void) mi_extra(file, HA_EXTRA_NO_WAIT_LOCK, 0);
 
   info(HA_STATUS_NO_LOCK | HA_STATUS_VARIABLE | HA_STATUS_CONST);
+
+  /*
+    Set data_file_name and index_file_name to point at the symlink value
+    if table is symlinked (Ie;  Real name is not same as generated name)
+  */
+  fn_format(name_buff, file->filename, "", MI_NAME_DEXT,
+            MY_APPEND_EXT | MY_UNPACK_FILENAME);
+  if (my_is_symlink(name_buff))
+  {
+    my_readlink(readlink_buf, name_buff, MYF(0));
+    data_file_name= strdup_root(&table->mem_root, readlink_buf);
+  }
+  else
+    data_file_name= 0;
+  fn_format(name_buff, file->filename, "", MI_NAME_IEXT,
+            MY_APPEND_EXT | MY_UNPACK_FILENAME);
+  if (my_is_symlink(name_buff))
+  {
+    my_readlink(readlink_buf, name_buff, MYF(0));
+    index_file_name= strdup_root(&table->mem_root, readlink_buf);
+  }
+  else
+    index_file_name= 0;
+
   if (!(test_if_locked & HA_OPEN_WAIT_IF_LOCKED))
     (void) mi_extra(file, HA_EXTRA_WAIT_LOCK, 0);
   if (!table->s->db_record_offset)
@@ -1847,7 +1872,6 @@ void ha_myisam::position(const uchar *record)
 int ha_myisam::info(uint flag)
 {
   MI_ISAMINFO misam_info;
-  char name_buff[FN_REFLEN];
 
   if (!table)
     return 1;
@@ -1895,27 +1919,6 @@ int ha_myisam::info(uint flag)
              sizeof(table->key_info[0].rec_per_key[0])*share->key_parts);
     if (table_share->tmp_table == NO_TMP_TABLE)
       mysql_mutex_unlock(&table_share->LOCK_share);
-
-   /*
-     Set data_file_name and index_file_name to point at the symlink value
-     if table is symlinked (Ie;  Real name is not same as generated name)
-   */
-    char buf[FN_REFLEN];
-    data_file_name= index_file_name= 0;
-    fn_format(name_buff, file->filename, "", MI_NAME_DEXT,
-              MY_APPEND_EXT | MY_UNPACK_FILENAME);
-    if (my_is_symlink(name_buff))
-    {
-      my_readlink(buf, name_buff, MYF(0));
-      data_file_name= ha_thd()->strdup(buf);
-    }
-    fn_format(name_buff, file->filename, "", MI_NAME_IEXT,
-              MY_APPEND_EXT | MY_UNPACK_FILENAME);
-    if (my_is_symlink(name_buff))
-    {
-      my_readlink(buf, name_buff, MYF(0));
-      index_file_name= ha_thd()->strdup(buf);
-    }
   }
   if (flag & HA_STATUS_ERRKEY)
   {