函数get_table_share

得到一个table_share

1)先从table_def_cache中查找, 如果有, 直接返回

2)如果没有找到,

   为table_share分配内存,初始化,打开.frm文件,并将share放置hashTable中

   如果hashTable的容量超过限制,根据lru策略,在oldest_unused_tables中循环删除table share

注意:

　　oldest_unused_tables中的元素是close_table时放入进去的

/*
  Get TABLE_SHARE for a table.

  get_table_share()
  thd            Thread handle
  table_list        Table that should be opened
  key            Table cache key
  key_length        Length of key
  db_flags        Flags to open_table_def():
            OPEN_VIEW
  error            out: Error code from open_table_def()

  IMPLEMENTATION
    Get a table definition from the table definition cache.
    If it doesn't exist, create a new from the table definition file.

  NOTES
    We must have wrlock on LOCK_open when we come here
    (To be changed later)

  RETURN
   0  Error
   #  Share for table
*/

TABLE_SHARE *get_table_share(THD *thd, TABLE_LIST *table_list, char *key,
                             uint key_length, uint db_flags, int *error,
                             my_hash_value_type hash_value)
{
  TABLE_SHARE *share;
  DBUG_ENTER("get_table_share");

  *error= ;

  /*
    To be able perform any operation on table we should own
    some kind of metadata lock on it.
  */
  DBUG_ASSERT(thd->mdl_context.is_lock_owner(MDL_key::TABLE,
                                             table_list->db,
                                             table_list->table_name,
                                             MDL_SHARED));

  /* Read table definition from cache */
  if ((share= (TABLE_SHARE*) my_hash_search_using_hash_value(&table_def_cache,
                                                             hash_value, (uchar*) key, key_length)))
    goto found;

  if (!(share= alloc_table_share(table_list, key, key_length)))
  {
    DBUG_RETURN();
  }

  /*
    We assign a new table id under the protection of LOCK_open.
    We do this instead of creating a new mutex
    and using it for the sole purpose of serializing accesses to a
    static variable, we assign the table id here. We assign it to the
    share before inserting it into the table_def_cache to be really
    sure that it cannot be read from the cache without having a table
    id assigned.

    CAVEAT. This means that the table cannot be used for
    binlogging/replication purposes, unless get_table_share() has been
    called directly or indirectly.
   */
  assign_new_table_id(share);

  if (my_hash_insert(&table_def_cache, (uchar*) share))
  {
    free_table_share(share);
    DBUG_RETURN();                // return error
  }
  if (open_table_def(thd, share, db_flags))
  {
    *error= share->error;
    (void) my_hash_delete(&table_def_cache, (uchar*) share);
    DBUG_RETURN();
  }
  share->ref_count++;                // Mark in use
  DBUG_PRINT("exit", ("share: 0x%lx  ref_count: %u",
                      (ulong) share, share->ref_count));
  DBUG_RETURN(share);

found:
  /*
     We found an existing table definition. Return it if we didn't get
     an error when reading the table definition from file.
  */
  if (share->error)
  {
    /* Table definition contained an error */
    open_table_error(share, share->error, share->open_errno, share->errarg);
    DBUG_RETURN();
  }
  if (share->is_view && !(db_flags & OPEN_VIEW))
  {
    open_table_error(share, , ENOENT, );
    DBUG_RETURN();
  }

  ++share->ref_count;

   && share->prev)
  {
    /*
      Share was not used before and it was in the old_unused_share list
      Unlink share from this list
    */
    DBUG_PRINT("info", ("Unlinking from not used list"));
    *share->prev= share->next;
    share->next->prev= share->prev;
    share->next= ;
    share->prev= ;
  }

   /* Free cache if too big */
  while (table_def_cache.records > table_def_size &&
         oldest_unused_share->next)
    my_hash_delete(&table_def_cache, (uchar*) oldest_unused_share);

  DBUG_PRINT("exit", ("share: 0x%lx  ref_count: %u",
                      (ulong) share, share->ref_count));
  DBUG_RETURN(share);
}