/*****************************************************************************

Copyright (c) 2020, Oracle and/or its affiliates. All Rights Reserved.
Copyright (c) 2020, MariaDB Corporation.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License, version 2.0, as published by the
Free Software Foundation.

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limited to OpenSSL) that is licensed under separate terms, as designated in a
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License, version 2.0,
for more details.

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this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA

*****************************************************************************/

#include "buf0block_hint.h"
namespace buf {

void Block_hint::buffer_fix_block_if_still_valid()
{
  /* We need to check if m_block points to one of chunks. For this to be
  meaningful we need to prevent freeing memory while we check, and until we
  buffer-fix the block. For this purpose it is enough to latch any of the many
  latches taken by buf_resize().
  However, for buffer-fixing to be meaningful, the block has to contain a page
  (as opposed to being already empty, which might mean that buf_pool_resize()
  can proceed and free it once we free the s-latch), so we confirm that the
  block contains a page. However, it is not sufficient to check that this is
  just any page, because just after we check it could get freed, unless we
  have a latch which prevents this. This is tricky because page_hash latches
  are sharded by page_id and we don't know the page_id until we look into the
  block. To solve this chicken-and-egg problem somewhat, we latch the shard
  for the m_page_id and compare block->page.id to it - so if is equal then we
  can be reasonably sure that we have the correct latch.
  There is still a theoretical problem here, where other threads might try
  to modify the m_block->page.id while we are comparing it, but the chance of
  accidentally causing the old space_id == m_page_id.m_space and the new
  page_no == m_page_id.m_page_no is minimal as compilers emit a single 8-byte
  comparison instruction to compare both at the same time atomically, and f()
    will probably double-check the block->page.id again, anyway.
  Finally, assuming that we have correct hash bucket latched, we should check if
  the state of the block is BUF_BLOCK_FILE_PAGE before buffer-fixing the block,
  as otherwise we risk buffer-fixing and operating on a block, which is already
  meant to be freed. In particular, buf_LRU_free_page() first calls
  buf_LRU_block_remove_hashed() under hash bucket latch protection to change the
  state to BUF_BLOCK_REMOVE_HASH and then releases the latch. Later it calls
  buf_LRU_block_free_hashed_page() without any latch to change the state to
  BUF_BLOCK_MEMORY and reset the page's id, which means buf_resize() can free it
  regardless of our buffer-fixing. */
  if (m_block)
  {
    const buf_pool_t *const buf_pool= buf_pool_get(m_page_id);
    rw_lock_t *latch= buf_page_hash_lock_get(buf_pool, m_page_id);
    rw_lock_s_lock(latch);
    /* If not own buf_pool_mutex, page_hash can be changed. */
    latch= buf_page_hash_lock_s_confirm(latch, buf_pool, m_page_id);
    if (buf_pool->is_block_field(m_block) &&
        m_page_id == m_block->page.id &&
        buf_block_get_state(m_block) == BUF_BLOCK_FILE_PAGE)
      buf_block_buf_fix_inc(m_block, __FILE__, __LINE__);
    else
      clear();
    rw_lock_s_unlock(latch);
  }
}
}  // namespace buf
