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+/*****************************************************************************
+
+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.
+
+This program is also distributed with certain software (including but not
+limited to OpenSSL) that is licensed under separate terms, as designated in a
+particular file or component or in included license documentation. The authors
+of MySQL hereby grant you an additional permission to link the program and
+your derivative works with the separately licensed software that they have
+included with MySQL.
+
+This program is distributed in the hope that it will be useful, but WITHOUT
+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.
+
+You should have received a copy of the GNU General Public License along with
+this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+
+*****************************************************************************/
+#pragma once
+#include "buf0buf.h"
+
+namespace buf {
+class Block_hint {
+public:
+ /** Stores the pointer to the block, which is currently buffer-fixed.
+ @param block a pointer to a buffer-fixed block to be stored */
+ inline void store(buf_block_t *block)
+ {
+ ut_ad(block->page.buf_fix_count());
+ m_block= block;
+ m_page_id= block->page.id();
+ }
+
+ /** Clears currently stored pointer. */
+ inline void clear() { m_block= nullptr; }
+
+ /** Invoke f on m_block(which may be null)
+ @param f The function to be executed. It will be passed the pointer.
+ If you wish to use the block pointer subsequently,
+ you need to ensure you buffer-fix it before returning from f.
+ @return the return value of f
+ */
+ template <typename F>
+ bool run_with_hint(const F &f)
+ {
+ buffer_fix_block_if_still_valid();
+ /* m_block could be changed during f() call, so we use local
+ variable to remember which block we need to unfix */
+ buf_block_t *block= m_block;
+ bool res= f(block);
+ if (block)
+ buf_block_buf_fix_dec(block);
+ return res;
+ }
+
+ buf_block_t *block() const { return m_block; }
+
+ private:
+ /** The block pointer stored by store(). */
+ buf_block_t *m_block= nullptr;
+ /** If m_block is non-null, the m_block->page.id at time it was stored. */
+ page_id_t m_page_id{0, 0};
+
+ /** A helper function which checks if m_block is not a dangling pointer and
+ still points to block with page with m_page_id and if so, buffer-fixes it,
+ otherwise clear()s it */
+ void buffer_fix_block_if_still_valid();
+};
+} // namespace buf