MDEV-23693 Failing assertion: my_atomic_load32_explicit(&lock->lock_word, MY_MEMORY_ORDER_RELAXED) == X_LOCK_DECR

InnoDB frees the block lock during buffer pool shrinking when other
thread is yet to release the block lock.  While shrinking the
buffer pool, InnoDB allows the page to be freed unless it is buffer
fixed. In some cases, InnoDB releases the latch after unfixing the
block.

Fix:
====
- InnoDB should unfix the block after releases the latch.

- Add more assertion to check buffer fix while accessing the page.

- Introduced block_hint structure to store buf_block_t pointer
and allow accessing the buf_block_t pointer only by passing a
functor. It returns original buf_block_t* pointer if it is valid
or nullptr if the pointer become stale.

- Replace buf_block_is_uncompressed() with
buf_pool_t::is_block_pointer()

This change is motivated by a change in mysql-5.7.32:
mysql/mysql-server@46e60de444a8fbd876cc6778a7e64a1d3426a48d
Bug #31036301 ASSERTION FAILURE: SYNC0RW.IC:429:LOCK->LOCK_WORD

1 files changed, 77 insertions, 0 deletions

diff --git a/storage/innobase/include/buf0block_hint.h b/storage/innobase/include/buf0block_hint.h
new file mode 100644
index 00000000000..2d681175b25
--- /dev/null
+++ b/storage/innobase/include/buf0block_hint.h
@@ -0,0 +1,77 @@
+/*****************************************************************************
+
+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:
+  Block_hint():m_block(NULL),m_page_id(0,0) {}
+  /** 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= NULL; }
+
+  /** 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;
+  /** If m_block is non-null, the m_block->page.id at time it was stored. */
+  page_id_t m_page_id;
+
+  /** 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