MDEV-23456 fixup: Fix mtr_t::get_fix_count()

Before commit 05fa455 (MDEV-22110) we have slot->type == MTR_MEMO_MODIFY that are unrelated to incrementing the buffer-fix count. FindBlock::operator(): In debug builds, skip MTR_MEMO_MODIFY entries. Also, simplify the code a little. This fixes an infinite loop in the tests innodb.innodb_defragment and innodb.innodb_wl6326_big.

Author: dr-m

storage/innobase/mtr/mtr0mtr.cc

@@ -313,25 +313,17 @@ mini-transaction */
 struct FindBlock
 {
  int32_t num_fix;
- buf_block_t *block;
+ const buf_block_t *const block;
+
+ FindBlock(const buf_block_t *block_buf): num_fix(0), block(block_buf) {}
 
- FindBlock(buf_block_t *block_buf): num_fix(0), block(block_buf) {}
  bool operator()(const mtr_memo_slot_t* slot)
  {
- if (slot->object != NULL)
- {
- buf_block_t *mtr_block= reinterpret_cast<buf_block_t*>(slot->object);
- if (mtr_block == block)
-num_fix++;
- }
-
+ if (slot->object == block)
+ ut_d(if (slot->type != MTR_MEMO_MODIFY))
+ num_fix++;
  return true;
  }
-
- int32_t get_num_fix()
- {
- return num_fix;
- }
 };
 
 /** Release a resource acquired by the mini-transaction. */
@@ -832,10 +824,9 @@ mtr_t::release_free_extents(ulint n_reserved)
 
 int32_t mtr_t::get_fix_count(buf_block_t *block)
 {
- struct FindBlock find_block(block);
- Iterate<FindBlock> iteration(find_block);
+ Iterate<FindBlock> iteration((FindBlock(block)));
  if (m_memo.for_each_block(iteration))
- return iteration.functor.get_num_fix();
+ return iteration.functor.num_fix;
  return 0;
 }