dict_check_sys_tables(): Do not rely on buf_page_optimistic_get()

We are supposed to commit and restart the mini-transaction between records. There is no point to store and restore the persistent cursor position otherwise. If buf_page_optimistic_get() is patched to always fail, the debug build would fail to start up due to trying to re-acquire an already S-latched block. This bug (which should not have visible impact to users, because the code is only executed during startup, while no other threads are accessing B-trees or causing pages to be evicted from the buffer pool) was caught as part of a debugging effort for something else. The debugging approach was: Make buf_page_optimistic_get() always return FALSE, and add ut_a(block->lock.lock_word == X_LOCK_DECR) to both buf_LRU_get_free_only() and buf_LRU_block_free_non_file_page(). This would catch misuse of the buffer pool. If it were not for buf_page_optimistic_get(), no buf_block_t::lock of any BUF_BLOCK_NOT_USED block would ever be acquired.

Author: dr-m

storage/innobase/dict/dict0load.cc

@@ -1388,6 +1388,7 @@ static ulint dict_check_sys_tables()
 
 for (rec = dict_startscan_system(&pcur, &mtr, SYS_TABLES);
  rec != NULL;
+ mtr.commit(), mtr.start(),
  rec = dict_getnext_system(&pcur, &mtr)) {
 const byte*	field;
 ulint	len;