FIL_PAGE_LSN is set in mtr_t::commit() when ReleaseBlocks::operator()(mtr_memo_slot_t* slot) is called. But in recv_recover_page() it's set when all hashed records are applied:

static void recv_recover_page(buf_block_t* block, mtr_t& mtr,                   

            const recv_sys_t::map::iterator& p,                                 

            fil_space_t* space = NULL,                                          

            mlog_init_t::init* init = NULL)                                     

{

...

  for (const log_rec_t* recv : p->second.log) {

...

    log_phys_t::apply_status a= l->apply(*block,                                

                 p->second.last_offset);

...

  }

...

  if (start_lsn) {                                                              

    ut_ad(end_lsn >= start_lsn);                                                

    mach_write_to_8(FIL_PAGE_LSN + frame, end_lsn);                             

    if (UNIV_LIKELY(frame == block->frame)) {                                   

      mach_write_to_8(srv_page_size                                             

          - FIL_PAGE_END_LSN_OLD_CHKSUM                                         

          + frame, end_lsn);                                                    

    }

...                                          

    mysql_mutex_lock(&log_sys.flush_order_mutex);                               

    buf_flush_note_modification(block, start_lsn, end_lsn);                     

    mysql_mutex_unlock(&log_sys.flush_order_mutex);                             

  }

...

  mtr.commit(); 

...

}

So we can't currently compare FIL_PAGE_LSN when CHECKSUM record is applied.

Pushed bb-10.5-MDEV-18976-redolog-crc branch. FIL_PAGE_LSN check is not implemented by the reason described above.

There is no clear description about what must happen if CRC does no match, so I implemented the version where warning is issued in error log.

I did not find the other way to test it except injecting debug code. My initial intention was to change pages FIL_PAGE_LSN to start recovery with OPTION CHECKSUM record, in this case page CRC would not match, but I did not find the way to pass/to count LSN of CHECKSUM record in mtr test.

The recursive page latches during page allocation and BLOB operations are making it challenging to implement this. I got an https://rr-project.org trace of a checksum mismatch. For the recovery of a mini-transaction that deletes a BLOB page by page during the ROLLBACK of an INSERT, there will be a checksum mismatch for an allocation bitmap page, because that page had already been modified in a ‘parent’ mini-transaction that did not write its log yet. The parent mini-transaction holds exclusive latches on the clustered index leaf page as well as the allocation bitmap page. I think that we must specially flag those sub-mini-transactions, so that checksum records will only be written in the parent mini-transaction.

There are also some challenges to implement this for ROW_FORMAT=COMPRESSED pages, because there are two copies of the page in the buffer pool.

There is an anomaly where OPT_PAGE_CHECKSUM records could be emitted after a FREE_PAGE record. In this case, recovery would fail, because a checksum would be computed on a page that was freed during recovery.

This can happen during DROP INDEX (or DROP TABLE or any table-rebuilding DDL in the system tablespace). To prevent the anomaly, we must revise the logic of MDEV-15528 and mark as freed any buffer pool pages that were modified by the mini-transaction before being freed in the mini-transaction. That in turn will depend on sux_lock_t::x_lock_upgraded() that was introduced in MDEV-24142. Furthermore, this will depend on some other changes that were made as part of MDEV-13542.